Added comments to support the theory that setDataMode numbers appear to be the same on the AVR as they are on the STM32, so I'll assume this is OK until someone tests it (somehow) and lets me know its not working ;-)

STM32F1XX/libraries/SPI/src/SPI.cpp

@@ -171,6 +171,33 @@ void SPIClass::setBitOrder(uint8_t bitOrder)
 }
 void SPIClass::setdataMode(uint8_t dataMode)
 {
+/* Notes.  As far as I can tell, the AVR numbers for dataMode appear to match the numbers required by the STM32
+
+From the AVR doc http://www.atmel.com/images/doc2585.pdf section 2.4
+
+SPI Mode 	CPOL 	CPHA 	Shift SCK-edge 	Capture SCK-edge
+0 			0 		0 		Falling 		Rising
+1 			0 		1 		Rising 			Falling
+2 			1 		0 		Rising 			Falling
+3 			1 		1 		Falling 		Rising
+ 
+ 
+On the STM32 it appears to be
+
+bit 1 - CPOL : Clock polarity
+
+    (This bit should not be changed when communication is ongoing)
+    0 : CLK to 0 when idle
+    1 : CLK to 1 when idle
+ 
+bit 0 - CPHA : Clock phase
+
+    (This bit should not be changed when communication is ongoing)
+    0 : The first clock transition is the first data capture edge
+    1 : The second clock transition is the first data capture edge
+ 
+If someone finds this is not the case or sees a logic error with this let me know ;-) 
+ */
 	_settings.dataMode = dataMode;
 	this->begin();
 }