Output a pulse of 6.5us upon pin B0 interrupt

[Einly]

Dear all,

I wish to do the following:
Whenever I receive a low to high interrupt on Pin B0, I will output a high pulse at Pin C0 and starts a timer 2 which will overflow after 6.5us. When timer 2 overflow, Pin C0 will be low again. This process repeats again when I receive another low to high interrupt. Below is my program:

#include <16f876.h>
#fuses hs,NOWDT,NOPROTECT,NOLVP
#use delay(clock=20000000)

#byte PORTA=5
#byte PORTB=6
#byte PORTC=7

#int_ext
void ads_isr()
{
output_high(PIN_C0);
setup_timer_2(T2_DIV_BY_1,32,1);
enable_interrupts(INT_TIMER2);
}

#int_timer2
timer2_isr()
{
output_low(PIN_C0);
disable_interrupts(INT_TIMER2);
}

void main()
{
set_tris_b(0x01);
set_tris_b(0);

ext_int_edge(l_to_h);
enable_interrupts(global);
enable_interrupts(int_ext);
}

1) Does this line reset the timer 2 value and recount for 6.5us?
setup_timer_2(T2_DIV_BY_1,32,1);
2) Does the program above look logical (meet the requirements)?
I might be wrong in placing this line at the wrong place
setup_timer_2(T2_DIV_BY_1,32,1);
enable_interrupts(INT_TIMER2);

Thanks a lot...
_________________
Einly

[Ttelmah]

The setup line, sets the prescaler, and the 'limit'. If the timer was already running, it could contain a count already at this point, but from power up, it shouldn't. You should be setting the timer up in main, then resetting the _count_ in the interrupt, clearing the interrupt flag (since it will already have been triggered if the timer was running), then enabling the interrupt.
However, your big problem is _latency_. You cannot do what you want with an interrupt. The problem is that when the interrupt occurs, a global interrupt handler is called. This saves a number of registers, and then checks what has interrupted, and eventually calls the handler itself. All this takes time. The typical handler on the 16 family processor, will reach the interrupt routine, about 30 instruction times after the event occured. Then returning from the interrupt typically takes another 25 instruction times. So in your external interrupt, once everything is 'set', 25 instruction times occur before the code returns to the main flow. Only 7 instructions latter, the timer triggers (the first time, on latter loops, the timer interrupt will already have been triggered because the timer was still running), then another 30 instruction times pass, before the edge is generated.
Look instead at using timer1, and the CCP module. The special event setting of this, allows it to clear a pin automatically when a count is reached. This however has to use the CCP pin, not pin_c0.
With the approach you are currently using, the shortest pulse possible, would be about 11uSec, which is the time taken to exit the interrupt routine, enter again, and operate the pin...
Given that the code results in an amost immediate second interrupt, which will itself take about 13uSec to handle, you might as well use the other 'normally avoided' approach, and just operate the pin with a delay in the interrupt handler. However this will still occur some 5uSec after the actual event that triggered the interrupt...

Best Wishes

Best Wishes

[Einly]

"Look instead at using timer1, and the CCP module. The special event setting of this, allows it to clear a pin automatically when a count is reached. This however has to use the CCP pin, not pin_c0. "

I tried to look at the Microchip AN594 application note and found out that there is 2 mode associated with the CCP module, which is compare and capture. For my application, how do I use the CCP module? There will be pulses coming into a pin (external pulses with a maximum frequency of 300kHz). Upon receiving a high to low pulse, I will output a 6.5us high pulse on a pin.

May I know what should I do to achieve that? Is it possible using CCP module? Do you have any example lines of code to do that? Which pins should I use? (I need 2 pins, 1 for receiving the high to low pulse, 1 to output the 6.5us).
_________________
Einly

[DragonPIC]

Use a CCP module in compare mode. When external interrupt occurs, setup the compare module to a 6.5us (plus an instruction cycle needed to set the output high) value and make it so it will clear the CCP output on match. Clear the timer1. Set the CCP output to high and when the compare match occurs, the output will return low. Then use the compare interrupt to turn the CCP of until you need it again.

Like Ttalmah said, you may be better off with:

[sseidman]

[DragonPIC]

I have used the decimal point before with the SX version with no problem. I used a constant and do not remember checking the accuracy. If needed, just use:

[sseidman]

[PCM programmer]

No, it can't take a float.

Compile the following program with PCM vs. 3.224:

[DragonPIC]

I beleive he is running at 20MHz. Retry with 20MHz. Your probably right anyway since it doesn't look right. The compiler should have come up with an error.
_________________
-Matt

[DragonPIC]