Xbee DOUT low voltage not low enough when using UART


I am attempting to connect an MSP430 with an MSP432 wirelessly by sending UART data through an Xbee. When the receiver Xbee’s DOUT line is not plugged into anything, UART is seen as expected. The problem is that when i connect the DOUT to the 432’s RX input, the “low” voltage level of DOUT jumps from 0 to around 2.5V. The “high” voltage of DOUT remains at 3.3V. So the UART data is seen going between 2.5V and 3.3V. This leads the 432 to not be able to read the UART data.

Does anybody know the cause for this problem or if there is any way to fix it?


The problem, at some level, is one of current flow; either too much entering the signal node from an unanticipated source, or not enough leaving it through the intended channel.

If a person inadvertently configured the '432s input as an output and set it high for example, it would disagree with the xbee output when the latter tried to pull the line low, and the voltage appearing on the line would end up somewhere in between valid logic levels based on the respective current limiting/protection circuitry on the I/O pins of the two devices. Or maybe there’s a ground lead somewhere that isn’t connected. Or maybe a person’s inadvertently changing something up in the process of making the measurements, and the problem actually isn’t what it seems to be…

It might be illustrative to toss a resistor in line between the two, and use it to figure out how much current is flowing in which direction when you change the logic states.

Hi @Lassley, what pinmux settings are you using on the MSP432 for this pin? It seems like it’s pull-up setting is configured too strongly… (Can we assume the XBee and MSP432 share the same GND?)


Thanks for the responses. I am using P1.2 with SEL1=0 and SEL0=1. This sets the pin to have the function UCA0RXD, the receieve for the communication module I am using. Would there be a way to change the pull up setting on this pin?

The problem does appear to be with current. When I power the Xbee with a supply that can provide more current, the problem goes away.

Still, I would like to be able to power it off the original supply if possible.

Thanks @Lassley, couple more questions, which MSP432 model (i’d like to lookup the pin settings)? What did you set for DIR/IE bits? Share GND between the MSP432 and XBee right?


I am using the MSP432p401r. I’m actually using a dev board for it, the red MSP-EXP432p401r.

I currently don’t set DIR or IE. The datasheet says DIR is don’t care for this selection of SEL. Nothing is said about IE, but I can try setting it to 0.

Yep, shared GND between them.

Thanks @Lassley, correct, on this part DIR is ignored and set elsewhere thru the eUSCI_A0 module.

What settings did you specify in XCTU for the XBee Module DOUT pins? (this screen shot is of a S2C Module)



I have the defaults for that, exactly what you showed in the pic.


Something must have been wrong with the 432. I switched it out for another and everything is working as intended.

Thanks for your time anyways