Intel 8253/54 Operational Modes - Microprocessor

Explain the operational modes of 8253/54?

8253/54 consists of 6 different modes which perform specific operations. In this chapter, we will discuss these operational modes.

Mode 0 ─ Interrupt on Terminal Count

  • It helps in generating an interrupt to the microprocessor after a specific interval.
  • Initially the output becomes low after the mode is set. The output becomes LOW when the count value is loaded into the counter.
  • The output becomes low as the loaded count value decreases per every cycle. This process continues till the terminal count is reached to zero and the output goes HIGH and is considered as an interrupt. It will remain high until a new count is reloaded.
  • The GATE signal would be high for normal counting. When GATE goes low, counting gets terminated and the current count will be latched till the GATE goes high again.

Mode 1 – Programmable One Shot

  • It can be used as a mono stable multi-vibrator.
  • The gate input is used as a trigger input and the output becomes high in this mode.
  • The output remains high at the end of the count that is loaded and a trigger is applied.

Mode 2 – Rate Generator

  • The output remains normally high after initialization.
  • Whenever the count comes to zero, generation of low pulse occurs at the output and reloading of the counter will take place.

Mode 3 – Square Wave Generator

  • This mode acts similar to that of mode2 except the output remains low for half of the timer period and other half of the period it should be high.

Mode 4 − Software Triggered Mode

  • In this mode, the output would remain high until the timer has to be counted to zero, at certain point the output would pulse low and then would go high again.
  • The count will become latched when the GATE signal goes LOW.
  • At the end of the count, the output will go low for one clock cycle then it would go HIGH. This low pulse can be used as a strobe.

Mode 5 – Hardware Triggered Mode

  • This mode produces a strobe with response to an externally generated signal.
  • This mode acts similar to that of mode 4 except that the counting was initiated by a signal at the gate input, which implies that it is hardware triggered instead of software triggered.
  • After it gets initialized, the output goes high.
  • When the count was reached at the end, the output would go low for one clock cycle.

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