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© Copr. 1949-1998 Hewlett-Packard Co.
Current Tracer: A New Way to Find LowImpedance Logic-Circuit Faults
By tracing current pulses, this sensitive probe helps locate so l d e r b r i d g e s , s hort ed c onduc t ors in ca b le s, sh o rts in voltage distribution networks, shorted 1C inputs and dead or stuck outputs, stuck wired-AND circuits, and stuck data buses. by John F. Beckwith
FOR TROUBLESHOOTING DIGITAL systems, instruments such as logic probes, logic pulsers, logic comparators, state analyzers, and computerbased board testers enable an operator to localize a system malfunction down to the faulty node, that is, down to a collection of 1C terminals and the network of printed circuit traces and/or wires that electrically tie the terminals together. However, after the faulty node is identified the operator still lacks the ultimate information needed to make the repair, namely, ex actly what part of the node has failed? Is the driving 1C dead, or has one of the driven ICs developed a shorted input? Has the interconnecting network shorted to another node, and if so, precisely where, or has the interconnecting network developed an open circuit? Except when the fault is due to an open circuit, vol tage-sensing instruments provide no further infor mation, simply because all points of the faulty node are constrained by the interconnecting network to be at the same voltage. At this stage, techniques such as cutting traces or lifting ICs are usually employed until the defective element is identified. This ap proach is time-consuming, tedious, and often damag ing to the printed circuit board and ICs. Although voltage provides no additional informa tion, there is a quantity whose variation about the node provides the information needed to pinpoint the faulty element. This quantity is current. To date, little use has been made of the information provided by the nodal current distribution simply because of the difficulty of measuring current flow. Traditional methods, such as cutting a trace and inserting an ammeter, or encircling the trace with a magnetic path, are clearly very awkward to use on printed cir cuit boards. The question thus arises whether there is a more convenient means to determine current flow in logic circuits. The new Hewlett-Packard Model 547A Current Tracer (Fig. I) was developed to meet this need. The current tracer is a self-contained, hand-held probe containing a single easily-viewed display lamp
Printed m U S A
whose intensity indicates the relative magnitude of current steps occurring in the vicinity of the current tracer's tip. The reference level for the display can be varied from one milliampere to one ampere by means of a conveniently placed sensitivity control. The 547A responds to the current changes normally preCover: A new troubleimt Sno°t'n9 team finds many â¢â¢*^ low-impedance faults that elude voltage-sensing in struments. Model 547 A Cur rent Tracer (right) reveals the presence and relative size of current steps by the bright ness of its built-in lamp. Model 546A Logic Pulser supplies the needed stimulus current if it isn't already present in the system under test.
In this Issue:
Current Tracer: A New Way to Find Low-Impedance Logic-Circuit Faults, by John F. Beckwith page 2 New Logic Probe Troubleshoots Many Logic Families, by Robert C. Quenelle page 9 A Multifunction, Multifamily Logic Pulser, by Barry Bronson and Anthony Y . C h a n p a g e
Probe Family Packaging, by David E. Gordon, page 16.
1 2
Multifamily Logic Clip Shows All Pin States Simultaneously, by Durward P r i e b e p a g e Interfacing a Parallel-Mode Logic State Analyzer to Serial Data, by Justin S . M o r r i l l , J r p a g e
1 8
2 1
©Hewlett-Packard Company 1976
© Copr. 1949-1998 Hewlett-Packard Co.
To operate the current tracer the user first places its tip near the driving point of the node, which is usually one of the 1C terminals, or near the tip of the logic pulser when the node needs an external stimulus. Then the sensitivity control of the current tracer is adjusted until the display lamp is between one-half and fully lit. At this point the operator can get an indication of the magnitude of the current flowing simply by noting the position of the sensitivity control. This in formation is often very useful in determining the nature of the fault. For example, if the current tracer indicates an abnormally high current, then the fault is due to a low impedance. The operator then moves the tip of the tracer along the conductive paths, and by noting the intensity of the indicator lamp, can tell if current is present near the tip. In this manner, he can follow the current directly to the fault. The cur rent tracer's tip need not make physical contact with the conductive path, so current can be followed in in sulated wires and along inner traces of multilayer boards.
The Current Tracer in Use
Fig. 1. The lamp near the tip of the new Model 547 A Current Tracer varies in brightness according to the relative magni tude of the current steps occurring near its tip. By observing how the lamp's brightness changes as the tracer is moved along the conductors the user can follow the flow of current.
Fault localization in digital systems is the major applications area for the current tracer, but there are others as well. Whenever a low-impedance fault exists, whether on a digital board or not, the shorted node can be stimulated with a logic pulser and the current followed by means of the current tracer. Some typical current tracer applications are described in the paragraphs that follow. ⢠Ground planes. An interesting application of the current tracer is designing ground planes and de termining their effectiveness by tracing current distribution through the plane. Current is injected into the ground plane using either a logic pulser or a pulse generator, and current flow is easily traced over the plane. Often the results are surprising in
547A Current Tracer
sent in digital circuits and typically does not require the injection of a stimulating signal. When a stimulus is required it may be provided by a logic pulser such as the HP Models 546A or 10526T. Synchronizing signals between pulser and current tracer are not needed. The 547A is compatible with all logic fami lies including CMOS. It responds only to current and possesses sufficient sensitivity and dynamic range to detect currents resulting from faults in any of the pres ently available families. Power for the current tracer may be derived from any dc source between 4.5 and 18 volts. No ground reference is required and a float ing battery may be used if desired.
Current tracer will indicate very little current at driver output
Jinn
Pulse Activity
Dead Driver Voltage stuck at some value
Fig. 2. Using the current tracer to determine that a stuck node is caused by a dead driver.
