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Topic: OE pin on TXS0108E bi-dirrectional logic converter (Read 10841 times) previous topic - next topic

budworthtdog

Okay so a bit of a newbie question here. I have used level shifter TE291 before and it worked fine. I ordered  model TXS0108E to have an 8-channel option. The 4 channel had two ground pins on it. One for the lower voltage side and one for the higher voltage side. This one has one pin marked GND and another marked OE.  I've read the data sheets and it had the following information listed for the OE pin.

"When the output-enable (OE) input is low, all outputs are placed in the high-impedance (Hi-Z) state. To ensure the Hi-Z state during power-up or power-down periods, tie OE to GND through a pull-down resistor. The minimum value of the resistor is determined by the current-sourcing capability of the driver."

Could someone explain that to me? For example I have a 5V Arduino Pro Mini and a 3.3v RFM69HW that I would like connected together. Would the ground for each side (5V and 3.3V) of the TXS0108E go to the same ground pin? What would I do with the OE pin in that case?

CrossRoads

If you wanted the part not drive any lines at times, you could use a 10K to 1K pulldown resistor, and drive the pin High when you wanted it to be able to drive lines. Like an active high chip select line.
If you just want it high all the time connect OE to 3.3V or 5V with a 1K pullup resistor.

Connect all grounds together.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

CrossRoads

Watch the drive capability of parts you are connecting. If I recall from the datasheet they must have 2mA drive to overcome the TXS0108 output to change it to an input. I was using a Maxim temp converter chip and the MISO line couldn't switch the line to an input:
In the TXB0108 issue it says:
8.3.2 Input Driver Requirements
Typical IIN vs VIN characteristics of the TXB0108 are shown in Figure 6. For proper operation, the device driving
the data I/Os of the TXB0108 must have drive strength of at least ±2 mA.

TXS0108 puts it differently:
8.3.2 Input Driver Requirements
The continuous DC-current sinking capability is determined by the external system-level open-drain (or push-pull)
drivers that are interfaced to the TXS0108E I/O pins. Because the high bandwidth of these bidirectional I/O
circuits is used to facilitate this fast change from an input to an output and an output to an input, they have a
modest DC-current sourcing capability of hundreds of micro-amperes, as determined by the internal pull-up
resistors.
The fall time (tfA, tfB) of a signal depends on the edge-rate and output impedance of the external device driving
TXS0108E data I/Os, as well as the capacitive loading on the data lines.
Similarly, the tPHL and maximum data rates also depend on the output impedance of the external driver. The
values for tfA, tfB, tPHL, and maximum data rates in the data sheet assume that the output impedance of the
external driver is less than 50 Ω.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

raschemmel

I have one of those chips and couldn't get it to work as a level converter between an Adafruit MAX31855 thermocouple breakout and an UNO. I had to resort to a CD4050. The reason is probably what you just described.

graynomad

I've had trouble with them as well. One problem is that they have a series resistor (4k IIRC) in line with each IO, that can cause problems if trying to drive too much current.
Rob Gray aka the GRAYnomad www.robgray.com

bellera

I'm trying to use the TXS0108E to connect RX/TX between NodeMCU and Arduino UNO.

The purpose is use NodeMCU as a bridge for WiFi/Serial communication with Arduino.

If I connect directly the [NodeMCU TX pin] to the [Arduino RX pin] it works perfectly. I can send orders from my laptop until the Arduino, using my WiFi.

But when I put TXS0108E in the middle I doesn't work. NodeMCU is on the A side. Arduino is on the B side.

If OE to 3.3V I receive strange characters. I OE at GND nothing is received.

I would like to use a Level-Shifting (Voltage Translator) solution for security.

I also want to connect [Arduino TX pin, 5V] to [NodeMCU RX pin 3.3V]. I can't do this directly.

Any idea? Thanks!



hitesh_solanki

Hi,

       I'm trying to connect TXS0108e to get output of 3.3V from input of 5V. Problem that i faced is, whenever i apply Vcca then i get same voltage on all pins of this side. Same problem occure at other side also. Can anyone explain me, why is it so?

       And also I'm not able to understand the function of 'OE' pin. I think this problem is due to connections. Please provide me proper connection of the same.

      Reply me as soon as possible.


Thanks,

bellera

http://www.ti.com/product/TXS0108E/datasheet

http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=TXS0108E&fileType=pdf

http://www.sunrom.com/p/8-way-level-conversion-board-txs0108e

OE, Output Enable.

Must be HIGH. From the datasheet:

When the output-enable (OE) input is low, all outputs are placed in the high-impedance (Hi-Z) state.

To ensure the Hi-Z state during power-up or power-down periods, tie OE to GND through a pull-down resistor.

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