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TB004
Automatic Calibration of the WDT Time-out Period
Author: Stan D'Souza Advanced Microcontroller Technology Division

CONCLUSION
The calibration of the WDT is simple and takes very little overhead in a program. The code in Appendix A is written for a PIC16C84 device, but can be translated to work on any PIC16CXXX product. The code in Appendix B is written for the PIC16C5X family. RAM Used: 2 Bytes ROM Used: 50 Words Execution time: Up to a max. of 132 ms from start-up.

INTRODUCTION
The WDT timer is a simple RC timer with a typical time-out period of about 18 ms. This time-out period is dependent on Voltage, Temperature and Silicon process variations. Hence the tolerance on the time-out period is very wide: Min. of 9 ms to a Max. of 33 ms (please refer to appropriate datasheet for device dependent value). There are applications where an additional timer would be useful as an approximate time keeper, hence getting a more precise value of the WDT time-out is useful. This Tech Brief implements an automatic calibration of the WDT time-out period on start-up.

FIGURE 1: PROGRAM FLOWCHART

START

CodeByte = 0xA5? No

Yes

IMPLEMENTATION
The hardware used for this brief is the PICDEM1 board. It is assumed that the main processor oscillator is an accurate crystal or ceramic resonator running at 4 MHz. The program flowchart is depicted in Figure 1. A check is made to see if a certain Codebyte exists, if it does not then a power-up is assumed and the calibration is executed. Note that instead of using the 8-bit wide CodeByte, the PD bit in the STATUS register could also be used for this very same purpose. The calibration uses an internal timed 1 ms interval. To get a better resolution, the WDT is postscaled by 4 in order to measure a longer time period . Every 1 ms interval is kept track by incrementing the WDTValue. The WDT will eventually time-out and cause a reset. The value in WDTValue in then divided by 4 to get the exact WDT time-out period and displayed as a binary number on PORTB. In order to check for repeatability, just press the reset button on the PICDEM1 board and the whole calibration process will be repeated. A power-down, followed by a power-up will also have the same effect. CodeByte = 0xA5 WDTValue = 0 OPTION = 0B11111010

Wait 1 ms increment WDTValue

WDT Reset

WDTValue PORTB

LOOP

© 1996 Microchip Technology Inc.

DS91003A-page 1

This document was created with FrameMaker 4 0 4

TB004
APPENDIX A:
MPC "C" COMPILER BC.193 22-Aug-1995 PAGE 1

/* In this example, a PIC16C84s WDT is calibrated on startup for better accuracy. Accuracy is exact to +/- 0.25mS. The code written in C works on a PICDEM1 board. The WDT value is written to PORTB at the end of the calibration. The value is in binary. */ #pragma option v #include #pragma option +1; #define MAXROM 1019 #pragma memory ROM [MAXROM] #pragma memory RAM [36] #pragma option +1; #include #pragma option +l; 000D 000E char WdtValue; chr CodeByte; void InitPorts(void) { PORTB = 0; //configure PORTB as output TRISB = 0; } void main(void) { InitPorts(); if(CodeByte != 0xA5)

03FB 0005 03FB 000C 0024

@ 0x05; @ 0x0C;

0005 0006 0007 0008 0009

1283 0186 1683 0186 0008

BCF CLRF BSF CLRF RETURN

STATUS,RP0 PORTB STATUS,RP0 PORTB

000A 000B 000C 000D 000E 000F 0010 0010 0011 0012 0013 0014 0015 0016 0017

2005 30A5 1283 0212 1903 281E 1283 0191 30A5 0092 30FA 1683 0081 0064

CALL MOVLW BCF SUBWF BTFSC GOTO BCF CLRF MOVLW MOVWF MOVLW BSF MOVWF CLRWDT

0005h A5h STATUS,RP0 TMR1L,W STATUS,Z 001Eh STATUS,RP0 PIR2 A5h TMR1L FAh STATUS,RP0 TMR0

{ WdtValue = 0; CodeByte = 0xA5; OPTION = 0B11111010; // set WDT post scaler = 4

CLRWDT(); while(1) { 01h 002Ah Delay_Ms_4MHz(1); // delay for 1mS // count mS

0018 3001 0019 202A 001A 001A 1283 001B 0A91 001C 2818 001D 2827 001E 1283 001F 0192

MOVLW CALL

BCF INCF GOTO GOTO BCF CLRF

STATUS,RP0 PIR2 0018h 0027h STATUS,RP0 TMR1L

WdtValue++; } } else { CodeByte = 0;

DS91003A-page 2

© 1996 Microchip Technology Inc.