© Copr. 1949-1998 Hewlett-Packard Co.
that current flows through only a few paths, or along edges. Vcc-to-ground shorts. Locating Vcc-to-ground shorts is an almost impossible task without the ability to trace current flow. To find the short, the user disconnects the power supply and pulses the power supply terminal using the logic pulser with the supply return connected to the GND lead of the pulser. Even if capacitors are connected between Vcc and ground, the current tracer will usually reveal the path carrying the greatest current. Stuck node caused by dead driver. Fig. 2 illus trates a frequently occurring troubleshooting symptom: a node has been identified on which the voltage is stuck high or low. Is the driver dead, or is something, such as a shorted input, clamping the node to a fixed value? This question is readily answered by tracing current from the driver to other elements on the node. If the driver is dead, the only current indicated by the tracer will be that caused by parasitic coupling from any nearby cur rents, and this will be much smaller than the nor mal current capability of the driver. On the other hand, if the driver is good, normal short circuit current will be present and can be traced to the circuit element clamping the node. Stuck node caused by input short. Fig. 3 illustrates this situation, which has exactly the same voltage symptoms as the previous case of a stuck node caused by a dead driver. However, the current tracer will now indicate a large current flowing from the driver, and will also make it possible to follow this current to the cause of the problem, the shorted input. The same procedure will also find the fault when the short is on the intercon necting path of the node â" for example, a solder bridge to another node.
Current tracer indicates large currents 547A Current Tracer
Stuck wired-AND node. Another difficult circuit problem to troubleshoot is the wired-AND node, a node formed by connecting several open-collector output structures. Fig. 4 shows how the current tracer is used to solve this problem. Each gate's output is placed in turn in its off or high-impedance state by forcing the inputs to an appropriate level (a jumper may be used) while a logic pulser is used to stimulate the output node. If the gate is good the current tracer will indicate only stray current at the gate output. Conversely, a stuck gate will re sult in a large current indication. The need to force the output of the gate to the off state by means of the jumper at the input can be eliminated if the duty cycle of the high-impedance state when the circuit is operating is not too low. When this is the case a logic pulser and current tracer may be used in the single-pulse mode. If the gate is not stuck the operator should observe a random presence and absence of current at the gate output while singlepulsing the node. If the gate is stuck, each pulse from the pulser will result in a large current indi cation from the tracer. Stuck three-state data bus. A stuck three-state bus, such as a microprocessor data or address bus, pre-
Logic Pulser
Gate with Shorted Input
Current tracer indicates large current at stuck gate
_n_n_n
Pulse Activity
Temporary jumper to ground Voltage stuck at some value
V
Bad gate stuck in LO state
Fig. 3. The current tracer shows that this stuck node is caused by an input short.
Fig. 4. The current tracer can often make use of normal cur rents in a circuit, but sometimes a logic pulser provides a needed stimulus, as in the difficult problem of a stuck wiredAND node.
© Copr. 1949-1998 Hewlett-Packard Co.
sents a very difficult troubleshooting problem, especially to voltage-sensing measurement tools. Because of the many bus terminals, it is very diffi cult to isolate the one bus element holding it in a stuck condition. However, if the current tracer indicates high current at several driver outputs, it is likely that one (and most likely onJy one) driver is stuck in a low-impedance state. The defective driver is located by placing one driver's control input line to the appropriate level for a highimpedance output state and noting whether high current flow persists at the output. This is repeated for each driver until the bad one is located. Al ternatively, if the low-impedance duty cycle of a driver is low, the node can be pulsed with the logic pulser, and the defective driver identified by noting whether every pulse from the pulser results in cur rent at the driver output. If the current tracer indi cates high currents at only two drivers the problem is a "bus fight", that is, both drivers trying to drive the bus at the same time. This is probably caused by improper control signals to the drivers. If the current tracer indicates the absence of abnormally high current activity at all drivers, yet the bus sig nals are known to be incorrect, then the problem is a driver stuck in the high-impedance state. It can be found by placing a low impedance on the bus, such as a short to ground, and using the current tracer to check for the driver that fails to show highcurrent activity. Efficient use of the current tracer usually requires a longer familiarization period than does the operation of voltage-sensing instruments. This is primarily be cause most operators are not used to thinking in terms of current and the information it provides, simply be cause this information has not been available conven iently. Also, it requires some skill to avoid the cross talk problem, that is, if a small current is being traced in a conductor that is very close to another conductor carrying a much larger current, the sensor at the tip of the current tracer may respond to the current in the
nearby trace. The current sensor has been designed to minimize this effect, but it can never be entirely elim inated. The operator can, however, by observing the variation of the current tracer's display as its tip is moved about the trace, learn to recognize interference or crosstalk from a nearby trace.
Inside the Current Tracer
Fig. 5 is a block diagram of the current tracer. Oper ation is as follows. A step change in current occurring at the current tracer's tip is coupled via mutual induc tance to the current-step sensor, which produces a voltage impulse output proportional to the magni tude of the current step. The size of the impulse de creases with increasing separation between the cur rent path and the tip of the tracer. The voltage impulse is fed to a variable-gain pre amplifier. The preamplifier gain is adjusted by the sensitivity control to produce an output of approxi mately 1 mV when the tracer's tip is placed next to the current path. The output of the preamplifier is further amplified and then stretched by two cascaded peak detectors to produce a pulse of sufficient height and width to cause a visible flash of the incandescent lamp that forms the current tracer's display. When the tip of the tracer is moved along the conducting path the display remains at the same brightness as long as the same current is still present. However, if the cur rent has changed to another path, the increased separ ation between the tip of the tracer and the current lowers the output of the current-step sensor and de creases the brightness of the display. Thus the opera tor is provided with sufficient information to track the current path. Current-Step Sensor Fig. 6 shows the physical construction and an equivalent circuit of the current-step sensor. Its oper ating principle is essentially that of a current trans former. A step change in current near the tip of the tracer attempts to induce an emf in the windings on the coil within the sensor. This coil is nearly shorted
Sensitivity Control
CurrentStep Sensor
rn
Gain Preamp
Bipolar Peak Detector
Display Driver
Second Peak Detector
Fig. 5. 547A Current Tracer block diagram. Current-step sensor detects current steps as small as 1mA and rejects magnetic flux that is not directly beneath the tracer's tip.
© Copr. 1949-1998 Hewlett-Packard Co.
Ferrite Core
Eddy-Current and Electrostatic Shield
Magnetic Flux Path
These criteria were successfully met by enclosing a very small (1.5-mm-diameter) H-shaped ferrite core within a relatively thick, highly conductive shield. The shield serves several purposes: it provides me chanical support and protection for the pickup core, it shields the core from external changing magnetic fields by means of induced eddy currents in the shield that oppose the changing flux, and it is elec trically grounded and thus protects the coil windings from electric fields caused by the voltages on the board. Protection from electric fields is adequate to provide the greater than 110 dB of shielding needed so the tracer's tip can be placed next to voltage steps of five volts and not respond to them, while at the same time responding to current steps of 1 mA. For a 1-mA current step the current-step sensor produces a 100-/xV output spike approximately 150 ns wide.
