; ------------------------------------------------------------------------ ; ; Title: ; ; PD65 -- PIC "4-pin" frequency divider (4 MHz to 1.667 Hz) ; ; Function: ; ; This PIC program implements a digital frequency divider: the external ; input clock is divided by a factor of 2,400,000. For example, if the ; input clock is 4 MHz then the output clock will be 1.666667 Hz. ; ; Diagram: ; ---__--- ; 5V (Vdd) +++++|1 8|===== Ground (Vss) ; input clock ---->|2 pD 7|----> output clock ; *|3 65 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: ; ; 04-Mar-2020 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 gpcopy ; shadow of output pins 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 clrf gpcopy ; initialize shadow output ; With an external 4 MHz PIC clock the (4:1) execution rate is 1 MIPS. ; A 300,000 instruction (1 us each) loop takes exactly 300 milliseconds. ; Output pins are toggled once per loop creating a square wave with ; pulse width 300 ms, period 600 ms, and frequency 1.666667 Hz. loop: movlw 0xFF ; -1 xorwf gpcopy,F ; toggle bits movf gpcopy,W ; gpcopy -> W movwf GPIO ; W -> output pin(s) movlw d'29' ; 290000 call DelayW10k ; delay W*10000 movlw d'99' ; 9900 call DelayW100 ; delay W*100 movlw d'91' ; 91 call DelayW1 ; delay (15 <= W <= 255) goto loop ; include delayw.asm ; precise delay functions end