problem with interrupt

[sefat]

i want interrupt every 20ms.this code is not invoking the isr. where is the problem ? plz help.
{
TCNT1=-625;
TCCR1A=0x00;
TCCR1B=0x04;
TIMSK|=(1<<TOIE1);
sei();

}

ISR (TIMER1_OVF_vect)
{
//code}

 

[internetuser2k12]

Which mcu are you using and posy the full code using code tags.

 

[alexan_e]

Why do you pass a negative value to TCNT1?

You are using a timer frequency of 1/256 of the core but what is the frequency of the core?

 

[alexxx]

i want interrupt every 20ms.this code is not invoking the isr. where is the problem ? plz help.

You didn't mention which MCU you are using. Obviously it's an AVR, but which one?
This code compiles and the ISR is not triggered? Because "TIMSK" seems that should be accessed as "TIMSK1", even though we still don't know which MCU is used.

 

[sefat]

i'm using atmega 32 with an external crystal of 8MHz.

the meaning of negative value is:
TCNT1H=0xFD;
TCNT1L=0x8F;

 

[alexan_e]

And why put a negative values instead of TCNT1=0xFD8F ?