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I have a hardware related question.
I'm using the Witty Pi 4 Mini with a Raspberry Pi Zero.
My understanding is that the supercap on board buffers the RTC when the Pi is not in operation. So my expectation would be that on brief (at least a few minutes) power loss (while Pi is not running), the RTC continues running and will wake the Pi at the next scheduled time. However, this is not what happens. Even when unplugging the USB-C Supply for a few seconds (10 Seconds), the Pi does not wake up at the next scheduled time (in my case: next day). Also the RTC seems to stop... when powering the system manually next day to check what went wrong, the RTC time is still stuck at the time of the power cut (i.e. in my case: the previous day's date).
What's going on here? Is the supercap defective? Or am I missing an important piece of information here?
Posted : 05/03/2024 11:20 am
The supercap keeps powering the RTC when the external power supply is off, and the RTC should keep running as long as the supercap has enough voltage to run the clock (>1.2V according to PCF85063A datasheet). You may use voltage meter to measure the actual voltage on the supercap, when external power supply is disconnected.
Please keep in mind that even if the RTC is still running, Witty Pi can not wake up your Raspberry Pi without external power supply.
You can always do such a test:
- schedule a startup time (e.g. in tomorrow)
- shutdown Raspberry Pi and unplug the USB-C power supply
- wait for a while
- plug back USB-C power supply and manually turn on Raspberry Pi
- run ./wittyPi.sh and check the current time and scheduled startup time
Posted : 05/03/2024 11:40 am