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PGA460: Noise & Low logic level on RX

Clint Spitzer
Prodigy 70 points
Part Number: PGA460

Hi -

I have two questions. My device is operating at 115,200 bps (1start, 8data, 2stop, no parity) over two-wire UART (Asynchronous). Both on the Evaluation card with the MSP430 and with my in house microcontroller I am seeing that the logic line of the RX line starts low. Then on a response back to the microcontroller there is a quick spike to bring it high before sending the diagnostic and additional fields back. The data I am seeing is good for the most part but sometimes there is a frame that does not make sense in terms of the diagnostic field. Is there a reason that the logic line of the RX is pulled low? And if so how do I make it high?

Secondly I am experiencing some noise/ghosting between the RX and TX lines - is there some things that may be causing this or to avoid it?

Thank you for your time.

Clint Spitzer

  • 0
    TI__Mastermind 27950 points
    Hi Clint,

    Please see this similar post for an explanation of the UART pin's initial idle state: e2e.ti.com/.../2922898

    I'm not entirely sure what you mean by noise on the UART lines. Do you mean the lines do not always show logic low (0V) or logic high (3.3V), such that the logic level may appear nondeterministic (1.65V)? Or do you mean that you occasionally see short inverse logic transients? A screen capture would help clarify.
  • 0 in reply to Akeem Whitehead
    Prodigy 70 points

    Here is a picture of my scoped data - Here on the PGA_TX line, which is the RX line of my microcontroller, there seems to be some small amounts of noise that will cause spikes in my digital frame. I am trying to minimize this noise, or reason for why this may be occurring?

    I am also curious why the PGA_TX line is held low when it should be high? Should there be a pull-up resistor on the PGA_TX line?

    Thank you for your assistance.

    best,

    Clint

  • 0 in reply to Clint Spitzer
    TI__Mastermind 27950 points

    Hi Clint,

    The reason for the PGA_TX line toggling high for a brief moment is also related to the tri-state nature of this pin.

    As you've noted, the UART_TX line should typically idle high. However, the PGA460 can be operated in either asynchronous (UART) or synchronous (SPI) mode at anytime, so the PGA_TX does not force itself high when inactive since the synchronous mode does not call for an idle high. It is only after an asynchronous command is received by the PGA460 that it forces an idle high immediately before a data low toggle. In the unintended case where the PGA_TX pin is idling low, the quick idle high force looks like a noise transient. Had PGA_TX been idle high, there would be no transient noise. To avoid this transient effect, place a pull up resistor at the PGA_TX pin.