Amplification
L
>
R
>
e o u ,
Fig. 6. Construction and equivalent circuit of the currentstep sensor, which acts like a current transformer. For each current step the sensor produces a voltage impulse at its output.
(R is small), so a current is induced in the coil whose magnetic flux nearly cancels the flux change caused by the unknown current step. Thus, for a fixed physi cal orientation, there will be a current step propor tional to the unknown current step induced in the pickup coil. This induced current flows through the resistor R, producing a voltage that forms the output signal of the sensor. The current in the pickup coil, and hence the output voltage, decays with a time con stant L/R. This time constant is such that the output of the current-step sensor is an impulse whose peak value is proportional to the change in the unknown current and is independent of the rise time of the current change up to about 200 nanoseconds. Although the current-step sensor provides an indication only of cur rent changes and not the total value of the current, it turns out that for fault finding in digital systems this information is sufficient. The current-step sensor had to meet a number of conflicting design criteria. It had to be physically small so currents in the closely spaced traces found on printed circuit boards could be distinguished. It had to be able to reject magnetic flux originating from locations other than the tracer's tip. It had to provide a detectable output for the smallest current steps of in terest (<1 mA). It could not respond to voltage changes found in digital systems. It had to be me chanically rugged and manufacturable at a reason able cost.
To make a 100-/xV 150-ns pulse light an incandes cent lamp long enough for it to be visible, consider able amplification is required. For the sake of operat ing convenience, it was decided to package all the necessary electronics in the probe body and to power the probe from the supplies typically present in a dig ital environment. This means that the circuitry has to operate from supply voltages of five volts or less, so nearly all commercially available linear ICs are in applicable. Also, the shape and small size of the probe body make physical isolation of the amplifier stages impractical, and the small volume within the probe also prevents the use of effective high-frequency de coupling capacitors because of their large size. It was possible, nevertheless, using two custom ICs, to pack age on a multilayer board approximately 1.3 cm wide by 10 cm long, 80 dB x 20 MHz of stable linear ampli fication. This was accomplished by very careful loca tion of components to minimize capacitive coupling, and by orienting the components to cancel destabiliz ing parasitic inductive coupling. Shielding of the sensitive amplifier from external electric and mag netic fields is provided by the probe housing, which is made of aluminum and makes an effective electro static and eddy-current shield. For maximum viewing angle, brightness, and con venience of location, the current tracer's display was chosen to be an incandescent lamp at the tip of the probe immediately behind the current-step sensor. This location creates a difficult stability problem: a pulse at the current-step sensor is amplified and re turned to the display, which is right next to the sen sor, and the energy content of the pulse that flashes the display can be greater than 1014 times that cap tured by the current-step sensor. It was possible, how ever, thanks to the efficiency of the shielding of the
© Copr. 1949-1998 Hewlett-Packard Co.
current-step sensor, the orientation of the amplifier components, and careful printed circuit board de sign to electrically decouple the input from the out put. In fact, the current pulse that drives the display lamp actually passes through the board area in which the amplifier is located, but by running the display lamp's supply and return traces on the inner layers of a multilayer board, one above the other and separated by a very thin insulating layer, and covering these with the outer-layer ground plane, it was possible to confine the fields from the display pulse to a very small vol ume. The ground plane also had to be carefully de signed so the charging currents of the parasitic capac itor formed by the grounded eddy-current shield and the trace or node under investigation can pass through the sensitive amplifier area without induc tively coupling unwanted signals to the amplifier. Fault currents in the various logic families and from pulse stimulators can vary from about one milliampere to one ampere. The tracer normalizes these to a common reference level so the operator sees only the value of the current at the tracer's tip relative to that at the driving point of the node. This 60-dB (1000:1) gain variation cannot be achieved with a potentiometer divider because the parasitic inductances and capacitances associated with a po tentiometer at the extreme positions of the wiper degrade the high-frequency response. Also, the gain control has to be in the initial stages of the amplifier chain because the requirement of operating from five-volt supplies severely restricts the linear operat ing range of the amplifiers. For these reasons the entire 60 dB of gain control was placed in the first amplifier stage. This was done by taking as the basic configuration a single transistor with an unbypassed emitter resistor approximately equal to the collector resistor (see Fig. 7). Current-controlled variable resis tors formed from Schottky and silicon diodes are ac-coupled to both the collector and emitter resistors. When the sensitivity control on the tracer is placed in the 1-mA position the variable emitter resistors are in their low-resistance state and the collector resistances are in their high-resistance state, thus forming a common-emitter amplifier of 20-dB gain. Conversely, when the sensitivity control is in the 1A position the collector resistance becomes very small and the emitter resistance large and the configuration forms a 40-dB attenuator. A temperature-compensated, precision-component network was required to pro vide control currents for the variable resistors to in sure that the maximum and minimum values of the amplifier gain were repeatable from unit to unit and stable with time. The noise performance required of the first ampli fier stage is quite severe. It must amplify, with a 20-MHz bandwidth, input pulses smaller than 100
Sensitivity Control
Fig. 7. Variable gain preamplifier provides 60 dB of gain variation as the sensitivity control is adjusted. The diodes function as current-controlled variable resistors.
microvolts and yet not introduce any noise peaks comparable in amplitude to the input pulses because the downstream circuitry detects each peak and stretches it for a sufficient time to light the display. Thus the rms noise introduced by the amplifier must be far below the 100-microvolt input pulse. This was achieved by using a low-noise, high-frequency transistor biased to optimize its noise figure.
