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Line Level Conversion

Dan McKenna
Prodigy 50 points
Other Parts Discussed in Thread: TMS320F28335, TXS0108E, OMAP3530, TXB0108, TXS0104E, SN74AVC8T245

Hey,
I'm trying to interface a OMAP 3530 over serial with a Delfino TMS320F28335.

The OMAP3530 is in the form of a Gumstix COM with the summit break outboard, and I am using the Spectrum Digital ezDSPf28335 kit. The OMAP processor operates on 1.8V and the Delfino operates on 3.3V logic. I was attempting to use a TXB0108 logic level shifter to shift the SCI (serial) channels and a TXS0108e chip to shift I2C and SPI channels.

Right now I'm only really concerned with making the TXB0108 work. When I power the Delfino and the Gumstix (OMAP) separately and scope the outputs, I get nice clean digital signals. When I hook up the two devices with a TXB0108 between them, I get high frequency high amplitude noise on both the driving and receiving lines. I would be happy to post a schematic of my circuit, but I was hoping that someone could tell me if this is a correct way to handle the level conversions.

If someone is willing to take the time to help me figure this out, I'd be happy to write it up so Gumstix and Beagleboard users don't have to go through the guessing games that I did when I designed my board.

Thanks for the help!

Daniel McKenna

  • 0
    TI__Intellectual 855 points

    Hi Dan,

    The TXB0108 is designed to drive short traces and light capacitive loads only.  If you have long traces or cables connected at the output of the TXB0108, it is possible that the outputs could oscillate.  If your signals are uni-directional, then a better option would be to use a level shifter like the SN74AVC8T245 which has strong DC drive capability.   If the signals you are passing through the level shifter are slower than 12MHz, then you could also consider using the TXS0104E which is capable of driving heavier capacitive loads. 

    --Prasad Dhond