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SN65HVD62: Using SYNCOUT to drive second device

Doane Arnold
Intellectual 335 points
Part Number: SN65HVD62


I'm using the SYNCOUT line from one SN65HVD62 to drive a second SN65HVD62. Per the EVM datasheet, the SYNCOUT has a 1K pull-up resistor to Vcc and the signal is connected to the slave device through a 10K resistor. The XTAL2 input of the slave deice is grounded. When I observe the SYNCOUT prior to the series 10K resistor, the signal swings from GND to Vcc (5VDC). At the XTAL1 input of the slave device (after the 10K resistor), the signal is about 1.1VPk-Pk and 1.5VDC off of ground.

The datasheet implies that a XTAL1 logic HI has to be a minimum of 0.7Vcc. Should the XTAL1 input attenuate the signal this much? Also, is the 10K series resistor necessary?

Thank you, Doane.

  • 0
    TI__Guru 74352 points
    Doane,

    Thanks for deciding to test our device for potential use! I'm a little confused though, looking at the EVM schematic (www.ti.com/.../sllu152.pdf), the SYNCOUT pins have the option to pull up to VCC like you said, but there is no connection through a 10k resistor that I see. I see on the XTAL1 and XTAL2 line that you have the option to either short XTAL1 to GND through a 10k resistor, short the to the other side of the crystal connected to XTAL2, or leave it open.

    Can you show what exactly you're looking at?

    And to answer your question, no the XTAL1 signal should not attenuate the signal that much, but my guess is incorrect connections are causing the GND to not be at a defined level and the signal is getting attenuated because it's being short to GND through a 10k resistor.

    Another thing to note is that on this EVM both of these devices are isolated, so to use them together it would be best to tie their GNDs and VCCs together.

    Regards,
  • 0 in reply to Eric Hackett
    Intellectual 335 points

    Hi Eric,

    I replaced the 10K series resistor with a short and the XTAL1 levels (from the Master SYNCOUT) are now correct. I'm at a loss to explain why I added the 10K series resistor, but it might have come from a competitors datasheet for a similar part:

    Multiple ******* devices can share the same crystal
    by using the SYNCOUT pin. One device acts as a master
    and provides the 8.704MHz clock signal to the slave
    device(s) through such a pin. To configure a device as a
    slave, XTAL2 should be connected to ground. The external
    clock coming from the master device feeds the XTAL1
    pin of the slave device through a series 10k resistor.
    Connect a 1k pullup resistor to VCC from the SYNCOUT
    pin of the master device.

    Thank you for your help Eric.

    Doane