Specifically, I'm using U8Glib and SDfat on the SPI bus (spi4teensy3, Arduino 1.6.5 and TD 1.24). With the hardware using CS pins 6 and 10 and specifying that in the appropriate code location, it works. I try pins 15 and 20 and the same hardware and changing the CS pin in the code, it doesn't work. I can bit bang U8Glib to use pin 20 but hardware SPI doesn't work. I can't get the SD card to work on pin 15 at all (no bit bang support in SDfat).
case U8G_COM_MSG_CHIP_SELECT:
if(arg_val == 0)
{
digitalWriteFast(_CS, HIGH);
}
else{
digitalWriteFast(_CS, LOW);
}
break;