Determining square-wave frequency input into pic

[PCM programmer]

To do that, you need to do these things:

1. Declare a global variable to indicate that a CCP interrupt occured.

2. Inside the CCP isr, set the global variable = TRUE.

3. At start of main(), set it = FALSE.

4. Then, down at bottom of the main while() loop, change the code
that displays the frequency so that it checks this global flag.
If the flag is TRUE, then display the frequency. If not, then display
a message indicating there's no signal present. Also, if the flag
is TRUE, then clear it.

Here's the modified code:

[acidice333]

What is needed to change for this to work on a 48mhz clock? (PLL 20mhz)

[PCM programmer]

I assume you have a board with a PIC such as the 18F4550 that you
want to run at 48 MHz, with a 20 MHz crystal. Your fuses statement
should look like this:

[acidice333]

Thanks I thought I did something wrong but its all correct.

I'm having a wierd probem- with it operating at 4mhz it seems fine and at 48mhz the reading spikes and is never stable. I will try running it at 20mhz tommorow.

[PCM programmer]

What's your PIC and your compiler version ?

[acidice333]

18f2550, and v4.009

4mhz crystal, with it set to 4mhz works fine.
4mhz crystal, with it set to 24mhz doesn't work (spikes)
20mhz crystal, with it set to 48mhz doesn't work (spikes)

Im measuring a frequency from a 4n25, ranging from 25hz to 200hz

[PCM programmer]

You want it to work at very low frequencies. That's different from just
making the demo work. It requires adjustments to the pre-scalers.
I'll work on it tomorrow.

[acidice333]

Yes. I was thinking it might be a prescalar problem, looking forward to your post tommorow (: Would be a bonus if it worked with 1hz (:

[PCM programmer]

Here's the answer to your question:

To make the tachometer example work with a 1 Hz input signal while the
PIC is running at 48 MHz, you need to extend Timer1 to 24 bits (currently
it's only 16 bits wide).

The reason for this is because at 48 MHz, the Timer1 clock is 12 MHz
(1/4 of the oscillator frequency). A 1 Hz input signal has a period of
one second. During that one second, the Timer1 will count up to 12
million. But Timer1 is only 16-bits wide, so it can only hold 65535.
By extending it to 24-bits, it can count up to 16777215 and so it's now
big enough to hold the count of 12,000,000 which occurs between the
positive edges of a 1 Hz input signal.

You could increase the pre-scaler on Timer1 to 8 (the maximum value
available), and this would reduce the timer clock to 1.5 MHz. But the
count during one second would still be 1.5 million, and that's way more
than the 65535 that the Timer1 hardware can count up to. So even
with a Timer1 prescaler, you still need to extend the Timer to 24-bits.

This thread has an alternate #int_ccp1 routine, which uses a Timer1
value which is extended to 24 bits.
http://www.ccsinfo.com/forum/viewtopic.php?t=906

Other changes:
In the frequency calculation in main(), you need to change the
numerator to 12000000L. That's the frequency of the Timer1 clock
(12 MHz). You also need to increase size of the global variable that
holds the current CCP to be an 'int32' (it was 16 bits). This will hold
the 24-bit extended Timer1 value. Also the 'current_ccp_delta' will
have to be change to an 'int32'.

The revised display code would look like this.

[deepakomanna]

Dear Programmer,
Here in #int_ccp1 interrupt you're calculating time interval between two rising pulses, but I have one query abt this interrupt.
Here your statement is,

[Ttelmah]

Hint.
Look at how long a signal has to be stable, before it is 'seen' by the CCP. This is dependant on the Tcy of the processor (and hence on the clock). I'd suspect your input signal has got some high frequency noise, and at the higher clock rates, the PIC is 'seeing' this, while at the lower rates it is not...
Figure 20-10, values 50, 51, and 52.
Try a small capacitor on the input used to detect the signal.

