; ------------------------------------------------------------------------ ; ; Title: ; ; PD84 -- PIC "4-pin" 18.432 MHz to 1PPS frequency divider (10 ms pulse) ; ; Function: ; ; This PIC program implements a digital frequency divider: the external ; 18.432 MHz input clock is divided to 1 Hz (1PPS). ; ; The output is a narrow 10 millisecond pulse (not a 50% square wave). ; ; Diagram: ; ---__--- ; 5V (Vdd) +++++|1 8|===== Ground (Vss) ; 10 MHz clock in ---->|2 pD 7|----> 1PPS out (10 ms) ; *|3 84 6|* ; o|4 5|o ; -------- ; Notes: ; ; Only 4 pins are required: power (2.0-5.5V), ground, input and output. ; * For added drive power, pin3/GP4 and pin6/GP1 are also outputs. ; o Tie input pin4/GP3 and pin5/GP2 to Vdd or Vss. ; Output frequency accuracy is the same as clock input accuracy. ; Output drive current is 25 mA maximum per pin. ; Coded for Microchip 12F675 but any '609 '615 '629 '635 '675 '683 works. ; ; Version: ; ; 14-Jan-2022 Tom Van Baak (tvb) www.LeapSecond.com/pic ; ; ------------------------------------------------------------------------ ; Microchip MPLAB IDE assembler code (mpasm). list p=pic12f675 include p12f675.inc __config _EC_OSC & _MCLRE_OFF & _WDT_OFF ; Register definitions. cblock 0x20 ; define register base endc ; One-time PIC 12F675 initialization. org 0 ; power-on entry here bcf STATUS,RP0 ; bank 0 clrf GPIO ; set all pins low movlw 07h ; set mode to turn movwf CMCON ; comparator off bsf STATUS,RP0 ; bank 1 clrf ANSEL-0x80 ; set digital IO (no analog A/D) movlw b'101100' ; set GP0,GP1,GP4 as output(0) and movwf TRISIO-0x80 ; other pins are input(1) bcf STATUS,RP0 ; bank 0 ; With an external 18.432 MHz PIC clock the (4:1) execution rate is ; 4,608,000 MIPS. The loop below takes exactly 1 second. ; The pins are pulsed once per loop to create a 1PPS output. ; Set output high for 10 ms = 46,080 Tcy at 18.432 MHz. rise: movlw 0xFF ; ( 1) high output movwf GPIO ; ( 1) W -> output pin(s) movlw d'4' ; ( 1) call DelayW10k ; ( 40000) delay W*10000 movlw d'60' ; ( 1) call DelayW100 ; ( 6000) delay W*100 movlw d'73' ; ( 1) call DelayW1 ; ( 73) delay (15 <= W <= 255) goto fall ; ( 2) ; Set output low for 990 ms = 4,561,920 Tcy at 18.432 MHz. fall: movlw 0x00 ; ( 1) low output movwf GPIO ; ( 1) W -> output pin(s) movlw d'250' ; ( 1) call DelayW10k ; (2500000) delay W*10000 movlw d'206' ; ( 1) call DelayW10k ; (2060000) delay W*10000 movlw d'18' ; ( 1) call DelayW100 ; ( 1800) delay W*100 movlw d'112' ; ( 1) call DelayW1 ; ( 112) delay (15 <= W <= 255) goto rise ; ( 2) include delayw.asm ; precise delay functions end