Avoiding the Cargo Cult: R19 and VBACKUP
Posted: Sun Sep 09, 2018 9:59 pm
I'm designing my own version of a MicroPython board based on the PYBv1.1 for my own home ecosystem of tinkering. [And it's an instructive exercise!]
When walking through the PYBv1.1 I see the note:
"R19 prevents a silicon failure from short circuiting the backup battery."
I imagine the reasoning is: There is little current draw on a backup battery (~3ma), so the drop across R19 should be acceptable and the power lost in it should be minimal. If the STM32F405 were to develop some kind of short, this prevents our coin cell from getting hot and causing problems as it dumps all its juice into the short limited only by its own internal resistance.
This raises the questions: Is this (The STM32 shorting internally) a problem which is/has been encountered often? Is this simply proactive good practice?
I don't see this concern addressed in the STM32F4xx reference manual section on the Backup Battery Domain.
I didn't want to just adopt this idiom into my design without getting a feeling for how it came to be.
When walking through the PYBv1.1 I see the note:
"R19 prevents a silicon failure from short circuiting the backup battery."
I imagine the reasoning is: There is little current draw on a backup battery (~3ma), so the drop across R19 should be acceptable and the power lost in it should be minimal. If the STM32F405 were to develop some kind of short, this prevents our coin cell from getting hot and causing problems as it dumps all its juice into the short limited only by its own internal resistance.
This raises the questions: Is this (The STM32 shorting internally) a problem which is/has been encountered often? Is this simply proactive good practice?
I don't see this concern addressed in the STM32F4xx reference manual section on the Backup Battery Domain.
I didn't want to just adopt this idiom into my design without getting a feeling for how it came to be.