Timers In Pic Tutorial Programming C

This tutorial help you Understand about Timers In Pic Programming C

Most microcontrollers provide hardware binary counters that allow a time interval measurement or count to be carried out separately from program execution. For example, a fixed period output pulse train can be generated while the program continues with another task. The features of the timers found in the typical PIC chip are represented in Figure 1.5 , but none of those in the ‘ 877 has all the features shown.

The count register most commonly is operated by driving it from the internal instruction clock to form a timer. This signal runs at one quarter of the clock frequency; that is, one instruction takes four cycles to execute. Therefore, with a 4-MHz clock, the timer counts in microseconds (1-MHz instruction clock). The number of bits in the timer (8 or 16). determines the maximum count (256 or 65536, respectively). When the timer register overflows and returns to zero, an overflow flag bit is set. This flag can be polled (tested) to check if an overflow has occurred or an interrupt generated, to trigger the required action.

To modify the count period, the timer register can be preloaded with a given number. For example, if an 8-bit register is preloaded with the value 156, a time-out occurs after 256 -156 =100 clocks. Many timer modules allow automatic preloading each time it is restarted, in which case the required value is stored in a preload register during timer initialization.

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A prescaler typically allows the timer input frequency to be divided by 2, 4, 8, 16, 32, 64, or 128. This extends the maximum count proportionately but at the expense of timer precision. For example, the 8-bit timer driven at 1 MHz with a prescale value of 4 counts up to 256 X 4 = 1024 μ s, at 4 μ s per bit. A postscaler has a similar effect, connected at the output of the counter.

In the compare mode, a separate period register stores a value that is compared with the current count after each clock and the status flag set when they match. This is a more elegant method of modifying the time-out period, which can be used in generating a pulse width modulated (PWM) output. A typical application is to control the output power to a current load, such as a small DC motor—more on this later. In the capture mode, the timer count is captured (copied to another register) at the point in time when an external signal changes at one of the MCU pins. This can be used to measure the length of an input pulse or the period of a waveform.

The ’ 877 has three counter/timer registers. Timer0 has an 8-bit counter and 8-bit prescaler. It can be clocked from the instruction clock or an external signal applied to RA4. The prescaler can also be used to extend the watchdog timer interval (see later), in which case it is not available for use with Timer0. Timer1 has a 16-bit counter and prescaler and can be clocked internally or externally as per Timer0. It offers capture and compare modes of operation. Timer2 is another 8-bit counter but has both a prescaler and
postscaler (up to 1:16) and a compare register for period control.

Further details are provided in Interfacing PIC Microcontrollers by the author and the MCU data books. When programming in C, only a limited knowledge of timer operation is necessary, as the C functions generally take care of the details.

Timer in Pic by Programming C In Pic