Raspberry PI CM4 3.3V output start up wierd pulses ?? (RTC loose time) - Page 1
Forum section: electronics / projects

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 22, 2025, 10:05:29 pm
content: Anyone using the 3.3V output from the CM4 to drive any external parts ?I find the power up waveform very much unusual,I think it is the main reason for my external RTC going nuts and loose its timing due to all the funny power on pulsing.my 5V input to the PI go nice and fine super stable up and down as it is supposed to.

author: thm_w
author_rank: Super Contributor
timestamp: August 23, 2025, 12:31:12 am
content: Oscilloscope screenshot would be nice.How much current are you drawing?Quotehttps://forums.raspberrypi.com/viewtopic.php?t=288840

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 23, 2025, 09:19:10 pm
content: I am super sorry about the missing picturelook heregreen is 5V input to PIyellow is 3.3V out from PIblue is one of the I2C lines i play with , not important herered is 3V capacitor for my RTC chip, the drops show supply take overi only load the 3.3 out under 100mA total

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 25, 2025, 11:01:00 am
content: feedback :it WAS indeed this wierd pulsed 3.3V from the PI CM4that confused our RTC chip, to make it loose its time,i added a linear 3.3V regulator, to make supply for the RTC, now the voltage go perfectly upand RTC timing is not erased,one could argue the RTC chip should be able to handle all sorts of funny unstable supply pulsing at power up and down,and just change over to battery usage of oscillator and clocks.but that is sadly not the case,maybe we found a weakness in the RTC chip,or maybe it is a normal problem with PI CM4 and RTC chips ??please comment on this, if you have any experiance with PI CM4 and RTC chips

author: Jeroen3
author_rank: Super Contributor
timestamp: August 25, 2025, 01:46:29 pm
content: Well no. RTC chips are not magic.If you provide them power outside of their specs they may do "undefined" things.If you provide power out of sequence it may also do weird things, eg: power on the IO pins before VddThe RTC chip will have an internal Vdd monitor to release reset, these will be above average tolerant.But these glitches are relatively slow...You typically avoid this by using a supervisor IC with a delay set during design (by you).Power supply sequencing is a real issue on large boards.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 25, 2025, 03:33:56 pm
content: i agree it is my fault alone for not measuring the 3.3V power up sequency out from a CM4 before making my own design using itthat said, and accepted..I still feel any RTC chip should be heawyly extra dumb proff in its power up and and switch over system,maybe if we picked another brand / type, we would not have seen any issues at all,we wanted to use a tiny super cap and get 5-6 month backup time,so it was specially selected for its ultra crasy low usage

author: thm_w
author_rank: Super Contributor
timestamp: August 26, 2025, 12:13:09 am
content: Thanks for update, what RTC was it?Probably doesn't matter too much as most have a minimum voltage of around 2-2.5V and it had dropped below that.Quote from: Jeroen3 on August 25, 2025, 01:46:29 pmSure but no one is expecting the raspi to do bizarre stuff like this.

author: Jeroen3
author_rank: Super Contributor
timestamp: August 26, 2025, 05:57:07 am
content: Are you sure it's the CM4 doing this and not a side effect of the PSU you are using to power it?Like plugging in the usb?

author: thm_w
author_rank: Super Contributor
timestamp: August 27, 2025, 12:00:35 am
content: Quote from: Jeroen3 on August 26, 2025, 05:57:07 amGreen trace was the 5V in