Resolve #3 by adding readGPIO function to read single bank of IO.

Author: tdicola

Adafruit_MCP23017.cpp

@@ -150,6 +150,25 @@ uint16_t Adafruit_MCP23017::readGPIOAB() {
 	return ba;
 }
 
+/**
+ * Read a single port, A or B, and return its current 8 bit value.
+ * Parameter b should be 0 for GPIOA, and 1 for GPIOB.
+ */
+uint8_t Adafruit_MCP23017::readGPIO(uint8_t b) {
+
+	// read the current GPIO output latches
+	Wire.beginTransmission(MCP23017_ADDRESS | i2caddr);
+	if (b == 0)
+		wiresend(MCP23017_GPIOA);
+	else {
+		wiresend(MCP23017_GPIOB);
+	}
+	Wire.endTransmission();
+
+	Wire.requestFrom(MCP23017_ADDRESS | i2caddr, 1);
+	return wirerecv();
+}
+
 /**
  * Writes all the pins in one go. This method is very useful if you are implementing a multiplexed matrix and want to get a decent refresh rate.
  */

Adafruit_MCP23017.h

@@ -27,6 +27,7 @@ class Adafruit_MCP23017 {
 
   void writeGPIOAB(uint16_t);
   uint16_t readGPIOAB();
+  uint8_t readGPIO(uint8_t b);
 
   void setupInterrupts(uint8_t mirroring, uint8_t open, uint8_t polarity);
   void setupInterruptPin(uint8_t p, uint8_t mode);