M. Amer Iqbal Qureshi
How does a timer operate?
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- Điều hướng trang này:
- Using prescaler in timer operating
- Using interrupt in timer operating
| How does a timer operate?
In practice, everything works as follows: pulses coming from quartz oscillator are once per each machine cycle directly or via pre-scaler brought to the circuit which increments number in the timer register. If one instruction (one machine cycle) lasts for four quartz oscillator periods then, by embedding quartz with the frequency of 4MHz, this number will be changed a million times per second (each microsecond). It is easy to measure short time intervals (up to 256 microseconds) in a way described above because it is the largest number that one register can contain. This obvious disadvantage may be easily overcome in several ways by using slower oscillator, registers with more bits, prescaler or interrupts. The first two solutions have some weaknesses so it is more recommended to use prescaler and/or interrupt. Using prescaler in timer operating A prescaler is an electronic device used to reduce a frequency by a pre-determined factor. Teach Yourself PIC Microcontrollers | www.electronicspk.com | 23 Meaning that in order to generate one pulse on its output, it is necessary to bring 1, 2 , 4 or more pulses to its input. One such circuit is built in the microcontroller and its division rate can be changed from within the program. It is used when it is necessary to measure longer periods of time. One prescaler is usually shared by timer and watch-dog timer, which means that it cannot be used by both of them simultaneously. Using interrupt in timer operating If the timer register consists of 8 bits, the largest number that can be written to it is 255 (for 16-bit registers it is the number 65535). If this number is exceeded, the timer will be automatically reset and counting will start from zero. This condition is called overflow. If enabled from within the program, such overflow can cause interrupt, which gives completely new possibilities. For example, the state of registers used for counting seconds, minutes or days can be changed in an interrupt routine. The whole this process (except interrupt routine) is automatically performed “in the background”, which enables main circuits of the microcontroller to perform other operations. tải về 3.06 Mb. Chia sẻ với bạn bè của bạn:
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