Circuit : Andy Collinson
Email :

Description
Faulting any high speed or logic circuit is not easy. These two circuits can be used to monitor an input or output or even the power supply rails. Any short duration spike, transient or glitch will be recorded and the LED will be permanently lit. As CMOS IC's work down to 3 Volts they can also monitor GPIO pins of a Raspberry Pi, Banana PI, Orange Pi or any other 3.3 Volt system.

The positive glitch detector is simply two NOR gates from a CMOS 4001 quad two input NOR gate. The gates are arranged as a SR (set reset) flip flop. Power is taken from the logic circuit to be monitored, or alternatively can be run from a power supply. If an external power supply is used, then its DC voltage must match that of the system under test and grounds need to be commoned.

On connection, press the reset switch and the LED (if lit) will go out. The probe is then ready to monitor any positive glitches. Any positive glitch will set the SR flip-flop and output will be latched high and the LED will light. The time taken is the transition time of the CMOS IC itself and from the CD4001 data-sheet this is typically 200 ns from a 5 Volt supply for the input to toggle low to high. Care must be taken so that the probe lead is as short as possible, as a long lead will add capacitance and slow down the response time.

The probe can be placed even on a ground line, or an output that is normally low. Any positive spike will be registered and the LED will light until reset.

The negative glitch detector is very similar and uses one extra gate as an inverter. The probe can be connected to an output or input pin that is normally held positive, or even the positive power supply bus can be monitored. Any negative spike, or transient will trigger the SR flip flop and the output LED will illuminate until reset.

All unused input pins should be connected to ground.