Peak Detectors
When the sensitivity control is set so the display is at its reference level, the output of the current-step sensor emerges from the amplifier chain with an am plitude of about 500 mV and a width of about 150 nano seconds. The polarity of this output will be either pos itive or negative, depending upon the polarity of the current step and the randomly chosen orientation be tween the current path and the pickup coil in the tracer. The operator is interested only in how the magnitude of this pulse varies as he moves the tip of the tracer from place to place, so the signal that drives the display lamp must be proportional to the ampli tude of the output of the amplifier chain, independent of its polarity, and of sufficient duration to be visible. The required signal processing is accomplished by means of two peak detectors. The first, or bipolar peak detector (Fig. 8), produces an output pulse of the same magnitude as the input pulse, but always of the same polarity, and stretches the peak of the input pulse from about 20 ns to about 40 /xs. A second peak detector stretches the output of the bipolar peak de tector to about 200 ms, which is long enough to be vis-
© Copr. 1949-1998 Hewlett-Packard Co.
loop. This configuration permits nearly all of the ex ternal supply voltage to be used when the external voltage is low. E
e
HP Model 547A Current Tracer
INPUT: SENSITIVITY: 1 mA lo 1A FREQUENCY RESPONSE: Light indicates: single-step current transitions; single for »50 ns in width; pulse trains to 10 MHz (typically 20 MHz for current pulses s?10 mA). RISETIME: Light indicates current transitions with risetimes «200 ns at 1 mA. POWER SUPPLY REQUIREMENTS: VOLTAGE: 4.5 to 18 Vdc INPUT CURRENT: s7S mA OVERVOLTAGE PROTECTION: ±25 Vdc for one minute TEMPERATURE: 0° to 55°C PRICE IN U.S.A.: $350 MANUFACTURING DIVISION: SANTA CLARA DIVISION 5301 Stevens Creek Boulevard Santa Clara, California 95050 U.S.A.
1A
SPECIFICATIONS
Fig. 8. The output of the bipolar peak detector is a timestretched version of the input, and is positive for both polarities of input.
ible. The output of the second peak detector is further amplified by a dc amplifier to provide the signal that lights the incandescent display lamp.
Voltage Regulator
The current tracer is designed to operate from any external dc supply between 4.5 and 18 volts and to withstand accidental reversals of supply voltage po larity. The internal tracer supply voltage is indepen dent of the magnitude of the external supply voltage. It is obtained by a feedback configuration using a spe cial PNP series-pass transistor that has a very high emitter-base breakdown voltage to assure re verse-polarity protection (see Fig. 9). The value of the internal supply voltage is set by the balancing of a "nonlinear bootstrap bridge", a bridge containing a diode in one arm and driven by the internal supply it self. Because of the nonlinearity of the diode only one value of the internal supply voltage will balance the bridge. Any unbalance is amplified and negatively fed back to the series-pass transistor to complete the
100 mAHP547A Typical Operating Range
10 mA -
1 mA 200 ns 2 fi Input Pulse Risetime
20 /is 200 /is
â" i â" â" r
Fig. 9. Voltage regulator output voltage is constant regard less of load current and input voltage as long as the input voltage exceeds the regulated output voltage by as little as the saturation voltage of the transistor.
John F. Beckwith P John Beckwith received his BSEE degree in 1 959 from Georgia Insti tute of Technology, and his MS __ _ _ degree in physics in 1967 from Florida State University. Before coming to HP in 1973, he de signed several commercial pro ducts, mainly computer terminals, gathering experience in digital, analog, broadband-linear, magnetic, and electromechanical design. He also spent two years in business for himself, developing an inventory control system. At HP, besides designing the 547A Cur rent Tracer, he's helped design a laser calibrator and an RF signal generator. John was born in New York City and raised in Florida. He's married, has two children, lives in Foster City, California, and likes to work on cars. He's also a student of economics, government, and law, and occasionally takes an active part in local government. Quantum electrodynamics is another of his continuing interests.
© Copr. 1949-1998 Hewlett-Packard Co.
New Logic Probe Troubleshoots Many Logic Families
The probe's single lamp indicator displays high or low logic levels, bad levels, and open circuits on digital circuit nodes. Testable logic families include TTL, DTL, CMOS, HTL, HNIL, and MOS.
by Robert C. Quenelle
DESIGNING AND TROUBLESHOOTING digital circuits poses some measurement problems not neatly solved by analog test equipment. The digital circuit typically has only two narrow voltage ranges, high and low, that represent logic ones and zeros. A voltmeter or oscilloscope will easily give the voltage to more accuracy than is required, but won't translate those voltages into logic levels, leaving that job to the user. Although the oscilloscope is sometimes indis pensable for checking ringing, skews, and so forth, in many instances a simple indication of logic levels will suffice. For several years the logic probe has comple mented the oscilloscope in helping to solve digital troubleshooting problems. HP's Model 10525T, for example, is small and light enough to be hand-held and features convenient fingertip display of ones and zeros and 10-nanosecond pulse-catching capability, independent of repetition rate. The 10525T translates voltages at the measurement tip to logic levels based on the specifications of TTL logic. With the expanding use of MOS and CMOS, it be came desirable to have more than TTL thresholds available. The new HP Model 545A CMOS-TTL Logic Probe (Fig. 1) offers switch-selectable TTL or CMOS thresholds for monitoring acitivity in most logic fam ilies, including DTL, TTL, HTL, HNIL, MOS, CMOS and discrete logic (but not ECL). The CMOS thresh olds are set at 30% and 70% of the supply voltage to which the 545A is connected. Most MOS parts are either TTL-compatible or have logic levels close to some power supply voltages. By selecting TTL thresholds for TTL-compatible parts or by using CMOS thresholds and connecting the probe to appropriate supply voltages, most MOS devices can be checked. The probe always displays the more negative logic level as a zero. HTL or HNIL parts are tested using CMOS thresholds. The input current is nominally 10 /zA (source and sink), compatible with high-impedance logic yet large enough to overcome leakage currents and prevent false readings. Besides displaying ones and zeros, the probe indicates vol
tages between valid logic levels, as well as open cir cuits or inputs not connected to outputs. The probe contains a latch that indicates and stores input activity, a useful feature when low repetition
^£
Fig. 1. Model 545A Logic Probe indicates logic levels and pulse activity at individual digital circuit nodes.
© Copr. 1949-1998 Hewlett-Packard Co.
