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TMUX1308: 3.3V UART MUX

daniel lin
Prodigy 220 points

Part Number: TMUX1308
Other Parts Discussed in Thread: CD74HCT4067, TMUX1237, TMUX1247

I plan to use one 3.3V UART of the MCU to connect with 8-set or 16 -set MCU (different MCU) with only one connection is active. Can you recomment the proper MUX ? Thanks.

  • 0
    TI__Guru 54042 points

    Hi Daniel,

    The best option we have for an 8:1 device for this situation is the TMUX1308, its a good general purpose 8:1 mux that can be used for digital or analog signals.

    For a 16:1 switch we have two possible options that may fit your needs the CD74HC4067  and the CD74HCT4067 are both 16:1 muxes that can work well at 3.3V. However they do have a smaller bandwidth (~89MHz) that you may have some data corruption if running UART at it's highest frequency.  If this is the case then you may need to look into a multi-chip solution using 3 Multiplexers.  See below for a basic explanation with parts for this application:

    The first stage is a 1:2 TMUX1237 or TMUX1247 (both will work equally well in this application), the second stage is 2 8:1 multiplexers using the TMUX1308. This solution still allows only one signal to be active at a time and will take on a similar truth table as a normal 16:1 multiplexer. This solution will connect the 16 set of multiplexers together.

    If you any other questions, concerns, or comments please let me know!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 220 points

    Can you tell me why CD74HC(T)4067's 89MHz may bring distortion when UART reaches its high frequency?

  • 0 in reply to daniel lin
    TI__Guru 54042 points

    Hi Daniel,

    It seems that it seems that I made a mistake  when I was looking at the maximum Baud Rates for UART and added an extra zero when I shouldn't have; I apologize for that. The CD74HC(T)4067 will be okay in the application. Generally the rule of thumb that I tend to follow is that  Baud Rate * (~ 3) < = Multiplexer Bandwidth for accurate data transmission. The max baud rate for UART is ~5Mbps but typically runs a lot lower so you'd need at least a 15MHz part to pass the data and mitigate data loss.

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 220 points

    As the Ron of CD74HC(T)4067 will be larger than 200 Ohms (for 3.3V application) and TMUX1308 350 Ohms, will these large Ron hurt the UART communication by losing too much voltage drop at these MUX/switch?

  • 0 in reply to daniel lin
    Guru 320720 points

    CMOS inputs have a very high impedance, so no DC current flows.

    A large Ron can limit the baud rate if there are too many parasitic capacitances in your board. If you have long traces or connectors, estimate their capacitance to determine what the limit of the resulting RC low-pass filter will be.

  • 0 in reply to daniel lin
    TI__Guru 54042 points

    Hi Daniel,

    That depends on the amount of current you are drawing for the application. Typically for digital applications with CMOS inputs you essentially have a high impedance load and the insertion loss from the switch is low because the application is drawing less current.

    However depending on the MCU that you are using there could be mA of current being used which could cause a voltage error loss. What current are you expecting on the bus?

    Best,

    Parker Dodson