Adding ability to send repeated starts in Wire library.

This adds an additional (boolean) parameter to both endTransmission() and requestFrom(), which specifies whether or not to send a stop condition after the corresponding transmission.  This defaults to true, as in the previous behavior of the library.

http://code.google.com/p/arduino/issues/detail?id=663

Author: damellis

libraries/Wire/Wire.cpp

@@ -15,6 +15,8 @@
   You should have received a copy of the GNU Lesser General Public
   License along with this library; if not, write to the Free Software
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ 
+  Modified 2012 by Todd Krein (todd@krein.org) to implement repeated starts
 */
 
 extern "C" {
@@ -73,24 +75,34 @@ void TwoWire::begin(int address)
   begin((uint8_t)address);
 }
 
-uint8_t TwoWire::requestFrom(uint8_t address, uint8_t quantity)
+uint8_t TwoWire::requestFrom(uint8_t address, uint8_t quantity, uint8_t sendStop)
 {
   // clamp to buffer length
   if(quantity > BUFFER_LENGTH){
     quantity = BUFFER_LENGTH;
   }
   // perform blocking read into buffer
-  uint8_t read = twi_readFrom(address, rxBuffer, quantity);
+  uint8_t read = twi_readFrom(address, rxBuffer, quantity, sendStop);
   // set rx buffer iterator vars
   rxBufferIndex = 0;
   rxBufferLength = read;
 
   return read;
 }
 
+uint8_t TwoWire::requestFrom(uint8_t address, uint8_t quantity)
+{
+  return requestFrom((uint8_t)address, (uint8_t)quantity, (uint8_t)true);
+}
+
 uint8_t TwoWire::requestFrom(int address, int quantity)
 {
-  return requestFrom((uint8_t)address, (uint8_t)quantity);
+  return requestFrom((uint8_t)address, (uint8_t)quantity, (uint8_t)true);
+}
+
+uint8_t TwoWire::requestFrom(int address, int quantity, int sendStop)
+{
+  return requestFrom((uint8_t)address, (uint8_t)quantity, (uint8_t)sendStop);
 }
 
 void TwoWire::beginTransmission(uint8_t address)
@@ -109,10 +121,23 @@ void TwoWire::beginTransmission(int address)
   beginTransmission((uint8_t)address);
 }
 
-uint8_t TwoWire::endTransmission(void)
+//
+//	Originally, 'endTransmission' was an f(void) function.
+//	It has been modified to take one parameter indicating
+//	whether or not a STOP should be performed on the bus.
+//	Calling endTransmission(false) allows a sketch to 
+//	perform a repeated start. 
+//
+//	WARNING: Nothing in the library keeps track of whether
+//	the bus tenure has been properly ended with a STOP. It
+//	is very possible to leave the bus in a hung state if
+//	no call to endTransmission(true) is made. Some I2C
+//	devices will behave oddly if they do not see a STOP.
+//
+uint8_t TwoWire::endTransmission(uint8_t sendStop)
 {
   // transmit buffer (blocking)
-  int8_t ret = twi_writeTo(txAddress, txBuffer, txBufferLength, 1);
+  int8_t ret = twi_writeTo(txAddress, txBuffer, txBufferLength, 1, sendStop);
   // reset tx buffer iterator vars
   txBufferIndex = 0;
   txBufferLength = 0;
@@ -121,6 +146,14 @@ uint8_t TwoWire::endTransmission(void)
   return ret;
 }
 
+//	This provides backwards compatibility with the original
+//	definition, and expected behaviour, of endTransmission
+//
+uint8_t TwoWire::endTransmission(void)
+{
+  return endTransmission(true);
+}
+
 // must be called in:
 // slave tx event callback
 // or after beginTransmission(address)

libraries/Wire/Wire.h

@@ -15,6 +15,8 @@
   You should have received a copy of the GNU Lesser General Public
   License along with this library; if not, write to the Free Software
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+
+  Modified 2012 by Todd Krein (todd@krein.org) to implement repeated starts
 */
 