A Smart Probe-Test System
Instrument test procedures can have substantial effects on factory cost, warranty cost, and customer satisfaction. Highvolume, high-technology instruments like the new logic pro ducts demand high test throughput and a high test confidence factor, the latter because warranty cost on a returned low-cost instrument represents a large portion of its price. The solution was found in a calculator-controlled test system. The system is a closed-loop, self-calibrating, fully automatic tester. Most of the hardware is off-the-shelf instrumentation: an HP 9830A Calculator as system controller, an HP 9866A Printer for failure analysis and statistical data, an HP 5328A Counter with built-in DVM for dc voltage measurements and a programmable input section for dynamic pulse measurements, an HP 59303A DAC to program the voltage on an HP 6824A Power Amplifier, another DAC to program dc probe tip voltages, an HP 1 900 Pulse Generator System to provide dc and dynamic signals, an HP 59301 A HP-IB-to-Parallel Converter to program the instrument interface test boxes and the 1900 Pulse Genera tor ranges, another 59301 A to program an 8-channel 6-bit DAC to set the verniers on the pulse generators, and two HP 59307A VHP Switches to route the VHP signals going be tween the generators, the two instrument interface test boxes, and the counter. The counter is also used to close the loop back to the calculator under software control for accurately setting the dc and dynamic stimulus circuitry. The counter's DVM port is used to get test-box control signals into the cal culator. The 5328A Counter is the only part of the system re quiring periodic off-line calibration. Everything else can be recalibrated automatically with correction factors stored in a software calibration file. Inside the two identical instrument interface test boxes are solenoids that push buttons and slide switches, microswitches that sense whether a probe, pulser, or current tracer is plugged in, phototransistors and amplifiers to measure the brightness of indicators, VHP relays for pulse line termination and load sens ing, reed relays for direct voltage measurement, and support electronics. In operation, all the operator need do is load the operating system tape cassette into the calculator, plug a probe or pulser into the left hole of either test box or a current tracer into the right hole, and press the test button. The system selects the box, identifies which instrument is plugged in, runs the proper test program, lights the pass or fail lamp upon completion of the test, and if the instrument fails, prints out a failure analysis and diagnostic message helpful in repairing the instrument. -Barry Bronson
supply. In the CMOS mode the probe's thresholds track the instantaneous supply voltage as does CMOS logic, thereby compensating for ripple to 1 kHz.
Using the Probe
The logic probe has two main display modes, static and dynamic. It indicates in a static circuit whether the voltage at a particular point is a logic zero, a one, or an indeterminate level. The lamp in the probe tip is off for a zero, bright for a one, and dim for a voltage be tween logic levels, an open circuit, or a high-impe dance point such as a floating input or a three-state output in the off state. In a dynamic circuit, the tip lamp will flash at about 10 Hz when the tip is touched to a point toggling between a one and zero at up to 80 MHz in TTL mode and 40 MHz in CMOS. It will flash on or off for about 50 ms for a single one or zero pulse. Pulses as short as 10 ns can be detected if the short ground lead is connected to a point near the pulse source. 20-ns pulses can typically be detected with out using the short ground lead. The probe can also be clipped into a circuit and the latch used to monitor infrequent activity. By displaying the dc logic levels in a circuit the logic probe can be used to verify combinatorial re lationships. Or, input levels can be overridden by a logic pulser and the logic probe will then test the functioning of a circuit without removing any normal connections. In clocked systems, many logic families respond only to clock transitions and are insensitive to the clock rate (up to their upper frequency specifi cations). This allows replacing normal clocking sig nals with a single cycle or slow clock (once again, a logic pulser may be used) and checking system se quencing at a convenient rate. Spikes and one-shot outputs can also be checked even at slow clock rates
1 Peak Detector , Comparator
rates or single-shot events are involved. A light-emit ting diode (LED) indicator turns on when a new one or zero level is detected and stays on until manually reset. To protect the 545A from accidental overloads, the power supply input is protected to ±25 volts for one minute and the tip can withstand momentary over loads to 250 volts. The probe is usable down to three volts, although the tip lamp gets dimmer below five volts. CMOS supplies often have considerable ripple, especially if electrically noisy devices use the same
10
© Copr. 1949-1998 Hewlett-Packard Co.
^ 0 Comparator D4
0 Reference Level
Fig. 2. 545A front-end has two fast peak detectors and two relatively slow, low-input-current comparators. The circuit can withstand accidental overloads of ±1 20V continuous and ±250V for 15 seconds.
because of the 545's pulse stretching display. The 545's dynamic capability allows checking cir cuits running at full clock rate. Dead nodes (stuck high, low, or bad-level/open) are common failures easily found with the probe. Spikes on normally quiet lines (such as reset) will also show up even if the duty cycle is very low. Often a technique similar to analog signal tracing can be used to find problems. Working back from an output to a point where a gate's inputs are moving but the output is not will localize many faults.
