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Rich Page 6/29/85 Yideo/Sound Timing Gate Array: VIG The VTG implements the horizontal and power supply sync signals, the sound clock, the system timer and the interfaces for the front desk bus modem and the clock chip. The horizontal and power supply sync signals are derived from the video dot rate clock divided by 32 (ie. 2.1888 MHz) using a six bit counter and write registers 10 thru 13 which provide control of the horizontal period, horizontal blank stop, horizontal sync start and horizontal sync stop. The default at reset provides a horizontal rate of 1!l.75KHz and a power supply sync at twice the horizo~tal period at 99.5KHz. The refresh/sound counter is fixed at four times the sound rate using a divide of 41.5 from the 3.6864 Mhz clock which yields a clock of 88.8 KHz. The system timer is a, 16 bit counter which counts at a rate of one fourth of the 3.6864 Mhz clock (ie. approximately once every microsecond). The timer counts upwards to zero. When the timer rolls over to zero, a time out interrupt is generated (ie. s,ets ~ROJ and the system timer is loaded from the the system timer latch (write registers 6 and 7). The front desk bus (FOB) interface is a five line interface which is controlled by write register 4. The five signals are STO, ST1, FS7 (data), FC (clock) and FINT. The, FOB shift register is located at register 2. --===-- ;z The real time clock (RTC) interface is a four line interface which is controlled by write register 5. The four signals are CINT (1 sec interrupt), RC (clock), CE (clock enable) and RS7 (data). The VTG is located at $FFFOOP0~ with a register spacing of $10. There are six read registers and ten write registers implemented as follows: Bead Register 0: Status $FFFOOOO 0 Bit 0 SRO J Status Reg 0, set by the RTC one second interrupt. Bit 1 SR1 ( Status Reg 1, set by the System Timer timeout. Bit 2 FINT . FOB Interrupt input. Bit 3 RSIN RTC serial data input. Bit 4 RA3 VTG Register Address bit 3 .. Bit 5 ES:E Enable ENC pin as sound counter enable. Bit 6 ErA Enable output buffer to RTC if set Bit 7 S10 Set if er~her SRO orSR1 is set. ..- 17.-"\ Bead Register 2: FDa Shifter $FFFk po~1..0 This register allows the FOB shift register to be read. Beacf Begister 4: Number..LQw $FFF~O This register allows the low byte of the number register to be read. Bead Register 5: Numberliigb $FFFdbso tou"S1J This register allows the high byte of the number register to be read. Read Register 6: Timer L~ $FFFD060 \?60 &0 This register allows the low byte of the sy~~em timer to be read.J7'~ Bead Register 7: Timer Hlgh $FFFD070 bClO 1 0 This register allows the high byte of