by Scott Dattalo
;----------------------------------------------- ;pulse width measurements with 3 Tcyc resolution ; ;The purpose of this routine is to accurately measure ;the width of a pulse. The resolution is 3 instruction ;cycles and the dynamic range is 19 bits or 3*2^19 cycles. ;(That's 1,572,864 cycles which is approximately pi/10 ;seconds on a 20Mhz pic.) ; ; btfsc ioport,iobit goto $-1 loop btfsc ioport,iobit ;If the pulse is over goto high0 ;then adjust the count ; movlw 1 ;Otherwise, intertwine the counter ;incrementing with the pulse checking btfsc ioport,iobit ; goto high1 ; ; addwf lo,f ;Increment the lo byte ; btfsc ioport,iobit ; goto high2 ; ; rlf upper,w ;Pick up the carry (if lo byte ;overflowed) btfsc ioport,iobit ; goto high3 ; ; addwf hi,f ;Increment the high byte ; btfsc ioport,iobit ; goto high4 ; ; skpc ;If the high byte rolls over btfsc ioport,iobit ;or the pulse is over goto high5_or_done ;then get out of the loop ; clrwdt ;Could be a nop or some inst. ; btfsc ioport,iobit ; goto high6 ; ; nop ; ; btfss ioport,iobit ;Note that we check the opp. level goto loop ; ;fall through... Add 7 to the total count incf upper,f high6: incf upper,f high5_or_done: skpnc ;If c=1 then we have an goto overflow ;overflow incf upper,f high4: incf upper,f high3: incf upper,f high2: decf lo,f ;Get rid of the extra ;increment of the lo byte incf upper,f high1: incf upper,f high0: rlf upper,f rlf lo,f rlf hi,f rlf upper,f rlf lo,f rlf hi,f rlf upper,f rlf lo,f rlf hi,f rlf upper,f swapf upper,w andlw 7 iorwf lo,f movlw 7 andwf upper,f retlw 0 overflow ;If we get here, then there was an overflow. ;it turns out that all three bytes of the ;counter are zero. Decrementing all three ;will set them to 0xff. decf upper,f decf hi,f decf lo,f retlw 0xff
Dwayne Reid [dwayner at planet.eon.net] of Trinity Electronics Systems Ltd Edmonton, AB, CANADA says
BTW: the code works VERY well. Its being used to measure the pulse duty cycle ratio from Analog Devices TMP03 / 04 temperature sensors and can resolve down to fractins of a degree (far more accuracy than the sensor). I cleaned the normalize part of the routine up slightly as follows at the end of this message.
The entire measurement routine (wait for HI, wait for LO, measure LO, measure HI, test for noisy signal, normalize everything, multiply HI pulse by constant, divide by LO pulse, average 16 samples) takes about 400 code spaces - left me lots of room for the rest of the project in a 12c508.;up to 19 bit pulse timer with 3 cycle resolution ;concept by Scott Dattalo, this version by Dwayne Reid ;now measure HI period clrf MH_low clrf MH_mid clrf MH_high ;used as known zero for main loop MH_loop btfss PULSE goto MH_1st movlw 1 btfss PULSE goto MH_2nd addwf MH_low,F btfss PULSE goto MH_3rd rlf MH_high,W ;get C into W lsb (add 0 or 1 to next byte) btfss PULSE goto MH_4th addwf MH_mid,F ;add previous carry btfss PULSE goto MH_5th ;use either line below (not both) btfss MH_mid,5 ;5 = 16 bits; 6 = 17 bits; 7 = 18; skpc = 19 ; skpc ;5 = 16 bits; 6 = 17 bits; 7 = 18; skpc = 19 btfss PULSE goto MH_6th nop ;spare cycle! btfss PULSE goto MH_7th clrwdt btfsc PULSE goto MH_loop MH_8th incf MH_high,F ;MH_high now used to accumulate LSBs MH_7th incf MH_high,F MH_6th incf MH_high,F MH_5th ;use either line below (not both) btfsc MH_mid,5 ;5 = 16 bits; 6 = 17 bits; 7 = 18; skpnc =19 ; skpnc ;5 = 16 bits; 6 = 17 bits; 7 = 18; skpnc =19 goto overflow ; subwf MH_mid,F ;undo increment, if any incf MH_high,F MH_4th incf MH_high,F MH_3rd decf MH_low,F ;undo increment incf MH_high,F MH_2nd incf MH_high,F MH_1st ;normalize high period movfw MH_high ;get LSB count into w clrf MH_high ; clrc ;make room for lower 3 LSBs rlf MH_low,F ; (shift everything to the left 3 bits) rlf MH_mid,F rlf MH_high,F ; rlf MH_low,F rlf MH_mid,F rlf MH_high,F rlf MH_low,F rlf MH_mid,F rlf MH_high,F ;none of this affects LSB count in w iorwf MH_low,F ;put LSBs into low byte ;done! Dwayne Reid <firstname.lastname@example.org> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 16 years of Engineering Innovation (1984 - 2000) * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email.
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<A HREF="http://www.sxlist.com/techref/microchip/pulsewidth-sd.htm"> PIC Microcontoller IO Method - Pulse Width Measurement</A>
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