Add function SPItransferAndRead()

Suggested by @MrBlinky
Same as SPItransfer() but also returns the byte received.
Provided for use with homemade or expanded units that have had
additional peripherals added to the SPI bus that are capable of
sending data.

keywords.txt

@@ -107,6 +107,7 @@ setTextColor	KEYWORD2
 setTextSize	KEYWORD2
 setTextWrap	KEYWORD2
 SPItransfer	KEYWORD2
+SPItransferAndRead	KEYWORD2
 systemButtons	KEYWORD2
 toggle	KEYWORD2
 waitNoButtons	KEYWORD2

src/Arduboy2Core.cpp

@@ -251,6 +251,13 @@ void Arduboy2Core::SPItransfer(uint8_t data)
   while (!(SPSR & _BV(SPIF))) { } // wait
 }
 
+// Write to and read from the SPI bus (out to MOSI pin, in from MISO pin)
+uint8_t Arduboy2Core::SPItransferAndRead(uint8_t data)
+{
+  SPItransfer(data);
+  return SPDR;
+}
+
 void Arduboy2Core::safeMode()
 {
   if (buttonsState() == UP_BUTTON)

src/Arduboy2Core.h

@@ -402,10 +402,32 @@ class Arduboy2Core : public Arduboy2NoUSB
      * or as data to be placed on the screen, depending on the command/data
      * mode.
      *
-     * \see LCDDataMode() LCDCommandMode() sendLCDCommand()
+     * \see LCDDataMode() LCDCommandMode() sendLCDCommand() SPItransferAndRead()
      */
     static void SPItransfer(uint8_t data);
 
+    /** \brief
+     * Transfer a byte to, and read a byte from, the SPI bus.
+     *
+     * \param data The byte to be sent.
+     *
+     * \return The byte that was received.
+     *
+     * \details
+     * This function does the same as the `SPItransfer()` function but also
+     * reads and returns the byte of data that was received during the
+     * transfer.
+     *
+     * This function is of no use for a standard Arduboy, since only the
+     * display is connected to the SPI bus and data cannot be received from
+     * the display. It has been provided for use with homemade or expanded
+     * units that have had additional peripherals added to the SPI bus that
+     * are capable of sending data.
+     *
+     * \see SPItransfer()
+     */
+    static uint8_t SPItransferAndRead(uint8_t data);
+
     /** \brief
      * Turn the display off.
      *