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[SOLVED] Quad 2 Channel UART Muxer

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ArticCynda

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Hi everybody,

I have a design with a PIC microcontroller and want to connect its UART (TX, RX, CTS, RTS) to a USB-UART bridge (MCP2200) or a bluetooth low energy module. The lines must be switched if the USB cable is plugged in.

I've been looking into multiplexers for this purpose, but wondering if there are perhaps better solutions for this purpose. I'm considering the following solutions:
- 74HC4052: dual 4 channel mux: from its datasheet I understand its bidirectional, but for 4 lines I would need 2 of them.
- 74HC157: quad 2 channel mux: would be ideal, but the datasheet doesn't mention if it's bidirectional.

Any suggestions?
 

FvM

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74HC157: quad 2 channel mux: would be ideal, but the datasheet doesn't mention if it's bidirectional.
The datasheet even has a logic diagram of the chip, so if yoi read beyond the title page, you should be able to understand the chip's internal operation.

Digital multiplexers are unidirectional by nature. You would need a demultiplexer for the inverse operation.
 

ArticCynda

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The datasheet even has a logic diagram of the chip, so if you read beyond the title page, you should be able to understand the chip's internal operation.
I saw the diagram yes, but the small letters mention that "this diagram is provided only for the understanding of logic operations and should not be used to estimate propagation delays" so it is unclear how the connection between input and output is physically made.

Digital multiplexers are unidirectional by nature. You would need a demultiplexer for the inverse operation.
Please take a look at fig. 3 on page 3 of the datasheet of the 74HC4052: https://www.nxp.com/documents/data_sheet/74HC_HCT4052_Q100.pdf
It appears to me that the connection between input and output is only made through MOSFET channels, so doesn't that mean that signals can propagate both ways? After all, current can flow through an open channel regardless of polarization as long as the Vgs treshold is met.
 

ArticCynda

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The solution to this problem is actually surprisingly simple, I found in the meanwhile: instead of using a (unidirectional) multiplexer, an analog multiplexer (analog switch matrix) is much better suited. The MC14551B is a quad dual channel analog multiplexer which allows me to switch 4 (RX, TX, CTS and RTS) and should conduct signals in both directions.

Thanks everybody for thinking along!
 

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Sorry for not stating this explicitely. I thought you already realized that an analog mux can work for the application.
 

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