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Six Easy Pieces (Non-Synchronous Circuit Tricks)
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Author: Peter Alfke Publication Date: 04/02/2003
Introduction
Most FPGA designs are purely digital, and are usually also synchronous. Occasionally, however,
there is a need for special asynchronous circuitry: a push-button debouncer, input Schmitt trigger,
low-frequency oscillator, frequency doubler, output-voltage booster, or truly asynchronous clock
multiplexer. These seemingly trivial circuits can lead to an inordinate amount of trouble if
implemented improperly. Here are six proven designs that e in handy in special
situations.
1. SPDT Switch Debouncer
Interfacing a switch to fast CMOS logic is not trivial. Any mechanical switch bounces when it is
being closed (and even when it opens). The bounce can last for milliseconds. The safest interface
is from a double-throw switch into the Set and Reset of a CMOS latch. The circuit below uses
feedback in the I/O as a latch, and thus requires only a single pin.
Configure the output with a low drive strength to minimize the short change-over current spike.
2. Adjustable Schmitt Trigger
This circuit provides adjustable input hysteresis. The difference between upward