GPIO Translated from 5V to 3.3 using TXB0108PWR

Naveen C K

Other Parts Discussed in Thread: AM3517, TXB0108, TXS0101

Hello,

I am using TXB0108PWR to translate SPI Signals and One GPIO from AM3517 Processor from 3.3V to 5V and vice-versa.

The GPIO is connected to the output of an Opto-Isolator (collector of FOD814A3SD) and pulled high to 5V using a 5.1K resistor. This 5V GPIO is converted to 3.3V using the TXB0108PWR and connected to AM3517 processor pin C24.

During testing it was observed that when the output of Opto-Isolator goes low, the 5V side of the GPIO goes completely low and comes back to 5V when the Opto-Isolator's output is high. However, the 3.3V side of the GPIO signal on TXB0108PWR does not come back up to 3.3V. It stays at around 1.415V.

The datasheet for the TXB0108PWR mentions that "If pullup or pulldown resistors are connected externally to the data I/Os, their values must be kept higher than 50 kΩ to ensure that they do not contend with the output drivers of the TXB0108."

1. Could the reason that I am using a 5.1K resistor to pull up the GPIO be causing the above mentioned problem ?

2. Should I use TXS0101DBVRG4 for the single GPIO ?

Please help with any inputs which might solve the problem. I am testing REV A of my board and I am in the process of coming up with REV B, I need to know what to do before I finish this revision.

The schematics for the opto-isolator and TXB0108PWR are as below:

Thanks and Regards,

Naveen

over 14 years ago

0 Hattie over 14 years ago

TI__Genius 9085 points

Hello Naveen,

My initial assumption is that the signal is not going high because the only thing driving the line high it is that pull-up resistor. The external driver must be strong enough to overpower the TXB0108 internal 4kOhm buffer. I think you are correct in saying the TXS0101 may be a better option. You may also be able to just switch to the TXS0108 for all the lines on the bus. Since the footprint is identical, it may save you some layout work.

I am a little bogged down right now, so it may be a day or two before I can get back to you in more detail, but below is an application note explaining the TXB devices more thoroughly.

http://focus.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=scea043&docCategoryId=1&familyId=707

0 Ryan Binns over 14 years ago

Prodigy 70 points

Hi Naveen,

Since you're opto-isolating the GPIO, why do you need level translation? Why don't you run the GPIO from the AM3517 directly into the output of the opto, and pull the line up to 3.3V rather than 5V?

Cheers,

Ryan

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GPIO Translated from 5V to 3.3 using TXB0108PWR