Inside the Probe
protection and the ability to operate at three volts, it was necessary to provide a protection circuit with low voltage drop. In Fig. 3, Ql is a series-pass transis tor that remains saturated in normal operation. The FET, Q2, provides base drive for Ql that is relatively independent of supply variation. When excessive positive supply voltages are encountered, zener diode Dl conducts, raising the potential at Q2's source and reducing Ql's base drive. Ql comes out of saturation and limits the voltage applied to the chip. Reverse protection is provided by Ql's high emitterbase breakdown voltage. S
The heart of the 545A Logic Probe is a custom HP integrated circuit containing input comparators, ref erence circuits, logic for controlling the tip lamp and latch LED, timing circuits, and a lamp driver for oper ating the lamp over a 3-to-18-volt power supply range. The input signal is connected to two compara tors through peak detectors and an external RC net work (see Fig. 2). The peak detectors use Schottky diodes (D3 and D4) for fast switching and make it possible to use relatively slow, low-input-current comparators. Protection from input overloads is pro vided by on-chip clamp diodes (Dl and D2) and the external network. Rl. the dc path, allows signals up to 250 volts to be momentarily applied while R2 and Cl provide a damped bypass for fast pulses. The logic section operates asynchronously, trig gered by data changes at the input. The internal state, indicated by the tip lamp, and the input history since the last display change determine the next state. A new zero or one signal starts a cycle that updates the display, then waits 50 ms to produce a stretching or toggling effect. The new zero or one signal also sets the memory indicator, which stays on until manually reset. If neither a zero nor a one is present and the tim ing cycle is complete, the logic displays a bad level (dim lamp) until a valid logic level arrives. To provide power supply overvoltage and reversal
HP Model 545A Logic Probe
INPUT CURRENT: « 1 5 /iA (source or sink) INPUT CAPACITANCE: s15 pF LOGIC THRESHOLDS: TTL: LOGIC ONE 2.0 + 0.4, -0.2 Vdc LOGIC ZERO 0.8 + 0.2, -0.4 Vdc CMOS: 3-10 Vdc supply LOGIC ONE: 0.7 : ^supply ±0.5 Vdc LOGIC ZERO: 0.3 x Vsupp|y ±0.5 Vdc CMOS: 910-18 Vdc supply LOGIC ONE: 0.7 x Vsupp|y ±1.0 Vdc LOGIC ZERO: 0.3 " Vsupjj|y ±1.0 Vdc INPUT MINIMUM PULSE WIDTH: 1 0 ns with ground lead (typically 20 ns without ground lead) INPUT MAXIMUM PULSE REPETITION FREQUENCY: TTL, 80 MHz; CMOS. 40 MHz INPUT OVERLOAD PROTECTION: ±120V continuous (dc to 1 kHz); ±250V for 15 seconds (dc to 1 kHz) PULSE MEMORY: Indicates first entry into valid logic level: also indicates re turn to initial valid level from bad level for pulse ^^ us wide. POWER REQUIREMENTS: TTL: 4. 5 to 15 Vdc CMOS: 3 to 18 Vdc MAXIMUM CURRENT: 70 mA OVERLOAD PROTECTION: ±25 Vdc for one min. TEMPERATURE: 0° to 55°C ACCESSORY INCLUDED: Ground cable and grabbers (allows connection to 1C pins). PRICE IN U.S.A.: $125. MANUFACTURING DIVISION: SANTA CLARA DIVISION 5301 Stevens Creek Boulevard Santa Clara. California 95050 U.S.A.
SPECIFICATIONS
v+
D1
02
-C
â¢R1
Fig. 3. Power supply protection circuit protects the 545A's single integrated circuit, its tip lamp, and its latch LED against overvoltage and reverse voltage to ±25V.
Robert C. Quenelle Bob Quenelle designed the 545A Logic Probe and put it into produc tion He's been with HP since 1973, and is now working on a JÃB?^ software development project. Jit\ *y jhf Bob was born in Tucson, Arizona. ^ -;»' He graduated from the University Ap / of Arizona at Tucson with a BSEE degree in 1970, and in 1973 he received his MSEE degree from Stanford University. He's married, lives in Santa Clara, California, and devotes part of his spare time to , , working with a local Boy Scout electronics Explorer post. He also ~ likes camping and bicycling, and working in his garden or on electronics projects.
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© Copr. 1949-1998 Hewlett-Packard Co.
A Multifunction, Multifamily Logic Pulser
This microprogrammed pulse generator in a probe can produce single pulses, pulse bursts, or pulse streams. Its output stage automatically adjusts for the type of logic being stimulated.
by Barry Bronson and Anthony Y. Chan
LOGIC PULSERS ARE VALUABLE tools for troubleshooting digital equipment. Using a pulser to inject pulses into logic nodes without cutting traces or removing ICs and monitoring the circuit response with a logic probe or clip is an effective method of locating logic, connective, or component faults. The pulser produces high-energy, shortduration voltage pulses of a logic state opposite to that of the node under stimulation. As a logic stimulus element, the new HP Model 546A Logic Pulser (Fig. 1) offers features never before available in a self-contained probe. Operating from a 3-to-18V logic supply, it is capable of overriding TTL, DTL, HTL, and CMOS logic nodes with narrow pulses of automatically controlled voltage, polarity, and width, and selectable rate and count. The new pulser retains much of the simplicity of its predecessor: one power connection, single push button control, completely self-contained elec tronics, a normally high-impedance output tip, and low power consumption. It expands the basic pulser concept by providing additional features that aid digital design and troubleshooting. These include 10-Hz and 100-Hz low-duty-cycle pulse streams that can be used for continuous clocking of circuits or as a current injector for a current tracer, a pulse-persecond mode for slow clocking circuits and following the action at human speeds, and recurring bursts of 10 and 100 pulses for presetting sequential circuits (e.g., counters, shift registers, sequencers) with a precise number of counts. An LED annunciator on the tip pro vides feedback to identify the output mode and allow pulse bursts to be detected and counted. By pro gramming single pulses and bursts of 10 and 100 while observing the LED, a predetermined number of outputs can be generated. A "smart" output stage takes care of pulse shaping and overload protection for compatability with all popular positive logic families. Serial encoding of the six function modes on a single push-slide switch (similar in operation to that of a flashlight) allows rapid eyes-off programming and the ability to lock the pulser in any mode for con tinuous hands-off operation. To program a mode, the
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© Copr. 1949-1998 Hewlett-Packard Co.
Fig. 1. Model 546 A Logic Pulser provides single pulses, pulse bursts, and pulse trains that automatically drive a digi tal circuit node to its opposite state
Single Pulses Continuous 100-Hz Pulse Stream 100-Pulse Bursts 10- Hz Pulse Stream 10-Pulse Bursts 1-Hz Pulse Stream One Press of Button Press and Hold
RAM. The user can examine the data set up in a busdriving 1C by pulsing its bus-enable line and using a logic probe on its output bus, and check system data error detecting or correcting performance by inject ing bit errors into a serial data transmission channel. When used with the 547A Current Tracer, the pulser can help locate faults on stuck buses.
An Instrument on a Chip
Fig. 2. The logic pulser is programmed by means of a single button, using the code shown.
button is pushed a predetermined number of times in rapid succession. Continuing to hold it down after the last push causes the selected function to be exe cuted (see Fig. 2). Sliding the button forward forces it to stay down and locks the pulser in the mode selected. All switch debounce and sequence timing are implemented internally.
