Support I2C request and add examples

examples/i2c_reply_datum/i2c_reply_datum.ino

@@ -0,0 +1,33 @@
+#include "I2CTransfer.h"
+
+
+I2CTransfer myTransfer;
+
+struct __attribute__((packed)) STRUCT {
+  char z;
+  float y;
+} testStruct;
+
+void I2C_RequestHandler(void)
+{
+  myTransfer.replyWithDatum(testStruct);
+}
+
+void setup()
+{
+  Serial.begin(115200);
+
+  Wire.begin(0x10); // Begin I2C as peripheral (slave) with address 0x10
+  Wire.onRequest(I2C_RequestHandler); //Setup I2C_RequestHandler function as I2C request callback
+
+  myTransfer.begin(Wire);
+
+  testStruct.z = '$';
+  testStruct.y = 4.5;
+}
+
+
+void loop()
+{
+  // Do nothing
+}

examples/i2c_request_datum/i2c_request_datum.ino

@@ -0,0 +1,47 @@
+#include "I2CTransfer.h"
+
+
+I2CTransfer myTransfer;
+
+struct __attribute__((packed)) STRUCT {
+  char z;
+  float y;
+} testStruct;
+
+
+/////////////////////////////////////////////////////////////////// Callbacks
+void hi()
+{
+  myTransfer.rxObj(testStruct);
+  Serial.print(testStruct.z);
+  Serial.println(testStruct.y);
+}
+
+// supplied as a reference - persistent allocation required
+const functionPtr callbackArr[] = { hi };
+///////////////////////////////////////////////////////////////////
+
+
+void setup()
+{
+  Serial.begin(115200);
+  Wire.begin(); // Begin I2C as controller (master)
+
+  ///////////////////////////////////////////////////////////////// Config Parameters
+  configST myConfig;
+  myConfig.debug        = true;
+  myConfig.callbacks    = callbackArr;
+  myConfig.callbacksLen = sizeof(callbackArr) / sizeof(functionPtr);
+  /////////////////////////////////////////////////////////////////
+
+  myTransfer.begin(Wire, myConfig);
+}
+
+
+void loop()
+{
+  // Request datum from connected peripheral at address 0x10
+  // Callback function hi() will be called on reception
+  myTransfer.requestDatum(sizeof(testStruct), 0x10);
+  delay(500);
+}

src/I2CTransfer.cpp

@@ -55,7 +55,8 @@ void I2CTransfer::begin(TwoWire& _port, const bool& _debug, Stream& _debugPort)
   to send as the payload in the next packet
   * const uint8_t &packetID - The packet 8-bit identifier
   * const uint8_t &targetAddress - I2C address to the device the packet
-      will be transmitted to
+      will be transmitted to. If targetAddress == 0xFF (invalid value in I2C),
+	  data is transmitted as a reply to an I2C request.
  Return:
  -------
   * uint8_t numBytesIncl - Number of payload bytes included in packet
@@ -66,11 +67,15 @@ uint8_t I2CTransfer::sendData(const uint16_t& messageLen, const uint8_t& packetI
 
 	numBytesIncl = packet.constructPacket(messageLen, packetID);
 
-	port->beginTransmission(targetAddress);
+	if (targetAddress < 0xFF)
+		port->beginTransmission(targetAddress);
+
 	port->write(packet.preamble, sizeof(packet.preamble));
 	port->write(packet.txBuff, numBytesIncl);
 	port->write(packet.postamble, sizeof(packet.postamble));
-	port->endTransmission();
+
+	if (targetAddress < 0xFF)
+		port->endTransmission();
 
 	return numBytesIncl;
 }

src/I2CTransfer.h

@@ -99,7 +99,54 @@ class I2CTransfer
 	template <typename T>
 	uint8_t sendDatum(const T& val, const uint8_t& packetID = 0, const uint8_t& targetAddress = 0, const uint16_t& len = sizeof(T))
 	{
-		return sendData(packet.txObj(val, packetID, len), packetID, targetAddress);
+		return sendData(packet.txObj(val, 0, len), packetID, targetAddress);
+	}
+
+	/*
+	 uint8_t I2CTransfer::requestDatum(const uint16_t &len, const uint8_t &packetID=0, const uint8_t &targetAddress=0)
+	 Description:
+	 ------------
+	  * Requests "len" number of bytes from the I2C peripheral at specified address. 
+	  * Call rxObj() or setup a rx callback to retrieve the requested data.
+	 Inputs:
+	 -------
+	  * const uint16_t &len - Number of bytes to request
+	  * const uint8_t &targetAddress - I2C address to the device the packet
+	  will be transmitted to
+	 Return:
+	 -------
+	  * uint8_t - Number of payload bytes received
+	*/
+	uint8_t requestDatum(const uint16_t& len, const uint8_t& targetAddress = 0)
+	{
+		uint8_t bytesRead = port->requestFrom(targetAddress, len + PREAMBLE_SIZE + POSTAMBLE_SIZE);
+		processData();
+		return bytesRead;
+	}
+
+	/*
+	 uint8_t I2CTransfer::replyWithDatum(const T &val, const uint8_t &packetID=0, const uint16_t &len=sizeof(T))
+	 Description:
+	 ------------
+	  * Reply to an I2C request (requestDatum) with an arbitrary object
+	  * Stuffs "len" number of bytes of an arbitrary object (byte, int,
+	  float, double, struct, etc...) into the transmit buffer (txBuff)
+	  starting at the index as specified by the argument "index" and
+	  automatically transmits the bytes in an individual packet
+	 Inputs:
+	 -------
+	  * const T &val - Pointer to the object to be copied to the
+	  transmit buffer (txBuff)
+	  * const uint8_t &packetID - The packet 8-bit identifier
+	  * const uint16_t &len - Number of bytes of the object "val" to transmit
+	 Return:
+	 -------
+	  * uint8_t - Number of payload bytes included in packet
+	*/
+	template <typename T>
+	uint8_t replyWithDatum(const T& val, const uint8_t& packetID = 0, const uint16_t& len = sizeof(T))
+	{
+		return sendDatum(val, packetID, 0xFF, len); //Send data with special I2C address 0xFF => don't initiate transmission
 	}