/OE on SN74AC245N (non-inverting buffer)

[banedon]

Hi guys

I haven't used buffers before, but they seem fairly basic to implement. However, I've hit a possible snag when it comes to the SN74AC245N by TI. The datasheet mentions on page 1 that /OE (as you'd expect) needs to be low to enable xfer of data between A and B or B and A (depending on DIR).
However, it states "To ensure the high impedance state during power up or power down, /OE should be tied to VCC through a resistor; the minimum value of the resistor is determined by the current-sinking capability of the driver.".

Despite this statement, all of the circuit diagrams that I can find seem to simply tie /OE to GND. Do I need to do as advised above and if so can someone give me an example of how to do this as I don't understand how I can use a pull up resistor and then pull it low when needed... unless this par is normally controlled by a microcontrollr or processor ... or some kind of power monitor? Also, I'm not sure (despite the explanation in the data sheet) of how to work out the "current sinking capability of the load"

[Paul Moir]

unless this par is normally controlled by a microcontrollr or processor

I think this is the reason for the statement; to deal with things that drive it that might start in high impedance mode like a programmable direction microcontroller pin.
If you definitely want it to output all the time, and the direction pin is defined at power up, there's no reason not to pull /OE to ground.

(Note that the '245 is often used rather than the 244 single direction buffer because of it's more useful pinout, so it's not unusual to see '245s with tied direction inputs.)

[banedon]

Hi Paul

That's exactly what I'm doing. The direction will never change and so DIR is going to be pulled high allowing A -> B xfer.
Thanks for the explanation .