Datasheet

PICkit™ 3 Debug Express

Line 5 sets bit 7, TMR0ON, of the T0CON register to turn on the timer so it begins

incrementing. Using one of the SFR unions to access bits, like T0CONbits.TMR0ON,

can change bits without affecting the other bits.

In the while(1) loop, the LED_Display global variable is updated to rotate the ‘1’ bit

based on the Direction variable value, and then LATD is updated.

The do{…}while() loop then polls the switch looking for a switch press while it waits

for the timer to overflow and set the TMR0IF flag bit. This is a simplistic example of how

using a timer allows the microcontroller to do work while waiting on a time delay,

instead of wasting processing time counting cycles in an instruction loop.

Once the switch it pressed, the Direction variable value is reversed. Follow the

if – else if logic flow in the do{…}while() loop to see how once the switch is

pressed, the direction is reversed only once until it is released and pressed again.

Lastly, once Timer0 overflows and sets the TMR0IF flag the do{…}while() loop is

exited. TMR0IF is then cleared in the software program so the next timer overflow can

be detected.

3.5.3 Build and Program the Lesson 5 Code

Build and program the Lesson 5 project. The LEDs will rotate, and pressing the Demo

Board button will reverse them.

3.5.4 Assigning the Timer0 Prescaler

Now we’ll go back to that commented-out line of code in the Timer0 setup statements.

Comment out the T0CON assignment statement, and un-comment the other statement

so the Timer0 setup code looks like this:

INTCONbits.TMR0IF = 0;

//T0CON = 0b00001000;

T0CON = 0b00000001;

TMR0H = 0;

TMR0L = 0;

T0CONbits.TMR0ON = 1;

Take a look at what this changes:

PSA = 0

The prescaler is now assigned to Timer0, and the values of T0PSx will set the

prescaler clock divider ratio.

T0PS2:T0PS0 = 001

This value sets the prescale value to 1:4, which means Timer0 will now increment

once every 4 instruction cycles instead of once every instruction cycle. It now takes

4 times as long for it to count up to 65536 – just over 1 second!

Rebuild and reprogram the Lesson 5 project with change in the source code. The LEDs

will rotate more slowly, 4 times slower to be exact, than before.

Note: Be aware that some cases using an SFR union to access a bit may affect

other bits. What actually happens during this instruction execution is the

operation is called Read-Modify-Write. If a bit reads a different value than

what it was last set as, this operation may affect register bits other than the

intended one. Check the SFR bit definitions carefully. In the case of

T0CON, all bits are Read/Write and all are set by software only; the hard-

ware will not affect any bit setting.