Beagleboard-xM main expansion header SPI electrical signaling


I'd like to use Beagleboard to control a 2-channel digital
potentiometer (Microchip MCP42050:
through SPI.

As the chip will only funcion with VDD above 2.7V, I'll power it using
the 5V power rail (pin 2). But its digital output reaches 4.5V
(high-level; 0.4V for low-level).

I have the following questions:

1. I'll have to protect Beagleboard SPI input (SOMI) against such a
"high" voltage, right?

2. To get it down to 1.8V is it enough to use a voltage divider
consisting of 10k (chip output to bb input) and 5k6 (bb input to
ground) resistors?

3. Is it better to get the voltage down to 2.7V (again using a voltage
divider, resistor values to be calculated) before getting to VDD pin,
and then from 2.2V (SOMI high-level output) to 1.8V using another

4. Should I worry about current ratings?

Sorry for the newbie questions. Thanks in advance for the answers.

Besides, should I worry about voltage levels on the other SPI pins

Now back to report what I’ve got.

I’ve got myself through experimentation (with just one LED burnt) the answers for my questions.

There’s not need to worry about MCP42xxx output, because that SO is just for cascading chips; there is no value in connecting it back to BB, as the internal potentiometers values cannot be read anyway.

On the other side, the MCP42xxx inputs (CS, SCK and SI) are only sensibilized above 3.5V (for logic high), much less than the 1.8V available from BeagleBoard-XM.

Next week I’ll try to get its level to something usable (using transistors or MOSFETs). Please share your thoughts if you have any ideas. Thanks.