Example 9-29 on page 364 shows code with timer 0 and XTAL = 10 M Hz that toggles

Question

Example 9-29 on page 364 shows code with timer 0 and XTAL = 10 M Hz that toggles PORTB.4 bit continuously every 50 ms.

Trythis code with three other timers: timer 1, timer 2, and timer 3. Show what the best you can have (say do you get exactly 50 ms or you get 49.5 ms, 50.2 ms, or you get ?s (microseconds) at the best)? Showthe code change you need to have for each of these timers (which lines must be added, deleted, or modified?).

void T0Delay()

{

T0CON=0x01;

TMR0H=0x85;

TMR0L=0xEE;

T0CONbits.TMR0ON=1;

while(INTCONbits.TMR0IF==0);

T0CONbits.TMR0ON=0;

INTCONbits.TMR0IF=0;

}

Explanation / Answer

For timer1

T1CON = 0xA0; //so that prescaler is 4 and clock is Fosc/4 and 16 bit timer

TMR1H = 0x85; //same as prescaler and clock are not changed

TMR1L = 0xEE; //same

T1CONbits.TMR1ON = 1;

while(INTCONbits.TMR1IF==0);

T1CONbits.TMR1ON = 0;

INTCONbits.TMR1IF= 0;

this will generate a delay of (0xFFFF-0x85EE+1)*4(prescaler)*400ns(1/(Fosc/4)) = 50ms

For timer2:

8 bit timer with 1,4,16 prescaler and 1-16 postscaler

using 16 post and 16 pre scalers we get a delay of 16*16*400ns = 102400ns = 102.4us for 1 count

So for 50ms we need a to count 488.28 times. This is not possible in a 8 bit timer(maximum of 255)

to achieve 50ms delay we split it into two and manage in the code for 50 ms.

for 25ms we need 244 counts which gives -> 24.985ms

the following code will generate 50 ms delay( approx 49.971ms)

T2CON = 0x7B; //so that prescaler is 16 and post scaler is 16

PR2 = 244;//match value

TMR2L = 0;//start from 0

T2CONbits.TMR2ON = 1;

while(INTCONbits.TMR2IF==0);

TMR2L = 0;//restart after 25ms

while(INTCONbits.TMR2IF==0);

INTCONbits.TMR1IF= 0;

T2CONbits.TMR2ON = 0; // end after 50 ms delay

for timer3:

same as timer0

T3CON = 0x20; //so that prescaler is 4 and clock is Fosc/4

TMR3H = 0x85; //same as prescaler and clock are not changed

TMR3L = 0xEE; //same

T3CONbits.TMR3ON = 1;

while(INTCONbits.TMR3IF==0);

T3CONbits.TMR3ON = 0;

INTCONbits.TMR3IF= 0;

Related Questions

Navigate

• Browse (All)
• Browse E
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.