external interrupt starting timer

[umka]

I want to turn on and off a transistor with the pin setup as IGBT_ON and IGBT_OFF. Kind of like PWM but the want it controlled by an external interrupt.

An external interrupt is triggered which should turn on the transistor, set a timer reload value as it is changing depending on the main code result, start the timer and then exit the interrupt and go do other stuff.

Once the timer interrupts I want the transistor to turn off, the timer not to interrupt again until it has been through the external interrupt routine again and then exit the interrupt.

This is a repeating cycle with the max time period for timer being about 13ms.

Is this code going to work correctly or is there a better way to do this as I am a bit concerned about having the timer setup each time I call the external isr?

Code snippet

[bwhiten]

Sorry, I have no answer for you, but I am also looking to perform a similar function, using an external interrupt to trigger an internal timer that triggers an ADC reading on time out. So this is a bump to get the question to the top again.

[umka]

Anyone got any input for us?

[Ttelmah]

I'd use a CCP.
You can program this to turn it's output off after a count.
You don't even need to interrupt. It is a hardware control, that will trigger an interrupt if required, but this doesn't have to be used. You start it in an interrupt, by resetting it's counter, and setting the line, then just exiting the interrupt. The line will turn off for you the programmed time latter.
I have used exactly this for such an application in the past.

Best Wishes

[umka]

the circuit i need it for is an AC phase controller using a IGBT to turn on the power and then off after a certain timer period (before the half sine wave of AC is complete). this is why i have the interupt so i know when the AC has just crossed the + to - half waves using a zero crossing circuit.

[Ttelmah]

Exactly. But you don't need a _second_ interrupt for the turn off.
You just tell the CCP to turn it's output line 'off' at the specified count, set the count, and the line automatically goes off the specified time latter.
The problem with using the interrupt, is that it takes a lot of time. Typically at least 30 instructions to enter the interrupt. Also, since it takes this long to get out of the first interrupt,your 'minimum' period, typically becomes something like 70 instruction times. With the CCP, the switch off is automatic, and the CCP can just be left running till the next time it is needed. Used like this, the CCP, can be thought of like a programmable monostable multivibrator, with you able to program the pulse width it generates.
You either reset the timer being used, and put the required count into the CCP, or just add the required count to the current timer value, and put this into the CCP. Set the line. Do this in your zero-crossing interrupt, and the hardware does the rest for you...

Best Wishes

[umka]

Cheers for the help Ttelmah

I am not totally sure if I am doing this right but do i want to use code similar to that in EX_CCP1S?

[Ttelmah]

The code you show, uses the CCP to set, the line, then the CCP again, to clear it. All you do, is put just the _clear_part into the zero crossing interrupt, and manually _set_ the line yourself here. The line goes high when you set it (logical), and clears automatically the required interval later. Since the only function is to clear the line, and the line is low, there is no re-triggering, till you yourself set the line in the next zero crossing interrupt. Hence no multiple pulses.

Best Wishes

[umka]

So the code would look something like this?

[umka]

Ok i tryed the above code and it doesnt work. i think problem is with the output_high.

[Ttelmah]

Can you accept a pulse that goes high, rather than low?. Otherwise it is just a 'tad' harder to do.
If you program the CCP to 'set on match', the code is:
[code]
#byte CCP1CON=0xFBD

#EXT_int
void isr()
{
int8 dummy;
dummy=CCP1CON;
CCP1CON=0;
set_timer1(0); // Reset timer 1
CCP1CON=dummy;
CCP_1 = ON_Period; // set CCP compare value
}

void main()
{
setup_ccp1(CCP_COMPARE_SET_ON_MATCH);
setup_timer_1(T1_INTERNAL); // Set up timer to instruction clk

while(TRUE)
{

// Calculate ON_Period

// Do stuff

}
}
{/code]
I have used this for basically exactly this application.

If you need to have the pulse positive going, then you set the pin, by using the code shown in your original example.

Best Wishes