What are glitches?


While trying to measure the speed of the outputs of a GPIO expander, some very short anomalies appear. In the example below, two of them last 4 ns each.

I can get rid of those anomalies by applying a glitch filter and setting the duration at eg. 8 ns.

  • However, what are those glitches?

The critical question here is:

  • Are they coming from the circuit I am analysing, or are they generated by the Saleae logic analyser hardware, firmware or software?

The first case may invite me to consider another IC. As the glitches are appearing randomly, using an oscilloscope to record a long time and spot the exact period when they appear may be difficult.

  • What recommendation, if any, to prevent those glitches from appearing?

In an other configuration with channel 5 and 6 used for I2C, I have noticed the signal from channels 4 and 7 are more prone to raise glitches.

  • Should shielded cables be used instead for the signals?

Thank you!

Configuration: Saleae 8 Pro, Logic 2.3.30 running on Debian

@rei_vilo Our Logic Pro 8 is implemented with comparators instead of standard CMOS buffers. That makes the input-low and input-high threshold voltages very close to each other (i.e. very minimal hysteresis). Therefore, glitches are most likely appearing because of rise/fall times on edges being slow to cross the comparator threshold.

The specific voltage thresholds are described below for each of our models:

I2C is the most common culprit due to the rise/fall times being affected by pull up resistors. Unfortunately, shielded cables would likely not help since the rise/fall times are typically the cause.

Given that our comparators have minimal built-in hysteresis, using the glitch filter would be the best way to prevent glitches from confusing an analyzer from decoding data properly.

Thank you for the answer. I’ll use the filters.

Note that grounding can make a difference and noisy power supplies in the device under test can be an issue.

Sometimes it is helpful to look at the signal as an analog signal. That won’t find 4ns glitches, but it will tell you about the quality of the signal and whether the voltage levels are correct. You can capture the same input as both an analog signal and as a digital signal.