 #ifndef TwoWire_h
@@ -50,8 +52,11 @@ class TwoWire : public Stream
     void beginTransmission(uint8_t);
     void beginTransmission(int);
     uint8_t endTransmission(void);
+    uint8_t endTransmission(uint8_t);
     uint8_t requestFrom(uint8_t, uint8_t);
+    uint8_t requestFrom(uint8_t, uint8_t, uint8_t);
     uint8_t requestFrom(int, int);
+    uint8_t requestFrom(int, int, int);
     virtual size_t write(uint8_t);
     virtual size_t write(const uint8_t *, size_t);
     virtual int available(void);

libraries/Wire/utility/twi.c

@@ -15,6 +15,8 @@
   You should have received a copy of the GNU Lesser General Public
   License along with this library; if not, write to the Free Software
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+
+  Modified 2012 by Todd Krein (todd@krein.org) to implement repeated starts
 */
 
 #include <math.h>
@@ -37,14 +39,16 @@
 #include "twi.h"
 
 static volatile uint8_t twi_state;
-static uint8_t twi_slarw;
+static volatile uint8_t twi_slarw;
+static volatile uint8_t twi_sendStop;			// should the transaction end with a stop
+static volatile uint8_t twi_inRepStart;			// in the middle of a repeated start
 
 static void (*twi_onSlaveTransmit)(void);
 static void (*twi_onSlaveReceive)(uint8_t*, int);
 
 static uint8_t twi_masterBuffer[TWI_BUFFER_LENGTH];
 static volatile uint8_t twi_masterBufferIndex;
-static uint8_t twi_masterBufferLength;
+static volatile uint8_t twi_masterBufferLength;
 
 static uint8_t twi_txBuffer[TWI_BUFFER_LENGTH];
 static volatile uint8_t twi_txBufferIndex;
@@ -65,6 +69,8 @@ void twi_init(void)
 {
   // initialize state
   twi_state = TWI_READY;
+  twi_sendStop = true;		// default value
+  twi_inRepStart = false;
 
   // activate internal pullups for twi.
   digitalWrite(SDA, 1);
@@ -103,9 +109,10 @@ void twi_setAddress(uint8_t address)
  * Input    address: 7bit i2c device address
  *          data: pointer to byte array
  *          length: number of bytes to read into array
+ *          sendStop: Boolean indicating whether to send a stop at the end
  * Output   number of bytes read
  */
-uint8_t twi_readFrom(uint8_t address, uint8_t* data, uint8_t length)
+uint8_t twi_readFrom(uint8_t address, uint8_t* data, uint8_t length, uint8_t sendStop)
 {
   uint8_t i;
 
@@ -119,6 +126,7 @@ uint8_t twi_readFrom(uint8_t address, uint8_t* data, uint8_t length)
     continue;
   }
   twi_state = TWI_MRX;
+  twi_sendStop = sendStop;
   // reset error state (0xFF.. no error occured)
   twi_error = 0xFF;
 
@@ -135,8 +143,20 @@ uint8_t twi_readFrom(uint8_t address, uint8_t* data, uint8_t length)
   twi_slarw = TW_READ;
   twi_slarw |= address << 1;
 
-  // send start condition
-  TWCR = _BV(TWEN) | _BV(TWIE) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);
+  if (true == twi_inRepStart) {
+    // if we're in the repeated start state, then we've already sent the start,
+    // (@@@ we hope), and the TWI statemachine is just waiting for the address byte.
+    // We need to remove ourselves from the repeated start state before we enable interrupts,
+    // since the ISR is ASYNC, and we could get confused if we hit the ISR before cleaning
+    // up. Also, don't enable the START interrupt. There may be one pending from the 
+    // repeated start that we sent outselves, and that would really confuse things.
+    twi_inRepStart = false;			// remember, we're dealing with an ASYNC ISR
+    TWDR = twi_slarw;
+    TWCR = _BV(TWINT) | _BV(TWEA) | _BV(TWEN) | _BV(TWIE);	// enable INTs, but not START
+  }
+  else
+    // send start condition
+    TWCR = _BV(TWEN) | _BV(TWIE) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);
 
