• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 21 subscribers
  • Views 88 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.

SN74HC595: SN74HC595 Abnormal communication level

Zhen Wang
Prodigy 20 points

Part Number: SN74HC595

 When SN74HC595 is in normal communication mode, a 5V voltage comes out from the MCU and enters the SRCLK of SN74HC595, where it only has a 4.8V voltage; when I pull down the MCU, there will still be a 1V voltage on RCLK; Is there any recommended resistance value for the serial path of SPI? The current 10K setting has been changed to 330R, and the voltage has returned to normal. Would you like to ask why this is the case? What caused this?image.png

  • 0
    TI__Genius 9780 points

    Hi Zhen Wang,

    This sounds like a problem with your inputs, is that correct?

    One problem that I see with this schematic is that you may be violating the input transition rise or fall time Δt/Δv. The datasheet specifies 200ns as the maximum input transition rise time. Looks like this design has an RC circuit to delay the input. Decreasing the size of both the resistor and the capacitor will fix the problem with the input transition rate.

    Please also consider changing the values of the resistor and capacitor on the OE input.

    Regards,

    Nikki

  • 0 in reply to Nikki Dengel
    TI__Genius 9780 points

    Hi Zhen Wang,

    By the way, are you using HCS595 or HC595? Your post says HC but the schematic says HCS. HCS has schmitt trigger inputs (no restriction on input transition rate).

    Regards,

    Nikki