Design Ideas September 28, 1995

If your design requires a precise monovibrator, you don't want to use the circuit in Fig 1. However, if sharp timing isn't crucial and you can afford just three spare PLD pins, this circuit is an alternative to using yet another 555. The circuit comprises asynchronous logic, an open-collector-like driver that controls the discharge of an external capacitor, and a feedback path for sensing capacitor voltage. When the capacitor charges beyond the threshold voltage, the circuit echoes a low level applied to the trigger input to the pulse output and also initiates capacitor discharge. The trigger input going high starts a charging phase, and the circuit ignores any trigger pulses that follow. Therefore, if the trigger input oscillates, the circuit produces a pulse whenever the delay time elapses.

Delay time depends on the values of the capacitor and resistor (some PLDs have internal pullup resistors). If the capacitor's value is too large or the triggering pulse is too short, then discharge is incomplete. Consequently, the delay time decreases because of a smaller voltage swing. Noise can easily influence an input signal that's near the threshold voltage, so period stability is generally poor.

The PLD equations in Listing 1 for a g16v8 are one implementation of this monovibrator with a fourth, and purely optional, gate input that enables or disables output pulses. You can also download the PLD equations from ( EDN BBS /DI_SIG#1770).