Question

Question 4 (20 marks) (a) A Timer 2 block diagram is shown in Fig.7. Timer 2 is used to generate a delay of 50 us. Determine the value of the PR2 if the TMR2 is 0, the prescaler and the postscaler of 1:1 are selected. Assume that the crystal clock is running at the frequency of 20 MHz. Ignore the overhead due to instructions in the calculation. [6 marks] (ii) Explain how the microcontroller knows the delay of 50 us has reached. [4 marks] 1:1 to 1:16 Postscaler Set TMR2IF TZOUTPS3:T2OUTPSO T2CKPS1:T2CKPSO ? TMR2 Output (to PWM or MSSP) TMR2 PR2 Match Fosc/4 1:1, 1:4, 1:16 Prescaler TMR2 A Comparator PR2 Internal Data Bus - Fig. 7

Answer 1

Answer:- Since pre-scalar and post-scalar both are 1:1, thus we can use 20/4 MHz = 5 MHz clock directly for the timer. The time period for the timer clock is-

$T=\frac{1}{5}\ \mu s$

Thus the number of count required is-

50 n = 1/5 = 250

Hence the value to be loaded in PR2 is 250 with TMR2 = 0.

When the TMR2 register count reaches to 250, it gets matched with PR2 register value. At this time, timer2 interrupt flag gets set i.e. TMR2IF is high. This interrupt causes micro-controller to know that 50 us delay has reached.