• State TI Thinks Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 18 subscribers
  • Views 304 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

SN74LVC1G07-Q1: Step down logic converter

skipper_the_penguin
Prodigy 10 points
Part Number: SN74LVC1G07-Q1
I'm trying to convert a 3V3 UART to 1V8 UART with an open drain buffer, but the output of the buffer is always high.

See the schematic below for the circuit. On the input to the buffer, there is a 0V - 3V3 UART, working exactly as it should. On the output, I'd expect a 0v to 1V8 UART. But I'm getting a 1V8 always high.

It's not a dead chip, as the same problem persists over multiple PCBs (10) - there are no shorts, the resistance values are correct and the supply rails are stable. For testing the output of the buffer is not connected to the downstream device.

What is going on?

The markings on the IC:

  • 0
    TI__Guru* 96591 points

    Hello,

    I don't see any issues with the schematic shown.

    Can you share a scope shot of the operation with the input, output, and supply signals shown?

    Preferably show a time scale of 100ns/div (or less) triggered on a falling edge from the input so we can see the output response. You can also show a larger timescale if your signals are slow, but I'd like to see at least on the same scale that the device operates.

  • 0 in reply to Emrys Maier
    Prodigy 10 points

    Here we have 100ns/div time scale - yellow is input, purple is output 

    In this screenshot i'd added an 10K pulldown to see if adding a pulldown would help the buffer work - hence why the voltage is at 0.9v not 1.8v


  • 0 in reply to skipper_the_penguin
    TI__Guru* 96591 points

    If the device is working you definitely won't need to help it with anything -- the output of the LVC1G07 is quite strong and should easily pull down a 10k resistor.

    Can you also take the same scope shot but showing the input + supply pin and input + ground pin (reference to ground at power input)?

    If all of these show as normal, then I would suspect either damage to the output or an issue with the device -- considering you have already verified other boards have the same issue likely ruling out damage, then it may be a bad batch of devices, which I would find very concerning considering TI tests every device, and these must have passed our tests (assuming these came from TI).

    Assuming your tests come back as 'good' for the supply and ground, I would recommend the next step of starting an FA / return through your supplier. There's likely nothing else I can do from here.

  • 0 in reply to Emrys Maier
    Prodigy 10 points
    Emrys Maier said:
    If the device is working you definitely won't need to help it with anything -- the output of the LVC1G07 is quite strong and should easily pull down a 10k resistor.

    Yea i was clutching at straws

    See below for the graph:

    I'm Very surprised that the rails are wobbling the way they are - I might try some additional filtering on the incoming RX to fix this. 
    It could also be cross talk on the scope lines - the scope is a slightly dubious chinese model


    I've also followed up with the assembler so they can verify the source of the ICs - To me counterfeit ICs seem likely