Electronic – Mux UART with 74xx Multiplexer

analogmicrocontrollermultiplexerttluart

I have a legacy LPC MCU with 2 UARTs only. One of these UARTs is reserved for different functions such as IAP and RS232. So basically, I have one UART available only.

For an experimental project I need 5 to 7 UARTs to add 5 to 7 SOC modules.

So, I plan a new hardware with any controller, but due to PCB space and controller limitations I do not expect to have many possibilities to add unlimited UARTs.

Planning some type of extender, I had the idea of using two 1:8 74xx multiplexers in order to switch between UARTs and a common VCC/GND shared by all of the participants in my SOC/MCU chain.

Can you switch safely between UARTs using analog muxes? Or are there ramps/peaks that might lead to UART periphery problems?

Best Answer

If all UART comms protocol originates at the Port of the MCU then this may be a workable solution to MUX to the various target devices.

On the other hand if UART comms can asynchronously originate from any of the target devices at any time then this multiplexing scheme is a non-starter.

Related Solutions

Electronic – How a 2-1 multiplexer (MUX) work

The best way to get a proper understanding is to write down the truth table completely. You have 3 inputs (A, B, S), so this will give you 2\$^3\$ = 8 combinations:

S  A  B | S' C  D | Z
--------+---------+--
0  0  0 | 1  0  0 | 0
0  0  1 | 1  0  0 | 0
0  1  0 | 1  1  0 | 1
0  1  1 | 1  1  0 | 1
1  0  0 | 0  0  0 | 0
1  0  1 | 0  0  1 | 1
1  1  0 | 0  0  0 | 0
1  1  1 | 0  0  1 | 1

It's often useful to add intermediate results to make things more clear. I added a term \$C = (A \land S')\$ and \$D = (B \land S)\$. Now it should be clear that \$Z = (C \lor D)\$.

Proper use of analog mux HEF4051B

The TOSLINK signal you are trying to switch is digital in nature and high-frequency, which a general-purpose analog switch does not handle very well. The datasheet of the transmitter does not seem to specify its input impedance, so it might be rather low, which would add to the problem.

I strongly suggest that you use a digital multiplexer instead of your analog one. For 3V3 I would suggest an 74AC151.

Best Answer

Related Solutions

Electronic – How a 2-1 multiplexer (MUX) work

Proper use of analog mux HEF4051B

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