Microprocessor-based dual timer features four outputs

Based on Freescale Semiconductor's MCC908QY 8-bit flash-memory microcomputer, the circuit in Figure 1 provides a low-cost, general-purpose dual timer that offers an alternative to one-shot circuits. You can modify the assembly-language software to meet specific applications. The circuit uses microprocessor IC₁'s internal 12.8-MHz clock oscillator. Internal division by four yields 3.2 MHz, which further divides by 64 with a timer prescaler to produce 50 kHz. A timer modulo counter divides by 50,000 to produce a 1-Hz timebase that generates a once-per-second real-time interrupt and master timing interval.

Two groups of four switches, S₃ through S₆ and S₇ through S₁₀, set time intervals t₁ and t₂, respectively, in increments of 1 to 16 sec. Although the figure shows individual DIP switches, you can also use hexadecimal-encoded rotary switches to set the time intervals. For demonstration purposes, LEDs D₁ through D₄ show the outputs' states during the timing cycle. Normally open pushbutton switches S₁ and S₂ start and stop the timers' operating cycles. The start function initiates the main timer and operates only when the timer stops. After activation, output Q₁ goes to logic one during interval t₁ (Figure 2). Output Q₂ complements Q₁ and remains at logic one until the next cycle starts. On Q₁'s trailing edge, output Q₃ goes to logic one for time interval t₂. After t₂ elapses, output Q₄ goes to logic one and remains there until the next cycle starts. You can use the stop switch, S₂, at any time to terminate a cycle and reset all four outputs to logic one.

Check out our Best of Design Ideas section!