T3.5 vs T3.2 pull-up resistor value?

paynterf

Well-known member
In researching for my current T3.5 project, I came across a reference that indicated a 100K value for the typical GPIO pull-up/pull-down resistance. I happened to have a T3.2 handy so I set up a pin as an input with INPUT_PULLUP, and then measured the pin voltage with a 27K resistor to ground. With that setup I measured 1.679V at the output, which *I think* means the internal pull-up value is more like 27K than 100K.

Am I missing something here? Maybe the T3.2 and T3.5 values are different?

TIA,

Frank
 
I recall measuring about 33K some time ago, which is pretty close to 27K.

You can expect on-chip resistors to change quite a lot with temperature and from chip to chip.
 
Paul,

Thanks for the info; somehow I got off track and wound up looking at the wrong reference manual. Once I found your list of datasheet references, they clearly state that the pullup/pulldown resistors are a min of 20K and a max of 50K. Sorry for not RTFM :(
 
They are really just FETs with long thin channels, rather than resistors proper, so the process variation is large.
True on chip resistors would be probably 100's of times the silicon real-estate which is why they are rarely used
in CMOS.

Also this means they are rather non-linear and not suitable for use on an analog input.
 
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