Sanity Check; analog input pullups

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Cosford

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Hi Guys,

quick sanity check if anyone knows the answer off the top of their head rather than trawling through the datasheets.

The Analog only inputs (A10-A13) on the T3.1, do they have pullups available? Or shall I just implement them in hardware? (For reference, I intend to use them as button inputs, to save digital pins for other purposes).

Thanks in advance,

Cosford.
 
You could try pinMode INPUT_PULLUP on them and then probe them with a multimeter set to DC volts - I doubt they do, to be honest I have been subjected to electronics at the hands of a pretty good EE for quite a while now (and I wasn't completely without experience to start with or I would not have acquired this position), we've dealt with making ADC readings more accurate (and less prone to noise) in various circuits and at no point whatso-ever has he suggested (or implemented) any form of pull-up on it.

I am willing to assume, without wiring thing one, that if the pull-up resistance is less than the impedance of the signal then the pull-up voltage will overwhelm the signal easily and if it is more then it would only drag the signal 'up' proportionately with the ratio of the two impedances.

If there is true merit to the idea of using a pull-up on a signal being read by an ADC (to facilitate/augment/quieten readings) then I will be flabbergasted that I have not come across it in a professional capacity in all this time after I find where Atmel or Freescale or TI or any other 'biggish' chip manufacturer which covers ADC offering instructions to enable their built-in pull-up on their ADC channel specific hardware.
 
I didn't necessarily expect it to to be part of the Teensy Core, just wondered if the hardware were capable of it, without going through the datasheet.

For the sake of a resistor, I'll save the effort and pop one on my boards.
Cheers.=)
 
pinMode INPUT_PULLUP works and is usefull on both 3.2 and LC analog pins.
Not sure of value. >10k at least.
 
Freescale's datasheet says the internal pullup resistors can be anywhere between 20K to 50K. Freescale's spec covers a wide -40 to +85 C temperature range, and the full power supply range of 1.71V to 3.6V. They probably allow for quite a bit of tolerance for the chip manufacturing process to vary.

I and others have measured about 30K to 35K when running at room temperature with the chip powered at 3.3V.
 
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