; ------------------------------------------------------------------------ ; ; Title: ; ; PD37 -- PIC 10 MHz to 1PPS divider, with sync and micro-step ; ; Function: ; ; This PIC program implements a digital frequency divider: the external ; 10 MHz input clock is divided by ten million to produce a 1PPS output. ; ; - The 1PPS pulse width is 100 ms (10% duty cycle). ; ; - Two Step inputs are checked once a second: ; to advance 1PPS by 10 us, hold StepA low ; to retard 1PPS by 100 us, hold StepB low ; to advance 1PPS by 1000 us, hold both StepA and StepB low ; ; - Two inputs support optional manual 1PPS synchronization. Pull Arm ; pin low for a second to stop divider. The output will synchronize ; to next rising edge of Sync pin (within one instruction cycle). ; ; Diagram: ; ---__--- ; 5V (Vdd) +++++|1 8|===== Ground (Vss) ; 10 MHz clock in ---->|2 pD 7|<+--- StepA ; 1PPS out <----|3 37 6|<+--- StepB ; Arm o--->|4 5|<+--- Sync ; -------- ; Notes: ; ; o External pull-up required on Arm input (pin4/GP3). ; + Step and Sync inputs have internal WPU. ; 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-Jun-2013 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 ; Define entry points. org 0 ; power-on entry goto init ; org 4 ; interrupt entry goto sync ; ; One-time PIC 12F675 initialization. init: bcf STATUS,RP0 ; bank 0 movlw 0x07 ; turn off comparator movwf CMCON ; movlw 0xFF ; set output latches high movwf GPIO ; bsf STATUS,RP0 ; bank 1 errorlevel -302 clrf ANSEL ; enable digital IO (no analog pins) movlw 0< output pin(s) call Delay5 ; (sync alignment) sync: call armed ; check for Arm request movlw d'24' ; call DelayW10k ; delay W*10000 movlw d'99' ; call DelayW100 ; delay W*100 movlw d'83' ; call DelayW1 ; delay (15 <= W <= 255) goto fall ; Set output low for 900 ms = 2,250,000 Tcy at 10 MHz. fall: movlw 0x00 ; low output movwf GPIO ; W -> output pin(s) movlw d'224' ; call DelayW10k ; delay W*10000 movlw d'74' ; call DelayW100 ; delay W*100 movlw d'58' ; call DelayW1 ; delay (15 <= W <= 255) call step ; alter loop period as necessary goto rise ; Use selected calls to delay functions to make 0-10-100-1000 us steps. step: movf GPIO,W ; get 2-bits of Step pins andlw 3 ; addwf PCL,F ; jump PCL+W (computed goto) goto step0 ; b'00' = advance 1PPS by 1000 us goto step1 ; b'01' = retard 1PPS by 100 us goto step2 ; b'10' = advance 1PPS by 10 us goto step3 ; b'11' = no 1PPS step (default) step1 call delay100us ; 100 us more delay than normal step3 call delay1000us ; consider this normal delay step0 call delay10us ; 1000 us less delay than normal return step2 call delay1000us ; 10 us less delay than normal return ; The following three functions delay: 10 us, 100 us, 1000 us. ; - Valid only for 10 MHz clock (Tcy 0.4 us). ; - Times include call/return. delay1000us: ; 1000 us is 2,500 instructions movlw d'24' ; call DelayW100 ; delay W*100 movlw d'94' ; call DelayW1 ; delay (15 <= W <= 255) return ; delay100us: ; 100 us is 250 instructions movlw d'245' ; call DelayW1 ; delay (15 <= W <= 255) return ; delay10us: ; 10 us is 25 instructions call Delay20 ; nop ; return ; ; Implement two-pin 1PPS Arm-Sync synchronization protocol. ; - Accept Arm (low) request when output(s) are high. ; - Use GP2/INT interrupt to keep accuracy within 1 Tcy. ; - Divider resets and resumes on rising edge of Sync pin. ; - Re-enter main loop late to compensate for interrupt/code latency. armed: btfsc GPIO,GP3 ; Arm pin low? return ; no, continue running movlw 1<