Necessity of Programmable Interval Timer
The 8253/54 solves one of most common problem in any microcomputer system, the generation of accurate time delays under software control. Instead of setting up timing loops in system software, the programmer configures the 8253/54 to match his requirements, initializes one of the counters of the 8253/54 with the desired quantity, then upon command the 8253/54 will count out the delay and interrupt the CPU when it has completed its tasks. It is easy to see that the software overhead is minimum and that multiple delays can be easily be maintained by assignment of priority levels.Also there is one more important problem. If the processor is busy in time delays using software control, we can't use the processor for performing any other action without the help of Interrupts. It is tedious job for a programmer. Also in the timing loops processor looses important time. So it is better to go for an Programmable Interval Timer.
The 8253/54 includes three identical 16 bit counters that can operate independently. To operate a counter, a 16-bit count is loaded in its register and, on command, it begins to decrement the count until it reaches 0. At the end of the count, it generates a pulse that can be used to interrupt the CPU. The counter can count either in binary or BCD. In addition, a count can be read by the CPU while the counter is decrementing. The 8254 is a superset of 8253. The functioning of these two ICs are almost similar along with the pin configuration. Only the differences are :
| 8253 | 8254 |
|---|---|
| Operating frequency 0 - 2.6 MHz. | Operating frequency 0 - 10 MHz. |
| Uses N-MOS technology. | Uses H-MOS technology. |
| Read-Back command not available. | Read-Back command available. |
| Reads and writes of the same counter can not be interleaved. | Reads and writes of the same counter can be interleaved |
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