• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 21 subscribers
  • Views 804 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN74AXC4T774: Slow ramp on CS pin

Roy Hsu
Genius 9350 points
Part Number: SN74AXC4T774


Hi Team,

My customer tested the SPI voltage translation (1.8V to 3.3V) with the SN74AXC4T774 EVM and found the ramp-up time of SS (A4 pin) was very slow. A pull-up resistor was added on the SS line per the design guide of the SoC. They tried 10kohm and 1kohm and the ramp-up patterns were pretty similar. Do you know what might be causing this?

Thanks!

Roy

  • 0
    Guru 317730 points

    The AXC has a very weak pulldown (100 kΩ). This cannot explain such a slow rampup.

    Is there anything else connected to the SS line?

    Can you add a curve for the power supply? I suspect that the SoC itself is not able to drive the line higher. (This can be proven by disconnecting SS from the AXC.)

  • 0 in reply to Clemens Ladisch
    TI__Genius 9350 points

    Hi Clemens,

    1. Nothing else is connected to the SS line.

    2. Below is the scopeshot of both VCCA (CH3)  and SS (CH4). SS line somehow got pulled low, I was thinking the SoC released it for some reason then it got pulled low by the internal pulldown resistor of 4T774. But I'm not sure why that happened.

    3. The SoC was able to drive the line high as you can see in the scopeshots below. We connected the SoC to an SPI flash (without the 4T774) and it worked fine.

    (Zoom-out of the above)

    Thanks!

    Roy

  • 0 in reply to Roy Hsu
    Guru 317730 points

    So the slow rampup is caused by the power supply.

    The AXC begins working at 0.65 V. At lower supply voltages, correct operation is not guaranteed. I'd guess that the direction control is not yet working.