TB004
0020 0811 0021 0022 0023 0024 0025 0026 008D 0C8D 0C8D 303F 050D 0086 MOVF MOVWF RRF RRF MOVLW ANDWF MOVWF PIR2,W PIR1 PIR1 PIR1 3Fh PIR1,W PORTB } while(1) CLRWDT(); } void Delay_Ms_4MHz(registerw delay) { #asm BCF STATUS, RP0 ;1 MOVWF __WImage ;1 DLMS4M1 002C 30F9 002D 0084 DLMS4M2 002E 0000 002F 0B84 0030 282E 0031 0B8D 0032 282C 0033 0008 0000 3000 0001 008A 0002 280A RETURN MOVLW MOVWF GOTO 00h PCLATH 000Ah NOP DECFSZ GOTO FSR DLMS4M2 RADIX DEC MOVLW 249 MOVWF FSR ;Use decimal values ;1 ;1 ; 4 cycles ;1 ;1 ;2 ;1 ;2 PORTB = WdtValue >> 2; ; // divide by 4 for // exact timeout value

0027 0064 0028 2827 0029 0008

CLRWDT GOTO 0027h RETURN

// loop forever

0000 002A 1283 002B 008D

DECFSZ __WImage GOTO DLMS4M1 #endasm }

ROM USAGE MAP 0000 to 0002 0005 to 0033 Total ROM used 0032

Errors Warnings

: :

0 0

© 1996 Microchip Technology Inc.

DS91003A-page 3

TB004
APPENDIX B:
MPC "C" COMPILER BC.193 22-Aug-1995 PAGE 1

/* In this example, a PIC16C54s WDT is calibrated on startup for better accuracy. Accuracy is exact to +/- 0.25 mS. The code written in C works on a PICDEM1 board. The WDT value is written to PORTB at the end of the calibration. The value is in binary. */ #pragma option v #include #pragma option +l; // Enable header to show branch islands #define MAXROM 512 #pragma memory ROM [MAXROM] @ 0x00; #pragma memory RAM [24] @ 0x08; // Reserve byte for ; // __WImage #pragma option +l; #include #pragma option +l;

0200 0000 0200 0008 0018

0009 000A

char WdtValue; char CodeByte;

0000 0001 0002 0003

0066 0C00 0006 0800

CLRF MOVLW TRIS RETLW

PORTB 00h PORTB 00h

void InitPorts(void) { PORTB = 0; //set PORTB as output __TRIS(0,PORTB); } void main(void) { InitPorts(); if (CodeByte != 0xA5)

0004 0005 0006 0007 0008 0009 0009 000A 000B 000C 000D 000E

0900 0CA5 008A 0643 0A14 0069 0CA5 002A 0CFA 0002 0004

CALL MOVLW SUBWF BTFSC GOTO CLRF MOVLW MOVWF MOVLW OPTION CLRWDT

0000h A5h 0A,W STATUS,Z 0014h 09 A5h 0A FAh

{ WdtValue = 0; CodeByte = 0xA5; __OPTION(0B11111010); // set WDT post scaler = 4 CLRWDT(); while(1) {

000F 0010 0011 0011 0012

0C01 091F 02A9 0A0F

MOVLW CALL INCF GOTO GOTO

01h 001Fh 09 000Fh 001Ch } else { CodeByte = 0; PORTB = WdtValue >> 2; Delay_Ms_4MHz(1); // delay for 1 mS WdtValue++; // count 1ms delays }

0013 0A1C

0014 006A 0015 0209 0016 0027 0017 0327

CLRF MOVF MOVWF RRF

0A 09,W PORTC 07

//divide by 4 for exact // timeout value

DS91003A-page 4

© 1996 Microchip Technology Inc.

TB004
0018 0019 001A 001B 0327 0C3F 0147 0026 RRF MOVLW ANDWF MOVWF 07 3Fh 07,W PORTB } while(1) CLRWDT(); // loop forever }

001C 0004 001D 0A1C 001E 0800

CLRWDT GOTO 001Ch RETLW 00h

0000 001F 0000 0020 0027

void Delay_Ms_4MHz(registerw delay) { #asm NOP ;1 MOVWF __WImage ;1 DLMS4M1 RADIX DEC ;Use decimal values MOVLW 249 ;1 MOVWF __FSRImage ;1 DLMS4M2 ;4 cycles NOP ;1 DECFSZ __FSRImage ;1 goto DLMS4M2 ;2 DECFSZ __WImage goto DLMS4M1 #endasm } ;1 ;2

0021 0CF9 0022 0028

0023 0000 0024 02E8 0025 0A23 0026 02E7 0027 0A21 0028 0800 01FF 0A04 RETLW GOTO 00h

0004h

ROM USAGE MAP 0000 to 0028 01FF to 01FF Total ROM used 002A

Errors Warnings

: :

0 0

© 1996 Microchip Technology Inc.

DS91003A-page 5

TB004
NOTES:

DS91003A-page 6

© 1996 Microchip Technology Inc.

TB004
NOTES:

© 1996 Microchip Technology Inc.

DS91003A-page 7

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11/25/96

All rights reserved. © 1996, Microchip Technology Incorporated, USA. 11/96
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DS91003A-page 8

© 1996 Microchip Technology Inc.