Best Wishes

[javi.ar]

Hi everyone, thanks in advance ...

what I am trying to find here is, where do I go wrong , I 've been wondering for a while how to fix and I could not. I have CCS 4.032 ver working with 18f4525 XT 10MHZ output PIN_E2
#define UNO 56
#define CERO 112
void main()
{
init(); //calls all the setup
lcd_init();
delay_ms(10);
// TODO: USER CODE!!
printf (lcd_putc,"working");

while (TRUE) {
send_bit(1);
send_bit(0);
}
}

int send_bit(unsigned int1 b)
{
if (b == 1){
output_high(OUT);
delay_us(UNO);
output_low(OUT);
delay_us(UNO);

}
else{
output_high(OUT);
delay_us(CERO);
output_low(OUT);
delay_us(CERO);
}
return 0;
}

code is slightly longer...

any way on the other end I have 16f628a input PIN_B3 / CCP1 with 10K resistor pulling down. (I test with small capacitor to see if could improve with no success)
with the following code...

int16 isr_ccp_delta;
int1 xccp_on = 0;

#int_ccp1
void ccp1_isr(void)
{
int16 current_ccp;
static int16 old_ccp = 0;

current_ccp = CCP_1; // From 16F628A.H file
isr_ccp_delta = current_ccp - old_ccp;
old_ccp = current_ccp;
xccp_on = 1;
}

int main(void)
{
init();

while(TRUE){
xccp_on = 0;

if (xccp_on == 1);
printf("%d ",read_bit());

}
return 0;

unsigned int1 read_bit(void)
{
int16 c_ccp_delta;

disable_interrupts(GLOBAL);
c_ccp_delta = isr_ccp_delta;
enable_interrupts(GLOBAL);
//printf ("%lu ", c_ccp_delta );
if ((c_ccp_delta > 11 )&& (c_ccp_delta < 19 )){
return 1;
}
else if ((c_ccp_delta > 25 ) && (c_ccp_delta < 35 )){
return 0;
}
}

and config (init())

int init(void){

setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
setup_timer_1(T1_INTERNAL|T1_DIV_BY_8);
setup_timer_2(T2_DISABLED,0,1);
setup_ccp1(CCP_CAPTURE_RE);
setup_comparator(NC_NC_NC_NC);
setup_vref(FALSE);
clear_interrupt(INT_CCP1);
enable_interrupts(INT_CCP1);
enable_interrupts(GLOBAL);
setup_oscillator(False);

return 0;
}

#include <16F628A.h>
#device *=16

#FUSES NOWDT //No Watch Dog Timer
#FUSES XT //Crystal osc <= 4mhz
#FUSES PUT //Power Up Timer
#FUSES NOPROTECT //Code not protected from reading
#FUSES NOBROWNOUT //No brownout reset
#FUSES NOMCLR //Master Clear pin used for I/O
#FUSES NOLVP //No low voltage prgming, B3(PIC16) or B5(PIC18) used for I/O
#FUSES NOCPD //No EE protection

#use delay(clock=4000000)
#use rs232(baud=2400,parity=N,xmit=PIN_B2,rcv=PIN_B1,bits=8,errors)

and here is part of the output
1 0 0 0 0 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 0 0 0 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 0 0 0 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 0 0 ...

but I was expecting to see 101010101010101010101010101010101010101...

the value of the c_ccp_delta for 4MHZ (timer1 div by 8 reading for 112us) is about 14/16

the wired thing is if the output is only 1s or 0s works fine, but with the changes...

I dont know what else to test. Thanks a lot. guys...

[PCM programmer]

What is the purpose of your code ?

What project are you trying to do ?

[javi.ar]

[Guest]

Hi

I dont have a scope, but I do have a DMM with frequency option, the frequency goes from 2.XX hz to 120.XX hz. Measuring the frequency with the DMM I get slow increasing +1hz values, with the 18f4550 I get unstable values for example the frequency could be 30.xx HZ and I printf it out and I see 27hz, 30hz, 31hz... just close.. what could be my problem?

I'm using 4mhz osc.