[FAQ] What Analog Switch / Multiplexer Should I use for UART Applications?

When looking for a UART analog switch / multiplexer for a microcontroller or other low voltage device, it's important to understand some of the specifications of the UART standard:

• 2-wire Universal Asynchronous Transmitter-Receiver
• Voltage Levels at Microcontroller Side: 3.3V and 5V
• Frequency: Data Rate Maxes out at ~5Mbps (at least 7.5MHz required, but 15MHz or greater preferred)

With the above knowledge and keeping in mind that at its core the UART protocol allows asynchronous data to be transmitted either in half-duplex, full-duplex, Tx only, or Rx only modes the following are 3 key specifications for your analog switch / multiplexer that will help pick the correct device for your system:

1) The configuration of the analog switch / multiplexer. As stated, UART is a 2-wire protocol, so a 2-channel device will be needed.  Depending on how many UART devices are going to be communicating is going to determine the configuration of the switch. If 1 device needs to talk to 2 other devices, then a 2:1 will be needed, if it is 4 devices, then a 4:1 will be needed. This pattern can continue indefinitely. One additional note on this is that most multiplexers have configurations in powers of two, so if 1 main device is talking to 5 other devices, an 8:1 to multiplexer will need to be used.  

2) UART is voltage level independent, as many different PHY’s can be used with the UART protocol. 3.3V or 5V are some common implementations of UART when near controllers, as many cannot handle above 3.3V to 5V. These voltage levels work very well with TI’s low voltage multiplexers and are a great piece to add to a UART expansion system.

3) The bandwidth is typically not going to be a limiting factor here. UART is relatively slow, with a device with 15MHz or greater bandwidth being able to pass UART data without issue, the majority of TI’s analog switch and multiplexer portfolio surpass the 15MHz bandwidth specification.

The below table is a recommendation for multiplexers that can pass UART signals on the data signal paths. These devices are controlled via GPIO.

Best,

Parker Dodson