Question

The Timer 0 module of a 16F873A is to be used to generate a regular interrupt. The interrupt must occur every 5 ms. Crystal oscillators are available at the following frequencies: 1.0 MHz, 1.8432 MHz, 2.0 MHz, 2.457 MHz, 3.276 MHz, 3.579 MHz and 4.0 MHz. Recommend one crystal, and indicate how the Option register should be set in order to achieve this objective

Answer 1

Answer:

For generating an interrupt every 5ms we should choose 3.276MHz crystal (actually 3.2768 MHz is better)

Now some explanation:

Recall the block diagram of Timer 0 of PIC16F873 from its datasheet. The signals relevant to solving this question are circled in red.

( A note to Chegg quality admin: Please note that using diagrams from semiconductor datasheet for educational purposes is not considered plagiarism. It is encouraged by the manufacturers actually. Thanks!)

To generate an interrupt every 5ms (i.e. at 200Hz), we need to input a clock signal to timer 0 which is integral multiple of 200Hz.

In this case if we select Fosc (the crystal frequency) 3.276 MHz, then CLKOUT = 0.819MHz

Then we set option bit T0CS to 0 so that CLKOUT signals is selected by the multiplexer and it reaches the prescalar.

We also set option bit PSA bit to 0 so that prescalar is assigned to timer 0 and not to watchdog timer.

We also need to select a prescalar of 256, for which the option bits PS2, PS1 and PS0 should be 111.

As the option register format is

Therefore its value to be set to 0 0 0 0 0 1 1 1 or 07 Hex

Thus the clock entering the 8-bit timer0 is 0.819MHz / 256 = 3199.22Hz. This corresponds to a time period of 1/3199.22 = 0.0003125 seconds.

As we need the interrupt to occur every 5ms, we need to (.005/.0003125) ticks of 3199.22Hz clock, that is 16 ticks.

Recall that timer 0 generates an interrupt when it overflows from max. count of 255 to 0, therefore we need to initialize it to a value of 256 - 16 = 240. This needs to be done every time it overflows. This will ensure that we get a continuous stream of interrupts every 5ms.

Hope this helps.