Explain 8254 programmable interval timer in detail.
8254 Programmable interval timer : In Microprocessor based system, we come across two important modes i.e timer -> to provide delay and counter -> to count incoming pulses. Presently the way we implement timer/counter in 8086 based system as studied in earlier has following draw backs :
- Delay : The 8086 can provide delays of any value, but it uses software to implement the delay. The instruction are arranged to waste time. The main disadvantage of this scheme is 8086 is executing some instructions. It means that it is busy in doing work.
- Counter : The 8086 can count number of pulses arriving at port. To implement this 8086 goes on checking port, if is active it increments counter by 1 and again goes on checking port. The same disadvantage, 8086 will have to execute instructions.
In large systems, where 8086 wastage time is critical, a separate timer IC 8254 ca be used. The 8254 consists of 3 identical 16 bit counters. These counters can work as counter or can provide accurate time delays. To operate as counter, a 16 bit count is loaded and the desired mode of operation is selected. The counter will work independently and generate the desired output. Now the 8086 job is to initialize and load counters.
Features of Programmable Interval Timer :
- Three independent 16 bit down counters.
- Counters can be programmed in 6 different programmable counter modes.
- Counting facility in both binary or BCD number system.
- Compatible with Intel and other microprocessors.
- Single +5v supply.
- It is completely TTL compatible.
- 24 pin dual in line package.
- Operating frequency range; for 8253-DC to 2.6 MHz; for 8254-DC to 10MHz.