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SN74LVCH8T245: SN74LVCH8T245

amnon pomerants
Intellectual 780 points
Part Number: SN74LVCH8T245
Other Parts Discussed in Thread: SN74LXC8T245

Hi

I use the SN74LVCH8T245 as a level shifter from my FPGA (1.8V) to my CPU (3.3V)

In the datasheet you wrote several times it is not recommended to use PullUp/ Down resistors

on the device inputs (this is correct for the device outputs as well??)

What is the cause for this request? It it because a "voltage divider" will appear between 

the device internal resistors and the board external Pull Up/ Down resistors?

However, on section 10 you wrote that when pin OEn is always low (always active) it is

better to use Pull Up/ Down on the device inputs.... Why is that?

What is the bottom line, with or without external PU/ PD ??

In case my OEn pin is connected to the POR (Power On Reset) signal (POR is "1" for 250mS

from power up and afterwards it is constantly low), is it still comply with section 10

and I should connect my inputs to PU/ PD ?  Please explain ...

Thanks

Amnon Pomerants

  • 0
    TI__Mastermind 43690 points

    Hi Amnon,

    Yes, PD/PUs can cause DC voltage  level issues at the I/O. It also defeats the purpose of using the Bus hold part if you are going to add PD/PUs to the I/O, so you are likely better off using the non bus-hold version. The bus hold circuitry is only on the data lines and not the control inputs so it doesn't pertain to the OE pin.

    If you don't need bus-hold I actually recommend you use the SN74LXC8T245 device.

  • 0 in reply to Dylan Hubbard
    Intellectual 780 points

    Hi Dylan

    Can you please explain me section 10 in the SN74LVCH8T245 datasheet, where it states (and I quote):

    "in a permanently enabled state (OE is always kept low) pull up resistors are recommended (!!) at the input"

    This confuses me, with the request not to use external pull up, I don't understand which of the two contradicted recommendations to follow...

     Please help

    Thanks

    Amnon

  • +1 in reply to amnon pomerants
    TI__Mastermind 43690 points

    Hey Amnon,

    It states the reason in the following sentence:

    Furthermore the app note goes into more details:

    Not being recommend, doesn't mean they can't be used. The best recommendation is to use the SN74LXC8T245 if you don't need bus-hold and want a robust power sequencing.