📖 9. Programmable Interval Timer/Counter: 8253

📖 From An Introduction To Microprocessor 8085 • 146 questions available

About 9. Programmable Interval Timer/Counter: 8253

Many real-world applications require precise timing—like generating a sound frequency, measuring the speed of a motor, or creating a real-time clock. The 8253 is a support chip that is like a sophisticated "stopwatch" or "metronome" for the microprocessor. It contains three independent 16-bit counters that the CPU can program to count down from a number to zero, generating an output signal when they finish. This takes the burden of counting and timing off the main CPU.

How It Works: Each of the three counters can be programmed in six different "modes." Mode 2 (Rate Generator) is used to create a continuous square wave (like a clock signal for a timer), while Mode 3 (Square Wave Generator) produces a symmetrical square wave. To use it, the CPU writes a Control Word to set the mode and then loads a 16-bit "count" value into the counter. The counter then starts counting down at a rate determined by an input clock signal (like 2MHz). When it hits zero, it toggles its output pin, creating a timing event.

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🔄 Last updated: 2026-07-30