The back serial port works at 3.3V.
The RX channel can be set in the CPU to GPIO mode or to alternative function (serial RX). By default, it's set to GPIO, as a keyboard line uses this (pin 26 of GPIO port D) HW wiki.
To enable the alternative function (RX), and disable the keyboard use, we have to write '1' to the proper position (so, the 32-bit value 2^26=0x04000000) of the PDFUNS register (0x10010344). I took this from the jz4725 PM.
We can set the value using the poke command running it as:
poke -32 0x10010344 0x04000000