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Efficient TMR0 INT routine?

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hhhsssmmm

New Member
Hello

I have a PIC18F2420 with 20MHz crystal and c18 compiler.

I have successfully performed the simple transformer coupled halogen lamp dimming via my PIC. The PIC achieves this by controlling the firing angle of the 230V 50Hz AC waveform on the transformer HT side. The TRIAC is snubberless BTB16600BW. Below is my code sample. The method of coding works and i get very good results....that is the bulb starts from very low level dim and then brightens up gradually all the way to MAX intensity. Also the transformer NEVER heats up and everything is smooth operation. However it is not good practice to stay inside an ISR for too long as can be seen in my TMR0 Interrupt routine where the program stays for 200us for the TRIAC latching. Anything less than 200us...and the halogen lamp flickers.

Code:
unsigned char bulbdim = 107; //initiallize at MIN intensity

//TMR0 is prescaled to 1:256

void main()
{

while(1); //loop forever

}

void LowInterrupt (void)
{

if(INTCONbits.INT0IF == 1) //if INT0 (ZERO CROSSing) interrupt
{

bulbdim++; //brighten up intensity

if(bulbdim == 215)//MAX intensity attained

bulbdim = 107; //reset to MIN intensity

TMR0L = bulbdim; //TRIAC firing time initiallize

T0CONbits.TMR0ON = 1; //Enables Timer0

INTCONbits.INT0IF = 0; //Reset the INT0 External Interrupt Flag bit

}

if(INTCONbits.TMR0IF == 1) //If TMR0 interrupt
{

LATBbits.LATB1 = 1; //turn on TRIAC

Delay10KTCYx(1); //200us TRIAC latching delay

LATBbits.LATB1 = 0; //turn off TRIAC

T0CONbits.TMR0ON = 1; //Disable Timer0

INTCONbits.TMR0IF = 0; //Reset the TMR0 Interrupt Flag bit

}

}//end of LowInterrupt ISR

To improve the efficiency, I then re-did my TMR0 Interrupt routine and optimized it so that the program does not stay inside for that long and yet the TRIAC can still be latched for the desired time. This TMR0 interrupt routine is given below. Sadly this version of the TMR0 Interrupt routine produces not a very good low level dimming result...meaning the bulb never dims too little as it SHOULD to get an overall effect of dimming.....AND the transformer heats up too quickly clearly indicating that the PIC is not in symmetry with the AC waveform.

Code:
if(INTCONbits.TMR0IF == 1) //If TMR0 interrupt
{

if(flag == 0) //200us will lapse
{

LATBbits.LATB1 = 1; //turn on TRIAC

flag = 1; //200us will lapse

TMR0L = 4; //initiallize for 200us TRIAC latching delay
//TMR0 is prescaled to 1:256

T0CONbits.TMR0ON = 1; //Enables Timer0

}

else //200us have lapsed
{

LATBbits.LATB1 = 0; //turn off TRIAC

flag = 0; //200us have lapsed

T0CONbits.TMR0ON = 0; //Disable Timer0

}

INTCONbits.TMR0IF = 0; //Reset the TMR0 Interrupt Flag bit

}

Please can someone explain this and help me to improve the above TMR0 interrupt routine to achieve perfect dim low to high level bulb intensity without the transformer heating up?

Thank you

Haseeb
 
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