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TS3A44159: input/output signal keep high when using UART with 3Mbps

Ben Kuo
Expert 7670 points

Part Number: TS3A44159
Other Parts Discussed in Thread: TMUX1574, TS3A24159

Dear Expert,

We use TS3A44159 in UART interface.

It can work well when using 230Kbps UART signal. But when baud rate up to 3Mbps, input and output keep high.

Attached waveform for your reference, looking forward for your feedback.(schematic is good as our EVM) 

Thank!

Ben

3Mbps

230Kbps

  • 0
    TI__Mastermind 45720 points

    Ben,

    The TS3A44159 has bandwidth of 35MHz which is more than enough for 3Mbps.  The scope shot you show above shows the switch working properly that is passing the signal from the COM to the NO pin.  The TS3A44159 is a passive FET switch and doesn't have the ability to hold the bus high but simply pass the signal from the input to the output. 

    One experiment you can try to see if the switch is the issue is to remove the switch and short the signal path to see if there is a difference.  I suspect there is something else on the signal path that is holding the bus high.  Do you have a schematic of the system showing the other devices? 

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    TI__Expert 7670 points
    Hi Adam,

    I have tried this before. It can work normally since we remove switch and short signal path.
    For schematic, I would like to discuss with you via mail.
    Thanks!
    Mail address: benkuo@ti.com

    Ben
  • 0 in reply to Ben Kuo
    TI__Mastermind 45720 points
    Ben,

    What does the waveform look like when the switch was removed comparted to when the switch was populated?

    I currently don't see any issue with the schematic.

    Which signal path is having issue DBG_UART_XX_1 or DBG_UART_XX_2?
    Where is DBG_UART_XX connected?

    I also noticed that U604 didn't have an external oscillator connected to it and Vcc = 3V. I believe there was a note in the U604 datasheet that Vcc must be > 4 V to use the internal oscillator.

    Thank you,
    Adam
  • 0 in reply to Adam Torma
    TI__Expert 7670 points
    Hi Adam,

    I will check with customer, and get feedback to you.
    Thanks for your support.

    -Ben
  • 0 in reply to Adam Torma
    Prodigy 10 points

    Hi Adam:

                     The waveform that without the analog switch and short the COM1 and NO1 is attached. UART Baud rate:3Mbps

    The issue is on signal paths pin NO1,NC1,NO2,NC2 on U610, the signal input path COM1 and COM2 come from LTE module.The signal input path COM3 and COM4 without control currently. (the U609 has the same issue).

    The VCC which on U603 FT232R is provided by USB VBUS that is 5V can meet the condition use the internal crystal.

    If the schematic is correct. Why it can't work normally??

    Thank you!!

    Spencer

  • 0 in reply to spencer chang
    TI__Mastermind 45720 points

    Spencer,

    Thank you for the additional information.  

    How many boards are showing this behavior out of how many tested? 

    Does replacing with a new IC work?  Does taking the suspect IC on a good board cause it to fail?  

    The TS3A44159 device is a passive FET switch with an NFET in parallel with a PFET.  This device doesn't have any source or sink capability and will simply pass whatever voltage it sees on one side to the other.  

    In the first image in this post you show "unexpected behavior" of the UART communication but it does show proper performance of the TS3A44159.  The switch is correctly passing the signal from the COM to the NO side but it just happens that the signal is a DC 1.8V. 

    1) You confirmed that the switch is not damaged because the communication works at lower communication speeds

    2) You confirmed the communication path works at the higher speed by removing the TS3A44159 and shorting the signal path

    3) You confirmed the control logic was set to connect COM to the correct path by stating the communication doesn't work with either NC or NO

    The difference I see between the test with the TS3A44159 on the signal path and you shorting the shorting the signal path is the TS3A44159 has series resistance (Ron) and parallel capacitance (Cio or Con) as seen below. 

    If the RC parasitic of the TS3A44159 was effecting your data I would expect to see data with rounded rising and falling edges but that isn't the case here.  I don't know if you have the board flexibility but there is a new TMUX1574 device with lower capacitance.

    Is the UART communicating with open drain or push-pull configuration? 

    Is the drive strength of the UART PHY strong enough to overcome the TS3A44159 164pF, board capacitance, and Rx capacitance? 

    Is there a timeout feature on the UART PHY

    I noticed there is another UART path with a similar switch TS3A24159.  Does this path have any issues?  What is the difference other than the switch in this circuit?   

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    TI__Prodigy 240 points
    After testing with customer, Rx has good enough driving capability to cover input cap of TS3A4419.
    But Tx is poor one and cause signal have wrong communication.
    May I learn from you how to improve driving capability with external circuit? Maybe add pull-high resistor, or you have other ideas.
    I will promote our TMUX1574 to customer at the same time. thanks!
  • 0 in reply to Whitney M Davis
    TI__Mastermind 45720 points
    Whitney,

    The Tx you are using that you provided me through email has a register setting to increase the drive strength which should help. Also that TMUX1574 is a much higher performing part and could be a good solution when you signal chain isn't optimal.

    Thank you,
    Adam