Pulser Applications
The 546A Logic Pulser can serve as a stimulus instrument for locating many common digital 1C problems. The response-measuring instrument can be a logic probe, a logic clip, or a current tracer. For example:
Problem Response Instrument
Shorted 1C Input or Output Stuck Data Bus Internal Open in 1C Solder Bridge Sequential Logic Fault Shorts to Vcc or Ground
Probe, Tracer Probe, Tracer Probe Tracer Clip Probe, Tracer
Most of the electronics in the 546A Logic Pulser is on a single custom HP large-scale integrated circuit. The "instrument on a chip" concept keeps the parts count, size, power, and cost low. On-chip logic is a micropower version of low-power Schottky logic, and includes a 256-bit read-only memory (ROM), 14 flip-flops, and 41 gates. An on-chip voltage regu lator contains a voltage and temperature compen sated reference and control circuit for generating the 2.7V that runs the internal logic. The total operat ing current for the logic is less than 25 mA. Total power dissipation for all circuits on the chip is less than 100 mW. Since most of the 1C operates at low voltage, two sets of layout rules were adopted to minimize total chip area. Using high-density cells for low-voltage and lower-density cells for higher-voltage circuits, we were able to use our standard bipolar single-level metal process to put approximately 1000 transistors on a 2.7X3.2-mm die, and still have overvoltage pro tection to ±25V. In Fig. 3 it can be seen that the 546A is a ROMbased system. The 2 56-bit ROM is a functionally opti mized structure occupying less than 10% of the chip area. A free-running 100-Hz RC-controlled voltage and temperature compensated clock circuit, operat ing at 2.7V, acts as a time base for the instrument. Using an external resistor and capacitor allows better
The pulser is a versatile design tool. It can be used to single-step breadboard designs, provide a substi tute system clock, or verify the integrity of ground and power buses. In sequential circuits such as flipflops, counters, and shift registers, the pulser can be used to preset, clear, or clock the circuit, or preload the circuit with a precise number of pulses. The pulser has many applications in micropro cessor and other bus-structured systems. For ex ample, it can force the CPU into the reset mode, in ject interrupts (single shot or 1, 10, or 100 per second), set flags and clear data latches in I/O ports, and force a memory write to occur at a selected address in
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© Copr. 1949-1998 Hewlett-Packard Co.
Fig. 3. 546A block diagram. Most circuits are on a single custom 1C chip, including the 256-bit ROM.
than ±10% accuracy over the full voltage and tem perature range of the instrument. The H-lOO counter acts as a sealer for generating pulse and LED output rates, a counter for the bursts, and a timer for program mode detection and switch debouncing. The switch input control debounces the single-contact switch, presets the -HlOO counter and burst control, generates a single-shot output signal, and increments the mode select control circuit. The mode select control determines whether the instru ment is programming or executing a mode, which output mode it is in, and when to return to the stand by mode. The burst control governs the burst timing, count (98 or 6 for the first burst*, 100 or 10 for successive ones), and pause interval between burst outputs. The LED annunciator circuit is a ROM-controlled direct-drive constant-current sink. It drives the highefficiency LED in the tip.
Automatic Output Control
An on-chip automatic pulse output control circuit effectively delivers the proper pulse polarity, vol tage, width, and fast edge necessary to overdrive the logic node at the tip. It can output a 3V, 10-mA, IO-/AS pulse to a 3V CMOS clock input node or a 5V,
"98 and the burst counts are necessary to subtract out the 2 and 4 pulses that output when the user pushes the button to program the 100-pulse and 10-pulse burst modes.
0.5A, 0.5-/U.S pulse to a TTL line driver output. About 20% of the active chip area and 40% of the dis crete components in the pulser are directly involved in this pulse output control task. Fig. 4 is a functional schematic of the output stage. Transistors QO and Ql are normally off, and SO and Si are open. The pulser presents a high impedance to the logic node at the tip. Residual charge on coup ling capacitor Cl has been bled off by Rl. When a signal from the ROM initiates an output cycle, the one-shot is triggered, and QO turns on via 10 (a 100-mA on-chip current source), GO, and the control 0 line. Immediately after QO saturates, SO is closed through delay element DO, an on-chip capaci tor/SCR circuit, and the 0 sense comparator is enabled. The load on the pulser tip is pulled toward ground through Cl, a low-impedance ceramic capacitor. Current through Cl gets reflected as a voltage (dv = Idt/Cl) on the sense line via input resistor R2 and speed-up capacitor C2. When the tip load current is low, as in CMOS, Cl charges slowly. Before it can reach 0.7V about ten microseconds elapses and the one-shot shuts off the 0 output circuit through GO. If a heavy load is present at the tip (e.g., a TTL buffer) charge develops rapidly on Cl. When the voltage across Cl reaches 0.7V, the 0 sense comparator fires the overload line causing the one-shot to retrigger and the 0 output circuit to turn off. The heavier the load, the shorter
From
Fig. automatically delivers schematic of 546A output stage. The circuit automatically delivers pulses ot the proper polarity, voltage, width, and transition time to override the logic node at the pulser's tip.
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© Copr. 1949-1998 Hewlett-Packard Co.
v-
Light Load Ground 10 /*s Typical
â¢â¢small
Heavy Load to V + Ground
vâ¢
will be the output pulse. Active turn-on and turn-off of QO assure fast pulse edges through logic transition regions. At the conclusion of a 0 output pulse, the 1 output circuit becomes active. Its operation is analogous to that of the 0 output circuit except for the variable threshold setting of the 1 sense circuit, necessary to adjust to the 3-to-18V operating range, and the inclu sion of charge storage capacitor C3 and charging re sistor R3. C3 sets the 1 output voltage pulse ampli tude and decouples potentially large current surges from the tested circuit's supply line. Between output cycles, current through R3 restores the charge lost from C3 during the previous 1 output. R3 and C3 also provide good power supply ripple tracking. This assures accurate instantaneous pulse amplitude con trol when used with a poorly regulated CMOS power supply. The pulse output cycle is complete when Ql turns off. Fig. 5 illustrates output waveforms under var ious loads. 2
Anthony Y. Chan Tony Chan designed the custom 1C chip for the 546A Logic Pulser. He joined HP in 1973 with four years' experience in linear and digital 1C design. Before taking on the pulser.lC design, he developed low-power low-voltage logic gates. Tony was born in Hong Kong. He received his BSEE degree from the University of California at Berkeley in 1969 and his MSEE degree from California State University at San Jose in 1974. He's married, has two chil dren and has a home in Sun nyvale, Californiathat he's currently remodeling. He also enjoys working with wood and with automobiles. Barry Bronson Barry Bronson, project leader for the 546A Logic Pulser, received his BS degree in engineering from UCLA in 1970 and his MSEE de gree from Stanford University in 1975. With HP since 1971, he's developed several automatic pro duction test systems and contri buted to the 5035T Logic Lab and the 5000A Logic Analyzer. Barry is a native of Los Angeles, California and now lives in San Jose. He and his wife have a one-year-old daughter. At home, Barry likes to work on electronics projects of his own, one of them a home-made microcomputer system. He also has a part-time video equipment business, dabbles in photog raphy and the stock market, and plays tennis.
