For example, if you preload the TMR0 register with the value 206, the Timer0 overflow would occur after 50 ?s.īesides, an eight bit programmable divider, known as prescaler, is also available to measure a longer time period. By preloading the TMR0 register with a suitable value, a smaller timer interval could be selected, with time out indicated by the timer interrupt. That means, the Timer0 will take 256 ?s to count from 0 to 0. The counter would then be clocked every 1 ?s exactly. Suppose, if you are running a PIC with a 4 MHz clock, the instruction clock will be 1 MHz (1 instruction cycle = 4 clock cycles, for PIC). The TOIF bit must be cleared by the interrupt service routine so that the timer interrupt can take place again. This interrupt would be the indication of the time out and will occur on the every overflow of the TMR0 register. The Timer0 interrupt is enabled by setting the T0IE bit (Timer0 Interrupt Enable) of the INTCON register along with the Global Interrupt Enable (GIE) bit. The T0IF bit set can trigger an interrupt (known as Timer0 Interrupt), if enabled. This register overflow is recorded by the T0IF (Timer0 Interrupt Flag) bit of the INTCON (Interrupt Control) register by being set to 1. When it reaches its maximum value, FF, and is incremented further, it rolls over to 00. As an 8-bit register, TMR0 can count from 00 to FF (255). In timer mode, the TMR0 register increments every instruction cycle. Timer mode is selected by clearing the TOCS bit (OPTION register, bit 5). We will discuss the two modes of Timer0 separately.īlock diagram of Timer0 Module (Source: Microchip) The Timer0 can also count external pulses arriving at the RA2 pin (11), which has the alternate name T0CKI (Timer Zero Clock Input). While counting the external pulses, it is said that it operates as a counter. When the Timer0 module is driven by the processor’s instruction clock, it is said to operate as a timer because it increments at a constant rate (defined by the processor clock) and if you know the number of pulses counted, you can derive how much time has elapsed. If you write a value to it, the counter will start incrementing from there. The Timer0 module in a PIC16F688 microcontroller is an 8-bit synchronous counter that stores the value for the counter in the special function register called TMR0. This register can be read or written at any time by software. It is simply an independent binary counter that can be configured to count instruction cycles or to count external clock pulses. The fundamental concept of a timer/counter module is discussed in Timers and Counters. Today’s lab lesson will explore the functionality of the Timer0 module. A PIC16F688 microcontroller has two built-in hardware timer modules: Timer0 and Timer1. Since it runs independently, it can count pulses concurrently with the main program execution.
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The main component of a timer module is a free running binary counter that increments for each incoming pulse. They can be used to perform a variety of time precision functions, such as generating events at specific times, measuring the duration of an event, keeping date and time record, counting events, etc. PIC microcontrollers are equipped with one or more precision timing systems known as Timers.