I thought all routines in a module would default to the same bank. At any rate, I removed the "using" and the problem remains.

I am running 3 captures, the problem occurrs even when only receiving pulses into one PCA channel.

I can't change the PCA clock, since I need to be able to read different frequencies simultaneously. The PCA is not highest priority, since I am also performing high baudrate serial.

The code I sent you was trimmed (I removed stuff unrelated to the problem) The complete module is below. I simplified the logic slightly and added a bunch of CF checks after I read some posts of yours on another board...... (something about clearing a CCF flag clearing a CF flag)

The idea is that when servicing the Capture ISR, if the MSB of the capture is greather than CH, a rollover has occurred after the capture, but before the ISR was serviced. Since we have serviced any CF flag, we just need to adjust the PCA extension value used for this capture.

code here:
// PCA.C
#include <reg51f.h>
#include <ssi.h>
#include <extern.h>

void PCA(void) interrupt 6
{
if(CCF0) PerformPCA0(); //
if(CCF1) PerformPCA1(); //
if(CCF2) PerformPCA2(); //
if(CCF3) PerformPCA3(); //
if(CCF4) PerformPCA4(); //
if(CF) PerformPCA_Overflow();

} // end PCA

void PerformPCA0(void) // Split
{
PCA0Val.C[1] = CCAP0L;
PCA0Val.C[0] = CCAP0H;

PCA0Val.I += Cal_MainTimeBaseAdjust;

CCAP0L = PCA0Val.C[1];
CCAP0H = PCA0Val.C[0];

OS_Task_1ms();

if(CF) PerformPCA_Overflow();
CCF0=0;
}// end PCA0

void PerformPCA1(void)
{
if(!CEX1)
{
if(PhotoBlankingTimer == 0)
{
Photo_Sensed = MainTimeBase;
Photo_Split = 0; //Main
PhotoBlankingTimer = Cal_PhotoBlankTime;
Serout_LaptimePacingDelay = 0; // force a laptime block
NewPhotoRecd = 1;
}
}
else
{
PCA_FalseTrip++;
}
if(CF) PerformPCA_Overflow();
CCF1=0;
}// end PCA1



void PerformPCA2(void)
{
if(CEX2)
{
RPM1_Period = RPM1_Ctr;
RPM1_Ctr = 0;
PCA1LongOld.L = PCA1Long.L;
PCA1Long.C[3] = CCAP2L;
PCA1Long.C[2] = CCAP2H;

if(CF) PerformPCA_Overflow();
if(PCA1Long.C[2] > CH) // rollover occurred after pulse received, but before ISR
{
PCA1Long.I[0] = PCA_Extension-1;
}
else
{
PCA1Long.I[0] = PCA_Extension;
}

PCA1LongDelta.L = PCA1Long.L - PCA1LongOld.L;
NewRPM1Recd = 1;
}
else
{
PCA_FalseTrip++;
}
if(CF) PerformPCA_Overflow();
CCF2=0;
}

void PerformPCA3(void) //RPM3 input
{
if(CEX3)
{
RPM2_Period = RPM2_Ctr;
RPM2_Ctr = 0;
PCA2LongOld.L = PCA2Long.L;
PCA2Long.C[3] = CCAP3L;
PCA2Long.C[2] = CCAP3H;

if(CF) PerformPCA_Overflow();
if(PCA2Long.C[2] > CH) // rollover occurred after pulse received
{
PCA2Long.I[0] = PCA_Extension-1;
}
else
{
PCA2Long.I[0] = PCA_Extension;
}
PCA2LongDelta.L = PCA2Long.L - PCA2LongOld.L;
NewRPM2Recd = 1;
}
else
{
PCA_FalseTrip++;
}
if(CF) PerformPCA_Overflow();
CCF3=0;
}// end PCA3


void PerformPCA4(void) // RPM1 input
{
if(CEX4)
{
RPM0_Period = RPM0_Ctr;
RPM0_Ctr = 0;
PCA0LongOld.L = PCA0Long.L;
PCA0Long.C[3] = CCAP4L;
PCA0Long.C[2] = CCAP4H;

if(CF) PerformPCA_Overflow();
if(PCA0Long.C[2] > CH) // rollover occurred after pulse received
{
PCA0Long.I[0] = PCA_Extension-1;
}
else
{
PCA0Long.I[0] = PCA_Extension;
}

PCA0LongDelta.L = PCA0Long.L - PCA0LongOld.L;
NewRPM0Recd = 1;
}
else
{
PCA_FalseTrip++;
}
if(CF) PerformPCA_Overflow();
CCF4=0;
}// end PCA4


void PerformPCA_Overflow(void)
{
CF=0;
PCA_Extension++;
}