Re: [Gumstix-users] SPI (MISO only) problem driving the input

[Chris G <chr...@qi...>]

Thanks very much for the verification. That led me to find a "fix," hopefully
it helps someone out in the future.

The problem is that while the ADS7846 lines are probably tristated
correctly, they're run through 1.8V-3V level translators/buffers (U8, U9,
U10 on the Chestnut schematic), and so the buffers themselves seem to be
loading those lines. That's why explicitly setting /CS0 high didn't help the
situation.

The "fix" is to lift U10 pin 11 to isolate the 1.8V side of the miso line.
You lose ADS7846 functionality by doing so, but with the buffer disconnected
the inputs from the header work as expected. Experiments showed that lifting
U10 pin 38 or cutting the trace to U7 pin 16 isn't sufficient, you need to
remove miso from U10 at the 1.8V side.

The output enable (/OE) on the buffer chips is tied to ground, making them
active. A better design would have either connected /OE to a GPIO for
software control or just directly connected it to /CS, since making the /OE
high tristates all the pins (according to the datasheet, haven't tried it).
You could do that yourself with some careful soldering if you wanted to
also.



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