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From @schwingkopf on February 13, 2013 19:59
when using the SPI library with the predefined SPI-CS0 and SPI-CS1 pins (digital pins 10 and 4), the voltage during SPI communication on these lines doesn't drop to 0V (only to 1.6V), when defining the digital pins for SPI-CS0 and SPI-CS1 prior as pinMode OUTPUT.
with the SPI-CS2 on digital pin 52 it worked fine for me.
for details please see http://arduino.cc/forum/index.php/topic,148463.0.html
Copied from original issue: arduino/Arduino#1280