I need to connect my K64 to a Telit DE910 module (via a NimbeLink or Janus baseboard) and it has a 1.8V max on some of the IO lines. I am taking care of the level conversion for the TTL signals with external components, but was hoping to reduce part count and utilize the processor's open drain configuration for some of the IO lines.
The specifications call for an open collector driver and that the module has an internal pullup resistor.
My question is what exactly the configuration is within the K64 when configured as Open Drain. The data sheet indicates that it must be pulled up externally to VCC (3.3V in my case) and the reference manual doesn't really explain in detail the configuration. However, through testing I have found that the output seems to be internally pulled up (seemingly a weak pullup) to VCC. Can anyone confirm this? Do we know what resistor value is being used or if the internal pullup can be disabled completely? Ideally I want a classic open drain driver without the pullup so I don't have to worry about exceeding the module's Vmax.
I have connected an external pullup to a 1.5V battery just for testing and found that with a 10K resistor the signal line reads over the battery voltage by ~0.15V and with a 1K it is about 0.5V over the battery voltage. This was done on a freedom board.
Using processor expert there doesn't seem to be a way to disable the internal pullup. Any help or insight would be much appreciated.