Determining square-wave frequency input into pic

[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.

[javi.ar]

In this case and only for dcc bit reading I solved this way without using interrupts

I start reading on L_to_H edge and after 90 microseconds (us) y read the port again if its high so must be a cero else must be one...
that is because a cero is 116us high and 116 us low, and one is 58us high and 58us low...

and it works!!!

[javi.ar]

[A]

Dear PCM programmer and another user.

I have some question about how to calculate about frequency. if current ccp is less than old_ccp;
example : when first time old_ccp is 65535 and when count up and when go to #int_ccp1 again current_ccp is 1000. what is the value of isr_ccp_delta and what is frequency of this loop.

And i want to know what is the limit of this sourcecode.
example : how many range of frequency that we can capture with this sourcecode and how to calculate about this limit. Please explain me about this sourcecode. Because i very confuse and doubt about when current_ccp is less than old_ccp and isr_ccp_delta will be minus value(-???). Thank you for your explaination.
Now i try to use this sourcecode to detect frequency of square wave and duty cycle about 50%. some time this sourcecode work. but when use this in some frequency. this sourcecode is not work. because i compare with scope to check value from micron.


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

current_ccp = CCP_1; // From 16F877.H file

isr_ccp_delta = current_ccp - old_ccp;
// example : isr_ccp_delta = (1000)-(65535); this value may be minus value; Is it true??

old_ccp = current_ccp;
}

[PCM programmer]

To get rid of limits, use a 24-bit timer (16-bit hardware timer with 8-bit
software extension). This is explained in a earlier post in this thread,
which has a link to the 24-bit timer code.

[cybanical]

PCM Programmer, I have a couple questions about your CCP1 code and frequency calculation:

[PCM programmer]

For this type of question, you can easily find the answer by yourself by
making a short test program and looking at the .LST file. Let's do that.

Here is a little test program. Compile it.