Level shifter for uart data on Boron LTE

I’m currently trying to read serial data from an NXP LPC2136 processor through a Boron LTE. I have had this working when powering the Boron off of 3.3v, but then found out my connection issues were due to underpowering the device and switched to a 5v power source. Once I did this, my serial data stopped showing up, I could still write to the NXP processor, but got nothing out of it; after doing some digging, I found out the LPC2136 is a 3.3v processor and is not 5v tolerant, so I ordered some level shifters from Adafruit and wired them in today. Same result, I can write data, but no reading. This is only an issue with the Boron, I can connect a photon or Argon with the same wiring (on 3.3v) and have no issues. Any help would be extremely appreciated.

Gen 3 devices, including the Boron and Argon are not 5V tolerant! Do not send 5V signals into any GPIO or port (including serial RX), as you can permanently damage the nRF52840 MCU.

Gen 2 devices including the Photon are 5V tolerant on most pins.

So Boron has 3.3V IO pins as well, so you should not need level shifters. You may have done some damage there already.

When you added the +5V supply, did you remember you need a common ground between the NXP and the Boron in addition to TX and RX?

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@rickkas7 I didn’t send 5v to any pins on the Boron except the VUSB, but when checking through issues trying to figure out why i lost comms, i realized the RX pin had 5v on it, this was while disconnected from the other system. The LPC2136 is 3.3v only so no way I could have even inadvertently sent 5v to it from there.

@bko i do have a common ground between the boron and the lpc2136, and now between those two and the level shifter, although it sounds like that may be unnecessary.

It looks like someone had a similar issue here, although not identical since he was communicating with a xenon. Could it be an issue where a Boron is powered via 5v, whether USB or on the VUSB pin, it shorts the RX pin?