ADuC7124 (eval board): UART problem / GP1CON


I'm trying to use the ADuC7124 UART from the pin headers on the eval board (J1). I set up P1 as follows:

GP1CON = 0x02220011;
GP1DAT = 0x8C0C0000;

On the same port I also use SPI for another peripheral - this works perfectly.

I'm using an FTDI TTL-232R-3V3 USB to UART converter with 3.3V logic, and have connected them accordingly. My problem is that when the FTDI is connected at the pin header, and I tx data from the PC, the observed voltage level on J1 is only 1.25Vpp. If I disconnect the cable and measure at the connector to the cable going to the PC, the voltage levels are again 3.3Vpp as they should be.

The drive strength of the FTDI converter is already at 12 mA. Reducing it to 4 mA using their FT-Prog program, the voltage levels observed when connected at J1 is about 600 mVpp.

What could possibly affect this?
VDDIO is not connected to the FTDI converter as it is powered by USB power. DGND however, IS connected.
My immediate thought was that GP1CON was actually not set, but it is!

Thanks for any hints.

Kind regards,

[edited by: tk-don at 4:51 PM (GMT -5) on 24 Nov 2021]