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THVD1450: Clarification on enable time

Mitchell
Expert 4915 points
Part Number: THVD1450

Hi Team,

My customer has a question on the behavior of the enable time for the THVD1450. 

It's our understanding that we can tie the drive enable and receive enable pins together and drive them from a single logic signal. But looking at the switching characteristics in section 7.8, and when you put it all together it seems like, if you use this part in half-duplex mode like this, it takes from 2.5-10 microseconds to switch into drive mode, and 4-14 to switch back into receive mode. But if you for some reason are enabling both at once, it only takes the entirely reasonable 100ns or less.

Can you help us understand if we are interpreting this correctly? It seems like the delay to get data around a multidrop system is way too high. 

Thanks,
Mitchell

  • 0
    TI__Guru 62920 points

    Hi Mitchell,

    I understand the concern, since the way this spec is written is ambiguous for this case (which is a common one).  When DE and /RE are tied together, the faster set of enable times applies.  The slower set only applies when the driver and receiver are both disabled for overlapping periods of time.  When that condition is encountered, the device starts a transition into a low-power mode.  Getting into and out of this mode takes some time, though, due to some internal filtering.  This ensures you don't enter the "sleep" mode when DE and /RE transition together, but it also means that if you are in this mode it takes some extra time to get back to a normal mode (which is where the longer set of tPZH/tPZL specs comes from).

    Regards,
    Max