MCF52235 Analog Input Pin Input impedance issue.

[LI_Dave]

I've run into a very strange issue with the AtoD inputs, or more correctly, one of the AtoD inputs.  I have a project where I am using both AN0 and AN1.  On each pin I have an external 10.0K Pull up to 3.3V and then a series filter of two 1.0K resistors with 1.0uF ceramic to ground in the middle between the pull up and the CPU. 

I am using the standard reference Coldfire lite project which does continuous conversions on all 8 inputs even though I only use AN0 and AN1.  AN0 works perfectly correctly, but I have an input impedance issue on AN1.  AN1 is behaving as though it has a pull up to VCC enabled internal to the part.  If I ground the input, the AN1 pin sources 24.9uA through my T network, introducing error in my measurements.  I plotted the input current on AN1 and the pin will source current into any input voltage below 3.3V.

Repeating the same test on AN1 I see very little current, only a 2 or 3 uA and as predicted the part sinks current if the input voltage is above mid scale and sources current if you are below the mid range.

Everything seems to point to AN1 having an internal pull up even in AtoD mode which according to the data sheet it doesn't have at all, regardless of mode.    I've tested a few boards and they all behave the same way.  AN0 is fine and AN1 has this odd pull up behavior.

Any thoughts?

Regards,

Dave

[TomE · Accepted solution]

Can you test this on a Development Board, just in case there's something wrong with your hardware? AN1 might be shorted to another GPIO pin somewhere.

Check PANPAR, just in case something wrote to it. Table 3, Note 2 says "All signals have a pull-up in GPIO mode."

The spec for the pullups is between 10uA and 130uA, so your measured current is within that range.

Can you try programming PANPAR to set AN1 to be a Digital input, and see if it now has the same pullup as AN1, and also if it still works as an ADC input, or whether that setting disconnects it?

Good luck.

Tom

[LI_Dave]

Tom,

Sometimes the obvious is not so obvious until someone points it out.  I had a resistor strap option to connect this input to UART pin U1_RXD.  Completely forgot it was still there and optioned in.   Glad I spent an hour or so graphing the voltage/current relationship of that pin!

In case anyone cares.  Here is what the V/I relationship on the pullups on this part look like:

Thanks.