# PIC: how to set timer0 to give an interrupt at every 1 sec?

I am programming a PIC18F67K22 and I've set it to use a 16X4Mhz Internal RC oscillator with PLL enabled.

• High level interrupt on TIMER0 overflow
• I am using MCC18 compiler.

So how to set TIMER0 to give an interrupt at every second?

• Give it a period that is slightly greater than 1s, then use CTC to tune it to 1s. – Ignacio Vazquez-Abrams Oct 3 '13 at 6:14
• I am not understanding what are you saying.please can you explain in brief?? – shail korat Oct 3 '13 at 7:09
• Do you understand how the timer modules work at all? – Connor Wolf Oct 3 '13 at 9:20

The TIMER0 is a 16bits timer, so to set to interrupt every 1 second you need to set:

• Prescaler @ 16

The equation:

$$T = \left(\frac{4}{Fosc}\right) \cdot Presc \cdot (Resolution - Preload)$$

Where:

• T = Period = 1s
• Fosc = Oscilator Frequency = 4MHz
• Presc = Prescaler = 16
• Resolution = 216 = 65535

Sample code:

// Timer0 Registers:16-Bit Mode; Prescaler=1:16; TMRH Preset=$BD; // TMRL Preset=$DC; Freq=1,00Hz; Period=1,00 s
T0CON.TMR0ON = 1;  // Timer0 On/Off Control bit: 1=Enables Timer0 / 0=Stops Timer0
T0CON.T08BIT = 0;  // Timer0 8-bit/16-bit Control bit: 1=8-bit timer/counter / 0=16-bit timer/counter
T0CON.T0CS   = 0;  // TMR0 Clock Source Select bit: 0=Internal Clock (CLKO) / 1=Transition on T0CKI pin
T0CON.T0SE   = 0;  // TMR0 Source Edge Select bit: 0=low/high / 1=high/low
T0CON.PSA    = 0;  // Prescaler Assignment bit: 0=Prescaler is assigned; 1=NOT assigned/bypassed
T0CON.T0PS2  = 0;  // bits 2-0  PS2:PS0: Prescaler Select bits
T0CON.T0PS1  = 1;
T0CON.T0PS0  = 1;
TMR0H = $BD; // preset for Timer0 MSB register TMR0L =$DC;        // preset for Timer0 LSB register

• Just wanted to let you know that an anonymous user had a comment regarding setting T0CON.PSA = 0 instead of T0CON.PSA = 1. They attempted to edit you answer instead of posting a comment to bring the issue to your attention, which was rejected. Their comment was the following, in case you can't access the link above: PSA must be zero for the prescaler to be considered. This code functions improperly when this bit is set to 1. – Ricardo Aug 19 '15 at 11:30