Can I use a diode from the +3.6V rail to the VCC of that part ? Diode would have say 0.7V drop, so would provide 2.9V to part.
Part is SPI interface. I could also use same diodes for the SCK and MOSI lines. MISO would go from part to controller, so that would not need dropping diode.
For the VCC supply, you can use the series diode. It will work, except for the fact that the voltage drop will not be exactly 0.7v. The lower the power consumption, the smaller the voltage drop. To be on the safe side, add a pulldown resistor from VCC_2.9v to GROUND. Something that always consumes about 1-2ma.
For the data signals however, you cannot simply add series diodes. Here you need pulldown resisters (not only for a current bias). The host cannot drive a logic LOW through the diode. You need resistors similar to the optional VCC resistor. The lower the value, the faster your SPI can go. Check with a scope on your final PCB layout to see what value gives best results.
All this stuff is very power consuming. If you are out for low-power stuff (battery operated), you should really check if you can replace the SPI component with a 3.6v compatible one.
Another option is to use a dedicated voltage regulator and logic level converter. This is more expensive than diodes + resistors, but saves a lot of power. Use a 74VHC bus driver to convert the 3.6v logic levels down to 3.0v.
If you can't afford this luxury, at least make sure that your SPI signals are LOW when inactive. This avoids current flow through the pulldown resistors.
jetmarc