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Explain Mode 0, and Mode 1 of 8254 Timer/Counter peripheral IC with help of timing diagram.
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Mode 0 (Interrupt on Terminal Count)

  • This mode id selected if (D3 D2 D1) of CWR is (0 0 0).
  • When this mode is selected OUT pin is initially low.
  • The count value is loaded. GATE pin is made high, so counting is enabled.
  • During counting, OUT pin remains low.
  • On Terminal Count (TC) the OUT pin goes high, and remains high.
  • During counting if GATE is made low, it disables counting. When GATE is made high, counting resumes.
  • Effect of Gate: When gate is low, counting is disables and when gate is high, counting Enables (Resumes).

Mode 0: Interrupt on Terminal Count

Mode 1 (Mono-stable Multi-vibrator)

  • This mode id selected if (D3 D2 D1) of CWR is (0 0 1).
  • When this mode is selected OUT pin is initially high.
  • The count value is loaded. Counting begins ONLY when a rising edge is applied to the GATE. OUT pin goes low and remains low during counting.
  • On Terminal Count (TC) the OUT pin goes high, and remains high.
  • During counting if GATE is made low, it has no effect on the Counting.
  • The GATE pin can be used as a Trigger. The Counter can be re-triggered by applying a rising edge on the GATE. This would Restart the counting, and hence re-trigger it.
  • Effect of Gate: when gate is low, no effect; when gate is high (Trigger) it tarts Counting, can also re-trigger it.

Mode 1: Monostable Multivibrator

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