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TMUX1108: Looking for solution to MUXING JTAG with digital select lines

Part Number: TMUX1108
Other Parts Discussed in Thread: SN74CB3Q3251

I am trying to implement a solution to mux a single JTAG programmer to 1 of 8 JTAG devices.  Our application does not allow a traditional chain.  My initial thought is using five TMUX1108 with the select lines in parallel.  Any advise on whether these can handle the clock speed and SI of JTAG reliably?  One of the primary desires is for physical select lines instead of sending SPI/I2C commands to change the channel.

  • Hi Mike,

    I have a couple questions:

    1. What is the highest expected JTAG frequency going to be employed in the application - the TMUX1108 only has a rated bandwidth of about 90MHz.

    2. What voltage levels are going to be expected from the JTAG signals

    3. What power supplies are going to be used with the Multiplexer?

    4. Is there anything particular that drew you to the TMUX1108 - as we usually don't see TMUX11xx parts being used in digital applications. 

    5. Can you verify the termination impedances on the lines? 

    My biggest concern right now is the signal bandwidth because this could affect SI negatively. If you could answer my above questions I should be able to either okay this design or have some other options. The TMUX11xx family tends to be a bit expensive as well - when you may be able to have this application done for much cheaper, using a different part. 

    Please let me know!

    Best,

    Parker Dodson

  • Parker,

    I believe your concern over the 90 MHZ is correct.  We could possibly have a 100MHz JTAG clock.  

    Our voltage levels are 3.3V

    Power would be fed from 3.3V rail

    My main priority is simple binary selection to MUX JTAG signals without having to write registers for selection.  This is to allow 1 program debug interface to be bidirectionally connected to one of 8, non-daisy chained, non-bussed devices.

  • Hi Mike,

    With that in mind - I think the best option we have is the SN74CB3Q3251. It is an 8:1 analog mux, GPIO controlled, at a typical On Resistance of ~4 ohms at a 3V supply. It does use a charge pump to achieve low on resistance and input voltage beyond supply - so there will be a higher power cost using this part (a few mA range)

    It has a bandwidth of 500MHz so this part has digital applications in mind. It is cheaper than the TMUX1108 - this is because the TMUX1108 has very low leakage - which isn't necessary for JTAG application. 

    However - the SN74CB3Q3251 is only rated up to 85C, where the 1108 is rated up to 125C. Is this a concern for the application?

    Best,

    Parker Dodson

  • Thanks Parker,

    The temp should be good.  I'll see about ordering some for the lab.