2 UART Connections to MCU using the Same RXD and TXD Lines

Question

I need to connect a GNSS module's RXD and TXD lines to my MCU. But the problem is I don't have any available UART connections on my MCU. There are 3 UART connections, one is used by flash, one is used for firmware uploads, and the last is connected to a LTE module.

Because I seldom update the firmware, this UART connection is somewhat wasted and I'd like to use it to connect to a GNSS module. Is there a way that I could use an array of transistors to only connect the RXD/TXD lines to firmware upload circuit when it is needed? Like when Vdd is detected at the micro-usb port of the MCU. The rest of the time the UART lines would be connected to the GNSS module.

Note that unless we are performing a firmware update, a battery is being used so any excessive power consumption is detrimental to the overall design. Below is the circuit used to upload firmware to the ESP through micro-USB which is then bridged to UART connection.

Answer 1

Use resistors to connect the GNSS module

When you want to program the device tie the GNSS end of the TX resistor high

^{simulate this circuit – Schematic created using CircuitLab}

connect the programmer at A and B and connec C to VDD so that the GNSS does not get confused by any responses sent by the MCU to the programmer.

Answer 2

With a 200 to 300 ohm impedance programmer cable and 50 ohm driver into 1k series to high Z // x pF Cin will cause horrible ringing (up to 50% amplitude) but only result in that being ignored as 5ns cycle ringing per meter is steady state if centred-bit sampling works as expected after start edge 1 to 0. Adding 170 Ohms to programmer side will eliminate ringing into 220 ohm twisted pair is a “nice to have”.

So the series 1k resistor must be added to the shared Rx side to prevent attenuation but 150 Ohms added to Tx side will be nice to have for jitter, ringing reduction at both ends of cable but not essential. Is programmer plugged in with GNSS activity? Or visa versa? Then Tx to programmer or GNSS Rx may need to be clamped

Idle Prog. Rx port must be PNP or Pch clamped high to prevent start bits (=0) when used by other UART.

All these problems may be corrected with 300 ohm CMOS 4016 or 4066 switches with ESD protection used as a DPDT switch enable by a jumper or connection voltage from programmer side when connected. ( no stinkin’ s/w needed)

Simulation proof of ringing on 1m cable 250 ohms

Answer 3

Thinking the best solution is a multiplexer such as TI's TMUX7219DGKR datasheet. For example for the RXD lines, S1 would come from GNSS RXD and S2 would be firmware update circuit's RXD and the single output of the chip (D) would go to the MCU's TXD.

The input will be selected by signaling High/Low at the select pin.