I'm using a DSP56F807 for years now. We are using the GPIO5...GPIO7 as input for safety relevant signals in one application.
Here we have 30kOhms in series with each port to guarantee low currents under every condition.
Looking at the data sheet this should be no problem with pullup/pulldown disabled. Up to now we never had any problems.
Now we found out that the behaviour at lower temperatures is very strange.
The input is driven by a 74HC1G14. At high signal everything is fine. But if the output of the HC1G14 is low, the level at the DSP stays at 1.6V for low temperatures (below 0°C). If the Pins are connected directly there the level is ok. It cannot be the internal pullup. If I activate it the level raises up to 2.4V which can be explained by the Input current low that is -210...-50µA.
I do not understand where this level comes from. For me it seems to be a leakage current that is not defined in the data sheet (without pullup/pulldown it should not exceed 1µA). Looking at the user manual of the DSP (figre 7-4 of Rev. 8) doesn't help. Here it seems that the pullup is activated by PMOS. If the figúre is right there shouldn't be any other source if the GPIO is configured as an input.
Has anyone an idea?