diego
Guest
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| CCP Problem |
 Posted: Wed Feb 22, 2006 12:51 pm |
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I have a problem with the compare function when trying to measure the RPM's of a motor.
The idea is to measure the period of the signal of the speed sensor.
So, when an interruption occurs, we start to count the time (by clearing TMR1),
and when the second rising edge interrupts we read the value of CCP_1.
Comparing my results against a commercial RPM meter, I found an important diference,
so I start to debug the proyect.
My results are that the value of CCP_1 variable at the time of the CCP interruption are
smaller that the real signal being measured.
To better ilustrate this, I run a simulation in MPLAB, with a signal of 60 Hz in the CCP input,
for this frecuency, the period in microseconds are ( 1 / 60 ) * 1000000 = 16666,66.
We try two options to read the period inside of the ISR:
period = get_timer1();
or
period = ccp_1;
The values at the moment of interruption are:
if period = get_timer1();
period 16667
if period = ccp_1;
period 16625
When calculate the RPM (for a 60 Hz input they are 60 * 60 = 3600), we have:
1/16667 * 60000000 = 3599.92 RPM
or with the CCP_1 value, 1/16625 * 60000000 = 3609.02 RPM
The difference is bigger when the frecuency of the input grows, at 120 Hz (7200 RPM):
if period = get_timer1();
period = 8334 => 1/8334 * 60000000 = 7199.42 RPM
if period = ccp_1;
period = 8292 => 7235.89 RPM
If you need more information (schematics, complete code, etc), please contact us.
Thank's in advance for your time.
Best Regard's
Diego Caraballo
| Code: |
#include <16F873A.h>
#device adc=8
#FUSES NOWDT //No Watch Dog Timer
#FUSES HS //High speed Osc (> 4mhz)
#FUSES PUT //Power Up Timer
#FUSES NOPROTECT //Code not protected from reading
#FUSES NODEBUG //No Debug mode for ICD
#FUSES noBROWNOUT //Reset when brownout detected
#FUSES NOLVP //No low voltage prgming, B3(PIC16) or B5(PIC18) used for I/O
#FUSES NOCPD //No EE protection
#FUSES NOWRT //Program memory not write protected
#use delay(clock=4000000)
#use rs232(baud=14400,parity=N,xmit=PIN_C6,rcv=PIN_C7,bits=8)
int32 periodo;
float rpm;
float frec;
int32 sumador = 0;
int sumador_index;
short measuring;
short ready;
#int_CCP1
CCP1_isr()
{
if (!measuring)
{
measuring = true;
set_timer1(0);
}
else if (measuring)
{
//we disable one of these:
periodo = get_timer1();
//periodo = ccp_1;
ready = true;
measuring = false;
}
}
void main()
{
setup_adc_ports(NO_ANALOGS);
setup_adc(ADC_OFF);
setup_spi(FALSE);
setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
setup_timer_1(T1_INTERNAL|T1_DIV_BY_1);
setup_timer_2(T2_DISABLED,0,1);
setup_ccp1(CCP_CAPTURE_RE);
setup_ccp2(CCP_OFF);
setup_comparator(NC_NC_NC_NC);
setup_vref(FALSE);
enable_interrupts(INT_CCP1);
enable_interrupts(GLOBAL);
delay_ms(500L);
measuring = false;
ready = false;
while(true)
{
}
}
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