TXB0304: I/O pull-down res

Chi Zhang2

Part Number: TXB0304

Dear team

my customer has the issue when using TXB0304.

When all the I/O are floating, only power 3.3V is on. They want get a default low signal. So connect a pull-down res 1K, but it is 1.57V.

my question is:

1. datasheet shows that it is better to use >20K res, and give the formula when use <20K. in our general understanding, pull-down will get low, pull-up will get high.

but in our datasheet, it is opposite. could you share the schematic for this? it is different from our competitor's socket.

2. do we support the case that all the I/O are floating, using res to force a low signal? because we find that the first time use multimeter is getting 1.57V, the second time is getting 0V, it is no matter with time, only multimeter different touch.

Chi

over 4 years ago

0 Chi Zhang2 over 4 years ago

TI__Intellectual 2690 points

Hi, All

add my reference in the datasheet. I couldn't understand the 1.5K res, could you share the schematic inside?

CHi

0 Clemens Ladisch over 4 years ago

Guru 316780 points

TXB I/O pins never float. The output buffers on both sides are always active. When there is a pull-down resistor and the TXB decides to drive high, the external resistor and the internal resistor form a voltage divider. With about 1 kΩ and 1 kΩ from 3.3 V, you indeed get about 1.65 V.

A pull-down resistor will set the voltage low when the TXB is completely disabled. To set the initial state after power up, try pull-down resistors on both sides.

What is the actual application? There are many situations where a direction-controlled translator is a better choice than the TXB.

0 Chi Zhang2 over 4 years ago in reply to Clemens Ladisch

TI__Intellectual 2690 points

Thanks for your reply.

First, the application is TXB0304 connecting to CPU and chip reset signal. now it is in MP process. it is better for us to clarify this issue.

but I am confused, setting pull-down on both side is for input and output? why are we different from Onsemi socket, do we have a P2P socket and same function?

BR,

Chi

0 Clemens Ladisch over 4 years ago in reply to Chi Zhang2

Guru 316780 points

I do not know what ONSemi device you are referring to.

All pins are both inputs and outputs at all times; the A and B sides are symmetrical. The initial state is takeb from a random pin.

If the signals are unidirectional, you should use a direction-controlled translator instead.

0 Chi Zhang2 over 4 years ago in reply to Clemens Ladisch

TI__Intellectual 2690 points

1. if we can't disable the socket through OE pin, how to get a low when power up?

2. if we have the pull-up res inside, how to explain Vol=VccX[1.5k/(1.5k+Pup)], it is easy to explain Voh=VccX[Pud/(1.5+Rud)]?

I think I didn't understand all the schematic, so may be in a confused mind. Thanks for your explaination

Chi

0 Clemens Ladisch over 4 years ago in reply to Chi Zhang2

Guru 316780 points

The internal resistor is not a pull-up resistor; it is in series with the output buffer. It behaves like a pull-up resistor when the output drives high, and like a pull-down resistor when the output drives low.

0 Chi Zhang2 over 4 years ago in reply to Clemens Ladisch

TI__Intellectual 2690 points

Hi,

Do you have email address? I couldn't find you through infolink

Chi

0 Clemens Ladisch over 4 years ago in reply to Chi Zhang2

Guru 316780 points

I am not a TI employee.

0 Chi Zhang2 over 4 years ago in reply to Clemens Ladisch

TI__Intellectual 2690 points

Thanks

do we have any way to get default low without disable OE?

BR,

Chi

0 Dylan Hubbard over 4 years ago in reply to Chi Zhang2

TI__Mastermind 45815 points

Hey Chi,

Not with this device, you may have an easier time with a direction controlled device like Clemens suggested.

Logic

Logic forum

TXB0304: I/O pull-down res