I2C pullup

after 2days battle with the DS3231 RTC I want to share my thoughts

If you get this kind of error

Uncaught InternalError: Timeout on I2C Write BUSY

It's most likely because I2C pull-up resistor value is too low, it should be 10k not 4,7k
4,7k is a default pull-up value in many schematics but it's not enough for stm32, of course in many cases 4,7k would do the job but if you want to avoid hazards always pull-up 10k on I2c.

In my case, a cheap DS3231 module had soldered 4,7k resistors, funny enough it was working on a breadboard but not on my custom made PCB lol....

oh here it is, my module

values on these tiny resistors are 432 = 4,3k so it's even worse..

and here is my solution (10k resistors soldered):

1 Attachment

Nice - thanks for the post (it's a nice looking little module!).

I'd really have thought that 4k resistors would work - the STM32 shouldn't be that picky. Perhaps it was something wrong with the resistors that came on the board itself?

Hi @Gordon

I don't think so, I've tested the module dozen of times, I think 4,7k would work but anything below 4,7 is hazardous, I assume it was working on a breadboard because of the extra resistance of pins, wires etc (my guess), anyway 10k is a safe option.

Often the I2C master [here, the stm32] is strong enough to force a line to 0V through a 4k7 resistor. I bet the problem here is the RTC itself which should probably have weak output drivers unable to push the lines to zero Volts (thus, very low current drained which is a good point for a RTC in term of power consumption).

Good point about the device on the end! I hadn't considered that.

Yes, the STM32 will push out 20mA or so per pin, so should easily be able to handle 1k or less - but the RTC itself is probably built for low power consumption so won't have powerful output drivers.

I haven't thought about it that way, good point

as I found, these tiny rtc modules are designed to work with Raspberry Pi (https://goo.gl/hwqD8I) moreover they all have soldered 4.3k pull-ups, I dont think they all manufactured broken with wrong pull up value.
I suspect 4,7k is okay for stm32 but anything below (like 4,3k) is not