开放drain UART

Hi,

I would like the DA14580 to communicate in UART with another chip that uses a lower I/O voltage (0-2V).
The DA14580's RX works fine at those voltages, but in the boost configuration I'm using, the TX's voltage goes all the way up to 2.7V.
As you can imagine, that causes a fair number of issues.

Typically I would solve that by configuring the DA14580's output to open drain mode, and add a pull-up resistor (which are luckily included in the other chip, and sw-controlled).
The thing is, I can't find how to configure the DA14580's UART output to open drain mode.
It does it by default for I2C, but I don't see the relevant register for UART.

Could you please point me in the right direction?

Thanks,
Jeremy