Heavy Load to Ground Ground
Fig. 5. Typical 546A output waveforms under various loads.
HP Model 546A Logic Pulser
OUTPUT: Logic Family TTL CMOS Logic Family TTL CMOS Pulse Width Power Supply Voltage 4.5-5.5 Vdc 3-15 Vdc Output Current «650 mA s100 mA Typical Output Voltage H i g h L o w 33 Vdc «0.8 Vdc *
SPECIFICATIONS
TIP IMPEDANCE: <2 ohms active; >1 megohm off SHORT CIRCUIT TIP PROTECTION: Continuous IMPEDANCE: <2 ohms active: -1 megohm off POWER SUPPLY REQUIREMENTS: OPERATING VOLTAGE RANGE: TTL: 4 .5-5.5 Vdc CMOS: 3-1 8 Vdc Operating current: «35 mA POWER SUPPLY INPUT PROTECTION: -25V for 1 min. TIME BASE ACCURACY: -10% OPERATING TEMPERATURE: 0" to 55'C ACCESSORY INCLUDED: Ground cable and grabbers PRICE IN U.S. A.: $150 MANUFACTURING DIVISION: SANTA CLARA DIVISION 5301 Stevens Creek Boulevard Santa Clara, CA 95050 U.S.A.
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© Copr. 1949-1998 Hewlett-Packard Co.
Probe Family Packaging
by David E. Gordon
Current Tracer
546A Logic Pul ser
Logic Probe
Fig. 1. Probe housing design makes it possible to produce three instruments with a minimum number of different parts. push this. Ordering, stocking, and handling costs are reduced by using common parts, and the increased production volume of those parts lowers the manufacturing costs. For low as sembly and service costs, all of the mechanical parts, with the exception of the crimp-on cable strain relief, are easy to fit to gether by hand without the use of nuts, bolts, washers, etc. (see Figs. 2 and 3). Custom switches for the logic probe and logic pulser are pro duced at very low cost. The lighted pulse memory reset switch (Fig. 4) and the latching pulser mode switch use plastic actua-
The problem was to design a second generation probe packaging system for three new logic test instruments, accom modating 100% more circuit area with minimal instrument size increase, 200% more heat dissipation, and electromagnetic shielding. The amount to be spent for tooling was strictly limited. It was desirable that all parts be inexpensive, easy to assemble and service, and highly reliable. Our solution was to use an aluminum extrusion coupled with simple molded plastic parts that snap, slide, and press together to provide a low cost, versatile instrument package that meets the functional requirements of the electronic design (see Fig. 1). To have low factory and selling costs and stay within the tool ing budget, the package had to be designed using a minimum number of parts that could be produced by easy manufacturing methods â" aluminum extrusion, plastic molding, and metal stamping processes offered the necessary tooling to accom-
Ground Ring
Fkj. 2. At top is an end view of the upper interlocking extru sion for the 546A Logic Pu/ser with the pulser switch in place. Internal slots serve as guides for the sliding switch. Slots in the lower extrusion serve as printed circuit board guides. Lower extrusion is from the 547 A Current Tracer, showing the faced and clear alodined end that provides grounding of the case for electromagnetic shielding.
Fig. 3. Ground ring provides a ground path from the bottom of the printed circuit board to the end of the extrusion shown in Fig. 2. Crimp-on strain relief pulls back into a cavity and locks the cable into place. Notches in the printed circuit board snap into the rear plastic support for structural ridigity.
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© Copr. 1949-1998 Hewlett-Packard Co.
Fig. 4. 545A Logic Probe pulse memory switch rotates into place over LED. Press-in switch contact touches printed cir cuit traces when the button is pushed. tors and printed circuit contacts to simplify manufacture and assembly. The instruments use an interconnect system that provides for connection of the power lead, probe/pulser tip, and ground pin to either a standard test pin, an 1C test clip, or an HP grabber (see Fig. 5). Aluminum extrusion allows for a very versatile package, since it can be cut to any length. With minor secondary operations, five different body halves are produced. Perhaps the aluminum extrusion's greatest attribute is its excellent heat transfer, which keeps component temperature to a minimum for higher instru ment reliability. Also, it provides electromagnetic shielding when grounded to the printed circuit board, making the sensitiv ity of black current tracer possible. The case surface is hard black anodized for the scratch resistance necessary for a high break down voltage, and the thin cross sectional area of the extrusion yields a larger circuit volume per unit length than previous plas tic packaging. Acknowledgments I would like to share design credit with Jim Marrocco, whose ideas and efforts contributed a great deal to the project. Also, considerable manufacturing assistance came from Steve Balog and John Lindahl
Fig. 5. Probe interconnect system provides for connection of power leads, probe tip, and ground pin to standard test pins, an 1C test clip, or HP grabbers.
David E. Gordon Dave Gordon received his BSME degree from California State Polytechnic College at Pomona in 1973, and joined HPa month later. Now group leader for printed cir cuit multilayer process engineer ing, he served until recently as - product designer for logic trouble shooting instruments. Dave was born in Alhambra, California, and now lives in Los Gatos, California, with his partner and her two chil dren. Family activities and the "in vigorating disciplines of medita tion and Hatha yoga" are major interests; he also enjoys backpacking and cycling.
1C Troubleshooting Kits
1C Troubleshooting instruments are available in kits consisting of various combinations of logic probe, logic pulser, logic clip, cur rent tracer, logic comparator, and carrying case. Model 5022A Kit (shown) contains the four instru ments described in this issue: the 545A Logic Probe, the 546A Logic Pulser, the 547A Current Tracer, and the 548A Logic Clip.
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© Copr. 1949-1998 Hewlett-Packard Co.