SN74AVC32T245 DIR and OE pin

Moving to Voltage Level Translation forum for support.

-Ryan

Hi, Ning

Thanks for asking.

SN74AVCA32T245 has four groups of IO. each group has its own DIR and OE pin.

Both DIR pins and OE pins are reference to the VCCA the VCCA voltage. and VIH, VIL is listed at the datasheet page 5. VIH min=VccA x0.7, and VIL max=VccA x0.3.

As you example, the DIR and OE pin will reference to VccA which is 3.3V logic.

and for the transfer direction, it relates to the DIR pin. when DIR pin is high, the direction is A to B, whent he DIR pin is low, the direction is B to A. while it is just A1 to B1 or B1 to A1 etc. not A1 to B2. at hte page3, it lists the logic diagram.

any further thing, please let me know.

BR
junjie

Junjie,

Let's say DIR is high, the direction is A to B. Then if B is connected to an output and A is connected to an input, what will the input pin see at side A while the output pin at side B is toggling between 0 and 1? A HiZ state?

Is there any product that doesn't specify the data direction? i.e, it is bi-directional simultaneously.

Thanks 

Hi, Ning

For your assumption, if keep the DIR high, while at the same time toggleing the B between 0 to 1, which will have two possilbe scenarios happen.

1. The B side is toggling between 0 to1 successfully, while since the A side output dirver is in high-impedance, there will be no inflence for the A side voltage, A side will keep the orginal state.

2. The B side can not be toggling as excepted , may in some middle voltage or not, and also the A side output dirver is in high-impedance, there will be no inflence for the A side voltage, A side will keep the orginal state.

I am curious why you don't change the DIR volage to low if you and the transfer from B to A.

We do have the direction autosening part which doesn't  need the DIR signal, which is our TXS010X and TXB010X and TXB0304, the maximum channel is 8 , no 32 channels.

You can check the datasheet and chose the one OK for customer.

any help needs. please let me know.

BR
Junjie 

Junjie,

Thanks for the suggestion.

The reason for auto direction sensing is the flexibility. SN74XXX requires 8 channels to be the same direction. It may work but I prefer to be able to arbitrarily define the direction for each channel. 

One thing I noticed for TXB0108 is VCCA must be no more than VCCB so for my application, VCCB must be the master and VCCA is the slave. However, OE is controlled by VCCA. If my VCCB is 3.3V and VCCA is 1.8V, I suppose I can connect OE to VCCB side without damage concern since VCCA can tolerate up to 3.6V. But will this cause malfunction? 

Thanks

Hi, Ning

Yes, for TXB0108, it needs the VccA <=VccB, and OE follows the VccA.  and you can put the OE pin up to 5.5V and no issue caused.

In the datasheet, you can see at page 4, it has the VIH inormaiton from 0.65VCCA to 5.5V.

BR
Junjie 

Junjie,

Thanks for the confirmation. I think TXB0108 is a better solution for my application.