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General Purpose Input/Output Labs
3.3.6
3.3.6.1
Lab 3: Simple Delays Using Timer0
NEW REGISTERS USED IN THIS LAB
To configure the peripherals used in this lab, the following registers are used:
1. Timer0 Module Register: TMR0
- Holds a count value of the number of selected edge transition of a clock
source.
2. OPTION Register: OPTION_REG (Register 5-1 in Section 5 of the PIC16F690
Data Sheet).
- Selects clock source used to increment TMR0 result register.
- Selects clock source edge transition to increment TMR0.
3. Software configurable prescaler to determine the number of clock source edge
transitions before incrementing TMR0 register value.
4. Interrupt Control Register: INTCON (Register 2-3 in Section 2 of the PIC16F690
Data Sheet).
- Contains a flag that when 1, indicates a TMR0 register overflow has occurred.
3.3.6.2
OVERVIEW
To implement a more accurate delay, the Timer0 peripheral can be used. Timer0 is an
8-bit timer/counter that uses a clock source to increment an 8-bit register called TMR0.
Since this register is 8 bits, it can increment up 2 8 = 256 times or 0 10 - 255 10
( 00000000 2 - 11111111 2 ) inclusive then rollover or overflow back to ‘ 0 ’. Whenever
TMR0 overflows, a Timer0 Overflow Flag (T0IF) in the OPTION register is set to ‘ 1 ’.
This register also features a prescaler that determines how many clock source cycles
it takes to increment TMR0 by ‘ 1 ’. In this way, simply by tracking the T0IF, very accurate
delays can be implemented. In this lab, the TMR0 register is configured to increment
on the low-to-high transition of an available internal instruction clock on the PIC16F690.
This internal instruction clock runs at the rate of the internal oscillator frequency F OSC
divided by 4. Therefore, when the PIC16F690 is configured to operate using the inter-
nal 4 MHz oscillator, this internal instruction clock runs at a rate of
F OSC /4 = 4MHz/4 = 1MHz. This is a period of 1/1MHz = 1 μ S. If it is known that TMR0
increments every 1 μ S, and it takes 256 internal instruction clock cycles to cause a
TMR0 overflow (i.e., 0-255 inclusive), then Equation 3-1 can be derived:
EQUATION 3-1:
TMR0 OVERFLOW PERIOD USING FOSC/4
TMR0 Overflow Period = (4/F OSC ) x 256 = 1 μ Second x 256 = 256 μ Seconds
As mentioned, Timer0 also features a prescaler that can be configured to increment the
value in TMR0 every 2, 4, 8, 16, 32, 64, 128, or 256 clock source transitions. Therefore,
EQUATION 3-2:
TMR0 OVERFLOW PERIOD WHEN INCLUDING THE
PRESCALER
TMR0 Overflow Period = (4/F OSC ) x 256 x prescaler
Using a 1:32 prescaler setting as an example and a 4 MHz internal oscillator
TMR0 Overflow Period = 1 μ S x 256 x 32 = 8.192mS
? 2009 Microchip Technology Inc.
DS41369A-page 29
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