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question on TXB0304

Vera Mao
Expert 5065 points
Other Parts Discussed in Thread: TXB0304

Hi dear supporting team,

my customer met one issue on TXB0304,  they are using it for BI-Direction level shifting btw CPU and TF card.  CPU side is 1.8V (left side), and TF side is 3.3v(right side), they pull it up through a 20K resistor.  (I asked them change to higher, they tried 80k, the issue is the same)

The issue is : when left side SD_CMD is 0V, right side H_SD_CMD is also 0V.

now if they pull the TF card, and in the mean time, they reset the left side CPU, when it is reset, SD_CMD should be high impedance.  while now H_SD_CMD is still low level. 

since currently both side has no driver, only right side has pulling up, the right side H_SD_CMD should be high, why it keeps low? could you help explain?

and I heard TXB0304  is not suitable for TF kind of applicaiton, is it right?

the sch is as below:

  • 0
    TI__Mastermind 31285 points
    Hello Vera,

    I think the issue is that when you reset the CPU and put the pin into high impedance state you are not providing enough current to overdrive the 4k buffer to change the output.

    TXB translators require a driver to be source/sink 2mA to initiate a direction change on the output. Because both I/O were at 0V before the CPU resets, the TF side will hold at 0V through the output the buffer. When you reset the CPU and put the line in high impedance the output will hold at last value (in this case 0V) until the buffer is over driven.