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TIR1000: The output state of the U-TXD and U-RXD when in reset

Part Number: TIR1000
Other Parts Discussed in Thread: SN74LVC1G125


I am developing a PCBA which uses this part. It connects between a TFB4711 and a ATSAMA5D27C CPU. All the UART ports are used up on the CPU, so I want to share the TIR1000 with the CPU's debug UART port. The debug UART port is connect connected to anything else except a test jig for R & D purposes. So it makes sense to utilise this port for the IR work in the product. 

So the question is: Are the U-TXD and U-RXD lines tristated when the chip is held in reset? That is I want the debug port to be usable when this chip is present.

I have placed zero ohm resistors to isolate this UART from the TIR1000 if required as a mechanical workaround. I can even use a tri-state buffer chip, but I want to keep parts count to a minimum and real estate is very tight. No point using a tristate buffer when we don't need one. The datasheet does state how these pins behave with RESET, other than the state machine in the chip is reset.

Best regards,


  • This is not so important now.

    I added a SN74LVC1G125 tristate buffer between the U-RXD and the debug port. Therefore, the IR circuit is entirely disabled using one line from the CPU (TFB4711 disabled, TIR1000 in reset and the SN74LVC1G125 is not output enabled, using a default pull-up reistor; so the the debug port is owned by the test jig by default. FOr normnal operation the IR can be enabled - we wont need the test jig then. 

    The TI cost is negligible, but the BOM count has increased by 1. The space is tight, but I believe I can fit the SN74LVC1G125DCKR.

    If anyone wants to answer about the state of the pins, great. If not, I will survive. However this answer might be useful for others, although it is an unusual application of sharing mutually exclusive devices on a UART port..



  • Hey David,

    I can place an order in for a device and see if I can put it on a breakout board to verify for you. My assumption though is that this device wouldn't tristate its output but rather leave it in an output HIGH state during reset. I'll see if I can get the parts in and test by the end of next week.

    I think your approach of placing a tristate buffer would be best if you need these pins to be HI-Z until later.


  • Hi Bobby.

    Thanks for your offer to try this, but I have used the tristate buffer. In quantity the TI buffer is only 3 as chips as we Aussies would say. I can fit it in too. And I think you are correct in that it would leave its output in a low impedance output.I have put the buffer in the U-RXD line on the TIR1000 which is a low impedance output from the TIR1000. I did not put one in on the U-TXD line which should be a high impedance input when the chip is in reset.