Reduce microcontroller clock speed so as to reduce noise susceptibility? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: October 13, 2017, 08:13:49 pm
content: Hello,Do you believe that an 8 bit PIC microcontroller that runs with a faster clock rate would be more susceptible to noise problems than one running at a lower clock speed?We are thinking that at a higher clock rate, there are more clock edges per second, and therefore, more chances of a narrow noise pulse corrupting an instruction, and crashing the software?...simply because there are more clock edges per second to be corrupted.Is it true that all microcontrollers should be run at the lowest possible clock rate, in order to reduce the chances of noise susceptibility?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 13, 2017, 08:42:47 pm
content: Quite often a µC will do it job and wait for the rest of the time in sleep mode with much of the clock turned off. So it is the same number of active clock cycles and thus the argument shown does not apply.Some µC have a HW lower limit because they use dynamic circuits - here at the lower limit they can be more sensitive.Usually it is a good Idea to run the µC not at it's maximum speed, as reserve will be lower there. But no need to go very low with the clock. It can help a little with EMI.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 13, 2017, 08:51:20 pm
content: Depends entirely on the noise, but you should really be dealing with the noise properly in hardware instead of trying to bodge around it.There could be an argument that in the presence of random,short noise spikes, running fast reduces the potential window of a susceptibility and might reduce the probability of a problem.

author: ataradov
author_rank: Super Contributor
timestamp: October 13, 2017, 09:53:23 pm
content: If it is external noise you are worried about, then what difference does the frequency make? If there is a short external spike, it will do something regardless of the internal clock speed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2017, 10:21:44 pm
content: If a crystal is used, the crystal impedance is higher at lower frequencies, therefore it is approximately constant susceptibility relative to a capacitively induced noise source.Tim

author: Bruce Abbott
author_rank: Frequent Contributor
timestamp: October 13, 2017, 11:15:28 pm
content: Quote from: treez on October 13, 2017, 08:13:49 pmYes, but significantly only when when running close to the upper limit (where voltage and timing tolerances are smaller).QuoteThe chances of a temporary glitch affecting operation at a critical time (eg. when data is sampled at the end of a bus cycle) is lessifthe noise pulses are infrequent and short relative to the clock. But it doesn't reduce the effect of permanent corruptions (eg. bits flipped in RAM) at other times. If the MCU has a particular job to do that takes a certain number of clock cycles, and sleeps at other times, then there isnoadvantage in lowering clock speed because the lower speed just exposes it to noise for a longer time.QuoteNo. Noise should be eliminatedbeforeit gets into the chip. And just reducing glitches isn't enough. The device should be designed tonevermalfunction - and recover gracefully in the (very unlikely) event it does. If actual glitching is detected thenyou have a problemthat needs to be addressed. Lowering clock speed until the glitches (hopefully) disappear is not a solution.An MCU with internal ROM and RAM should be pretty much immune to outside interference, provided that the power supply is stable and pin voltages are kept within spec. This seems to be true in practice, since MCU manufacturers are not recommending lower clock speeds for noisy environments. If it was a problem then they would, because reliable operation is a top priority for embedded systems (much more so than eg. a gaming PC, which is run at the ragged edge to get best possible performance).