please help

[virtual]

I am writting follwing code to toggle bit on port b each time one clock is

counted using external timer-1 .frequency at external timer is 256KiloHz


but frequency at port b1 is only 36KiloHzWhich should be 256KiloHz .


MCU PIC16F877A ,Crystal 20Mhz.What is problem.


#include "D:\Timer\Freq.h"
//#bit TimerOn = 0x10.0
//#bit TimerFlag = 0x0C.0
//#byte TimerLow = 0x0E
//#BYTE TimerHigh = 0x0F
#bit Clk = 0x6.0
unsigned char Flag = 0;
void Check(void);
#int_TIMER1
TIMER1_isr()
{
Flag = 1;
output_bit( PIN_B1, 1);
set_timer1(0xFFFF);


}



void main()
{

setup_adc_ports(NO_ANALOGS);
setup_adc(ADC_OFF);
setup_psp(PSP_DISABLED);
setup_spi(FALSE);
setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
setup_timer_1(T1_EXTERNAL|T1_DIV_BY_1);
setup_timer_2(T2_DISABLED,0,1);
enable_interrupts(INT_TIMER1);
enable_interrupts(GLOBAL);
//TimerOn = 0;
SET_TRIS_B(0xF0 );
SET_TRIS_D(0xFF );

set_timer1(0xFFFF);

//TimerHigh = 0xFF;
//TimerLow = 0xFF;

//Check();
Flag = 0;
output_bit( PIN_B0, 0);
output_bit( PIN_B1, 0);
//while(Clk);
do
{
do

{

}while(!Flag);

Flag = 0;
output_bit( PIN_B1, 0);
}while(1);
}
void Check(void)
{
int i;
for(i = 1 ;i<100;i++)
{
output_bit( PIN_B0, i%2);
delay_ms(100);
}

}

[Ttelmah]

Calling an interrupt, has a significant overhead associated with it. Typically perhaps a total of about 60 instructions, to call an interrupt and return. At 20MHz, this is about 12uSec. Now you are not showing 'fast_io', so a single bit input or output instruction, itself involves about 3 instruction times, while the loop itself, also involves another couple of instruction times. So as it stands, you will get:

Event happens.
Appproximately 6uSec latter, you will arrive in the interrupt event.
About 1uSec latter, you set the timer. At this point, you will have missed one, and possibly two of the event edges. You then setthe counter, which itself prevents counting from taking place while the value is being changed, another 1 to 1.5uSec. The interrupt flag is cleared. Some 6uSec latter you arrive 'back' in the main code, and another interrupt event will have occured. You then execute one instruction, and immediately call the interrupt handler again.
Basically, you cannot handle an event at this sort of frequency, with an interrupt, on a system at 20MHz (in assembler, with care, it could possibly just be done, an you might be able to get 'close', with a custom 'int_global' handler, but I doubt it.
Start by not using the timer fo this. This brings the significant overhead, of needing to be reset to see the next event. The normal 'INT' input, is a better check for an edge. Then don't handle this usng an interrupt handler at all, but simply poll for the interrupt being set. Also use fast_io, to reduce the overhead on each I/O operation.
So:
(Learn to use the 'code' button, makes ce much easier to read)

[virtual]

Hello

I am happy for quick reply But microchip asks upto 50 Mhz frequency couting can be done with timer 1 how is it possible as my rate is 255 KILOhZ
ONLY

[ckielstra]

[rberek]

Read Ttelmah's post again.

Your problem has nothing to do with the speed of your timer, but rather the speed of your program. Your program is written in such a way that it cannot react to every edge of the timer so your toggle rate of port B is a fraction of the timer's rate.

Don't forget that instructions take time to execute, and if your program takes longer than a timer period to toggle port b, the apparent toggle rate will be slower than the timer rate.

r.b.

[Ttelmah]