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2.7 Free-run timer TMR0 Timers are usually most complicated parts of a microcontroller, so it is necessary to set aside more time for their explaining. With their application it is possible to create relations between a real dimension such as "time" and a variable which represents status of a timer within a microcontroller. Physically, timer is a register whose value is continually increasing to 255, and then it starts all over again: 0, 1, 2, 3, 4...255....0,1, 2, 3......etc.
This incrementing is done in the background of everything a microcontroller does. It is up to programmer to "think up a way" how he will take advantage of this characteristic for his needs. One of the ways is increasing some variable on each timer overflow. If we know how much time a timer needs to make one complete round, then multiplying the value of a variable by that time will yield the total amount of elapsed time.
After each count up to 255, timer resets its value to zero and starts with a new cycle of counting to 255. During each transition from 255 to zero, T0IF bit in INTCOM register is set. If interrupts are allowed to occur, this can be taken advantage of in generating interrupts and in processing interrupt routine. It is up to programmer to reset T0IF bit in interrupt routine, so that new interrupt, or new overflow could be detected. Beside the internal oscillator clock, timer status can also be increased by the external clock on RA4/TOCKI pin. Choosing one of these two options is done in OPTION register through T0CS bit. If this option of external clock was selected, it would be possible to define the edge of a signal (rising or falling), on which timer would increase its value.
In practice, one of the typical example that is solved via external clock and a timer is counting full turns of an
axis of some production machine, like transformer winder for instance. Let's wind four metal screws on the
axis of a winder. These four screws will represent metal convexity. Let's place now the inductive sensor at a distance of 5mm from the head of a screw. Inductive sensor will generate the falling signal every time the head of the screw is parallel with sensor head. Each signal will represent one fourth of a full turn, and the sum of all full turns will be found in TMR0 timer. Program can easily read this data from the timer through a data bus.
Prescaler can be assigned either timer TMR0 or a watchdog. Watchdog is a mechanism which microcontroller uses to defend itself against programs getting stuck. As with any other electrical circuit, so with a microcontroller too can occur failure, or some work impairment. Unfortunately, microcontroller also has
program where problems can occur as well. When this happens, microcontroller will stop working and will remain in that state until someone resets it. Because of this, watchdog mechanism has been introduced. After a certain period of time, watchdog resets the microcontroller (microcontroller in fact resets itself). Watchdog works on a simple principle: if timer overflow occurs, microcontroller is reset, and it starts executing a program all over again. In this way, reset will occur in case of both correct and incorrect functioning. Next step is preventing reset in case of correct functioning, which is done by writing zero in WDT register (instruction CLRWDT) every time it nears its overflow. Thus program will prevent a reset as long as it's executing correctly. Once it gets stuck, zero will
not be written, overflow of WDT timer and a reset will occur which will bring the microcontroller back to correct functioning again.
OPTION Control Register
Bit 0:2 PS0, PS1, PS2 (Prescaler Rate Select bit)
bit 3 PSA (Prescaler Assignment bit)
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