Intel 8253/54 - Operational Modes


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8253/54 can be operated in 6 different modes. In this chapter, we will discuss these operational modes.

Mode 0 ─ Interrupt on Terminal Count

  • It is used to generate an interrupt to the microprocessor after a certain interval.

  • Initially the output is low after the mode is set. The output remains LOW after the count value is loaded into the counter.

  • The process of decrementing the counter continues till the terminal count is reached, i.e., the count become zero and the output goes HIGH and will remain high until it reloads a new count.

  • The GATE signal is high for normal counting. When GATE goes low, counting is terminated and the current count is 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 in this mode.

  • The output remains high until the count is loaded and a trigger is applied.

Mode 2 – Rate Generator

  • The output is normally high after initialization.

  • Whenever the count becomes zero, another low pulse is generated at the output and the counter will be reloaded.

Mode 3 – Square Wave Generator

  • This mode is similar to Mode 2 except the output remains low for half of the timer period and high for the other half of the period.

Mode 4 − Software Triggered Mode

  • In this mode, the output will remain high until the timer has counted to zero, at which point the output will pulse low and then go high again.

  • The count is latched when the GATE signal goes LOW.

  • On the terminal count, the output goes low for one clock cycle then goes HIGH. This low pulse can be used as a strobe.

Mode 5 – Hardware Triggered Mode

  • This mode generates a strobe in response to an externally generated signal.

  • This mode is similar to mode 4 except that the counting is initiated by a signal at the gate input, which means it is hardware triggered instead of software triggered.

  • After it is initialized, the output goes high.

  • When the terminal count is reached, the output goes low for one clock cycle.

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