OSC_IN, OSC_OUT as inputs

lil-vince2 · ‎2010-05-18

Posted on May 18, 2010 at 11:38

swhite2 · ‎2011-05-17

Posted on May 17, 2011 at 13:51

Not sure about the STM32F101C6T8 but on some devices the PD0/PD1 pins are 5V tolerant whereas OSC_IN/OSC_OUT aren't. On other devices the pins are the same. I'd check the datasheet for your exact device to be sure.

Generally I don't recommend allowing 5V into non-5V tolerant pins but in this case it's probably ok since you have 10K pullup resistors greatly reducing any input current due to the internal protection diodes conducting (at 3.3V + 0.3V). It'd be interesting to measure the voltage on PD0/PD1. If they're 5V tolerant they should measure 5V. If not they'll be ~3.6V.

lil-vince2 · ‎2011-05-17

Posted on May 17, 2011 at 13:51

Hi,

Thanks for your answer,

It seems that they are not 5V tolerant (I/O level is not ''FT'' in pin´s description table and in the ''external clock source characteristic'' table OSC_IN input high level voltage MAX is Vdd ) but as you said, for OSC_IN the resistor should limit the current and then protect the input.

But for the OSC_OUT pin I haven´t found any indication on hardware characteristics and I´ve no idea how is made the OSC hardware.

And OSC_OUT is directly connected to the ADS1251 digital output (for which I have not found the maximum current it can source).

When the STM32 is not programmed, I measure 4V on Vdd (which is supply by a 3,3V classic linear regulator), 4,4V on OSC_IN and 4,7V on OSC_OUT...

Thanks in advance for help

swhite2 · ‎2011-05-17

Posted on May 17, 2011 at 13:51

In that case I'd put a resistor (e.g. 1K) between OSC_OUT and the ADS1251 pin. That should limit the injection current to a safe value.

Vdd measuring 4V isn't a good thing. I think the max is 3.6V.