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[ DS92LX1621/22 ] GPIO for UART

Kenichiro
Mastermind 9165 points
Other Parts Discussed in Thread: DS92LX1621

[ DS92LX1621/22 ] GPIO for UART

Hi,

My customer would like to use GPIO port of DS92LX1621/22 for UART communication. (see drawing below)

According to the datasheet, maxim speed of uplink GPIO communication is limited to 66KHz. Can you please let me know how this limitation come from? My customer would like to make sure whether UART (9600bps or faster) can be ensured over back channel communication.

Also do you have the number of variation of this maximum 66KHz like +/- x % and/or +/- y KHz?

Finally, in order to make sure UART Rx device can receive signal correctly, do you have any suggestions how to secure the customer's UART communication work well? Since there is a limitation on maximum speed (66KHz), there could be some requirements like minimum pulse width and so on. 

Thank you for your support in advance, please let me know, if you need more information.

Regards,

Ken

  • 0
    TI__Mastermind 29635 points
    Hi Kenichiro-san,

    You are correct. The maximum speed of GPIO communication along the back-channel (BCC) is limited to a maximum of 66 kHz. This limitation comes from the internal oscillator that is always on, irrespective of PCLK. The oscillator speed in the back-channel direction is approximately 2.5 MHz for a 30-bit frame. To determine the frequency of one bit, we must then down-convert by 30:

    2.5 MHz / 30 bits = 83.3 kHz for one bit (12 us per bit)

    The tolerance of the internal oscillator is +/- 20%. Therefore, in a worst-case scenario, the frequency per bit is:

    (2.5 x 0.8) MHz / 30 bits = 66.7 kHz for one bit

    This is where the datasheet spec limitation value comes from. Due to internal oscillator tolerance of +/-20%, we choose 66.7 kHz as the maximum frequency that can be assured. Note that 66.7 KHz is the value determined by [internal oscillator - 20%]. Nominally, the max speed is closer to 83.3 kHz.

    If you send data along the back-channel, there will be approximately a 12-24 us latency delay, depending on when the back channel buffers the signal over the back-channel. Therefore, to ensure that communication works well, I think your customer should also ensure that they can handle this delay, which will result in a reduction in the overall effective throughput.

    For resources regarding back-channel communication, I recommend referencing the following app note, starting from page 10: www.ti.com/.../snla222.pdf

    Hope this helps!

    Michael
  • 0 in reply to Michael Lu (Santa Clara)
    Mastermind 9165 points
    Hi Michael,

    Thank you for your support on this matter!!
    Your answer is really helpful!

    Regards,
    Ken