Hi,
The capture module can be configured as one rising edge detector. If TMR1 is
running free and the capture module is armed, CCPx can take two snapshots of
TMR1. The pulse width can be calculated applying 16 bit unsigned math:

PULSE_WDTH = NEW_CAPT - OLD_CAPT

TMR1 is configured in a way that each unit values one microsecond.
Since I don't have a stable wave generator to test it, I've done a simple
ON-OFF 50% duty oscillator with a 16F873. Using Andy Warren loop
generator (625 Inst-cy) achieves full precision in MPLAB simulator, so I use
some crystals to change the frequency and I've obtained these captures:
20Mhz = 250 and  251 (us)
10Mhz = 500 and  501
 4Mhz =1250 and 1251

Q1: What means that jumping from 0 to 1 and 1 to 0?
Q2: If it is normal, has a name?
Q3: CCPx has lost one pulse from TMR1?

Configuration:
16mhz crystal
TMR1 1:4
CCP2 prescaler 1:1
CCP2 rising edge
The whole process is fired once every 100ms, by now.

;======================================================================
; CCP2 ISR : PIE2,CCP2IE bandera en PIR2,CCP2IF
;======================================================================
PIR2_CCP2IF_Handler:
        BSF    STATUS,RP0
        BCF    PIE2,CCP2IE
        BCF    STATUS,RP0
        BCF    PIR2,CCP2IF

;Test if it is the first pulse
        btfsc  PWMErrorFlags,SnapOnGo
        goto   SndSnap
;First pulse
        BSF    PWMErrorFlags,SnapOnGo
        movfw   CCPR2H
        movwf   CCP_H_OLD
        movfw   CCPR2L
        movwf   CCP_L_OLD

        BSF    STATUS,RP0
        BSF    PIE2,CCP2IE
        BCF    STATUS,RP0
        GOTO    _Interrupt_handler

SndSnap:
        BCF     INTCON,PEIE
        BCF     PWMErrorFlags,SnapOnGo

        movfw   CCPR2H
        movwf   CCP_H_NEW
        movfw   CCPR2L
        movwf   CCP_L_NEW

;Measure the width
        movf    CCP_L_OLD, w
        subwf   CCP_L_NEW, f
        btfss   STATUS, C
        decf    CCP_H_NEW, f
        movf    CCP_H_OLD, w
        subwf   CCP_H_NEW, f
        bsf     PWMErrorFlags,SnapNewCap

        GOTO    _Interrupt_handler


Thank you in advance.

Dennis Crawley
Argentina

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