Discussion Overview
The discussion revolves around the safety and typicality of voltage glitches observed on the 3.3V supply of a CPLD used as an IO expander. Participants explore the implications of these glitches, potential causes, and solutions related to power supply decoupling in digital circuits.
Discussion Character
- Technical explanation
- Debate/contested
- Experimental/applied
Main Points Raised
- One participant reports observing +/- 200mV glitches on the 3.3V supply when switching output pins and questions whether these are typical or safe.
- Another participant emphasizes the importance of power supply decoupling and suggests using a 0.1uF decoupling capacitor for each Vdd input pin, placed close to the pins with proper grounding techniques.
- A participant confirms using 0.1uF decoupling capacitors and questions whether the observed glitches are typical for digital outputs.
- Concerns are raised about the number of outputs switching simultaneously and the potential limitations of the CPLD regarding simultaneous switching, prompting a check of the datasheet.
- One participant speculates about the possibility of timing violations causing contention when enabling output drivers, suggesting the use of a Z-lead probe to improve measurement accuracy.
- Another participant acknowledges not using a Z-lead probe and expresses uncertainty about its effect on the observed glitches, while also considering the possibility of timing violations during bus driving.
- After improving grounding for the probe, a participant notes that the glitches are reduced to +150mV/-110mV and suspects they could be even lower with a Z-lead probe.
- A participant shares findings from Altera's website suggesting the use of 0.01uF decoupling capacitors instead of 0.1uF, indicating a potential change in their approach.
- Another participant supports the idea of using a mix of 0.1uF and 0.01uF capacitors while stressing the importance of low series inductance in connections.
Areas of Agreement / Disagreement
Participants express varying opinions on the typicality and safety of the observed glitches, with some suggesting that they may be excessive while others propose that they could be normal. There is no consensus on the best approach to mitigate the glitches, as different decoupling capacitor values are discussed.
Contextual Notes
Participants reference the importance of proper grounding techniques and the potential impact of simultaneous output switching on voltage stability. The discussion also highlights the need for careful measurement practices when assessing power supply integrity.