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LSF0108: Trouble with 3.3V to 5V SSI at low frequency 40kHz with 6 ft cable

Henry Tonoyan69
Prodigy 20 points
Part Number: LSF0108
Other Parts Discussed in Thread: TMS320F28069, SN74LVC2T45

Hi,

I'm interfacing to an RLS RM08 rotary encoder which outputs 5V SSI. For those not familiar with SSI it is basically SPI communication but only in one direction, so there is only a clock and a single data line. The board I am using has a LSF0108 between the microcontroller (TI TMS320f28069, at 3.3V) and the output connector. I was able to get the encoder working just fine using a short length of cable, about 12 inches, however, it seemed extremely susceptible to extra capacitance, since when I would use my oscilloscope on the clock line my readings would be somewhat garbled and useless.

I moved to a 6 ft cable and the readings were garbled in the same way. I tried lowering the clock rate down to 40kHz which is about the slowest that the encoder can handle, thinking that this would solve the problem but I the output is still garbled. Moreover, it still seems very sensitive to changes in capacitance like my oscilloscope probe. I am using a 2k pull-up resistor on the 5V side. See schematic below. The encoder is a 5V single-ended push-pull output.

My question is this: Could the LSF0108 be the cause of my issues? Do I have it set up incorrectly or out of spec? I have been trying to narrow down the root cause of this issue and I still have no idea what's going on, but for some reason I suspect the LSF0108. When scoping the 6 ft cable near the board side, however, the signals look fine and undistorted, so that is even more confusing to me. Any help or ideas for troubleshooting/debugging would be appreciated.

Thanks

  • 0
    TI__Guru* 97046 points
    Hi Henry,
    The short answer is yes, the LSF is likely the cause of your issues.

    We always recommend to keep capacitive loading to a minimum on the high-side of an LSF translator, and it's preferable to have the LSF0108 on the same board as the 5V device, usually within a centimeter or two. There's a short video that explains the reasoning why located here: training.ti.com/TLM-LSF-Up

    In this case, could you instead use a dual supply buffer like SN74LVC2T45? This is a direction controlled 2-bit translator that has active drive outputs that's definitely more likely to work in your application.

    Adding a 6-foot cable to any device is problematic though -- is the sensor module the only thing at the distant end, or is it part of a system board there? You might consider adding a small board at the end to act as a transceiver and convert data back and forth.