SN74AXC4T774: 1.177V on the B-side inputs when it should be 3.3

Phillip Lam1

Part Number: SN74AXC4T774

We designed a board where we are using the B-side as inputs and A-side as outputs. However we see 1.177V at the B-side. Can you help check to see if this is correct?

over 4 years ago

0 Phillip Lam1 over 4 years ago

Prodigy 90 points

Here is some additional information:

Regardless whether OE_N is high or low, I still see the same measurements. Seems like 1.177V for one board, another board shows 1.212V.

The JTAG_xxx signals are hooked up to a JTAG connector, with a 100k pull-up to 3.3V, which is not being driven. All measurements on A-side or B-side are done on floating nets.

1. If VCCA is powered down to 0-300mV and VCCB is 3.3V, B-side shows 1.2V, A-side shows 0V.

2. If VCCA is powered on to 1.8V and VCCB is 3.3V, B-side shows 1.2V, A-side shows 0V.

3. If VCCA is powered on to 1.8V and VCCB is 0V, B-side shows 1.2V, A-side shows 0V.

The strange part is the ~1.2V from the B-side. I am expecting 3.3V because of the pull-ups I have on here.

Part

Footprint

I will update more as I get more status.

0 Phillip Lam1 over 4 years ago

Prodigy 90 points

0 Clemens Ladisch over 4 years ago

Guru 283370 points

Everybody knows that the I/Os have internal weak pull-down resistors. (And with "everybody", I actually mean "nobody", because the datasheet does not mention it.)

When either power supply is off, all I/Os are high impedance.

0 Phillip Lam1 over 4 years ago in reply to Clemens Ladisch

Prodigy 90 points

So does this mean that I should depopulate the 100k ohm resistors?

0 Clemens Ladisch over 4 years ago in reply to Phillip Lam1

Guru 283370 points

If you want the lines to be pulled down.

If the lines need to be pulled up, you need stronger pull-up resistors.

0 Dylan Hubbard over 4 years ago in reply to Clemens Ladisch

TI__Mastermind 44765 points

Hey Phillip,

I would recommend 10 kohm pull-ups if possible. However my concern is with the fact that this issue is apparent whether or not the I/Os are disabled. When the I/Os are disabled you should not have this issue with the glitch suppression circuitry acting as a weak pull down.

Let me know if you are able to retest with 10 Kohm and if it fixes the issue.

0 Phillip Lam1 over 4 years ago in reply to Dylan Hubbard

Prodigy 90 points

Your suggestion for the 10k ohms is correct. I validated it. Thank you

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

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SN74AXC4T774: 1.177V on the B-side inputs when it should be 3.3