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74LVC4245A & DIR signal

WILLIAM STEPHENS
Prodigy 20 points

Having problems with 74LVC4245A switching the DIR signal between a 3V & 5V bus. When the DIR signal is changed to drive from 3V side to 5V side a great deal of noise/ringing is seen on the 5V data bus.

The spec does not explicity say the DIR signal can be changed during operation - can it be?

Thanks for any help.

Bill

  • 0
    Mastermind 7395 points

    The direction can be changed anytime.... but

    1 Check that you drive DIR and OE with 5V

    2 Check that there are no collisions on the bus

    3 The ringing could be because of bad impedance matching of the bus

    Hope this helps

    Avi

  • 0 in reply to Avi Chami1
    Prodigy 20 points

    Hi Avi - Thanks for the response!

    1 - DIR & OE are driven by 5V

    2 - No collisions.  Last device asserting on the bus is shut off ~300-400nS before 74LVC4245 drives the bus.

    3- Bad impedance match is possible - but when the ringing occurs, it does not 'die down' over time.

    Interesting to note, this only happens when 6-8 of the bits driven are 'HI'.  In fact, the problem has been 'fixed' by driving one bit at a time ~30nS apart.  All noise/oscillation is gone when just turning on one bit at a time!

    That is, all bits are driven 'LO' at the beginning of the read - because there is never a problem when the bits driven are LO.  And then the bits are driven to their proper state for that read cycle one at a time.

    Also very interesting that no DIR timing is discussed in the spec, while this timing is documented in other '245 variants I've used in the past.

    And, the spec states:  Depending on the direction of the data path, DIR can be high or low. If DIR high is needed (A data to B bus), ramp it with Vcca.  OTHERWISE, KEEP DIR LOW.

    Anyway - I greatly appreciate your feedback on this.  Thanks again!

    Bill