   // wait for read operation to complete
   while(TWI_MRX == twi_state){
@@ -162,13 +182,14 @@ uint8_t twi_readFrom(uint8_t address, uint8_t* data, uint8_t length)
  *          data: pointer to byte array
  *          length: number of bytes in array
  *          wait: boolean indicating to wait for write or not
+ *          sendStop: boolean indicating whether or not to send a stop at the end
  * Output   0 .. success
  *          1 .. length to long for buffer
  *          2 .. address send, NACK received
  *          3 .. data send, NACK received
  *          4 .. other twi error (lost bus arbitration, bus error, ..)
  */
-uint8_t twi_writeTo(uint8_t address, uint8_t* data, uint8_t length, uint8_t wait)
+uint8_t twi_writeTo(uint8_t address, uint8_t* data, uint8_t length, uint8_t wait, uint8_t sendStop)
 {
   uint8_t i;
 
@@ -182,6 +203,7 @@ uint8_t twi_writeTo(uint8_t address, uint8_t* data, uint8_t length, uint8_t wait
     continue;
   }
   twi_state = TWI_MTX;
+  twi_sendStop = sendStop;
   // reset error state (0xFF.. no error occured)
   twi_error = 0xFF;
 
@@ -198,8 +220,23 @@ uint8_t twi_writeTo(uint8_t address, uint8_t* data, uint8_t length, uint8_t wait
   twi_slarw = TW_WRITE;
   twi_slarw |= address << 1;
 
-  // send start condition
-  TWCR = _BV(TWEN) | _BV(TWIE) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);
+  // if we're in a repeated start, then we've already sent the START
+  // in the ISR. Don't do it again.
+  //
+  if (true == twi_inRepStart) {
+    // if we're in the repeated start state, then we've already sent the start,
+    // (@@@ we hope), and the TWI statemachine is just waiting for the address byte.
+    // We need to remove ourselves from the repeated start state before we enable interrupts,
+    // since the ISR is ASYNC, and we could get confused if we hit the ISR before cleaning
+    // up. Also, don't enable the START interrupt. There may be one pending from the 
+    // repeated start that we sent outselves, and that would really confuse things.
+    twi_inRepStart = false;			// remember, we're dealing with an ASYNC ISR
+    TWDR = twi_slarw;				
+    TWCR = _BV(TWINT) | _BV(TWEA) | _BV(TWEN) | _BV(TWIE);	// enable INTs, but not START
+  }
+  else
+    // send start condition
+    TWCR = _BV(TWINT) | _BV(TWEA) | _BV(TWEN) | _BV(TWIE) | _BV(TWSTA);	// enable INTs
 
   // wait for write operation to complete
   while(wait && (TWI_MTX == twi_state)){
@@ -343,7 +380,16 @@ SIGNAL(TWI_vect)
         TWDR = twi_masterBuffer[twi_masterBufferIndex++];
         twi_reply(1);
       }else{
-        twi_stop();
+	if (twi_sendStop)
+          twi_stop();
+	else {
+	  twi_inRepStart = true;	// we're gonna send the START
+	  // don't enable the interrupt. We'll generate the start, but we 
+	  // avoid handling the interrupt until we're in the next transaction,
+	  // at the point where we would normally issue the start.
+	  TWCR = _BV(TWINT) | _BV(TWSTA)| _BV(TWEN) ;
+	  twi_state = TWI_READY;
+	}
       }
       break;
     case TW_MT_SLA_NACK:  // address sent, nack received
@@ -374,6 +420,17 @@ SIGNAL(TWI_vect)
     case TW_MR_DATA_NACK: // data received, nack sent
       // put final byte into buffer
       twi_masterBuffer[twi_masterBufferIndex++] = TWDR;
+	if (twi_sendStop)
+          twi_stop();
+	else {
+	  twi_inRepStart = true;	// we're gonna send the START
+	  // don't enable the interrupt. We'll generate the start, but we 
+	  // avoid handling the interrupt until we're in the next transaction,
+	  // at the point where we would normally issue the start.
+	  TWCR = _BV(TWINT) | _BV(TWSTA)| _BV(TWEN) ;
+	  twi_state = TWI_READY;
+	}    
+	break;
     case TW_MR_SLA_NACK: // address sent, nack received
       twi_stop();
       break;

libraries/Wire/utility/twi.h

@@ -40,8 +40,8 @@
 
   void twi_init(void);
   void twi_setAddress(uint8_t);
-  uint8_t twi_readFrom(uint8_t, uint8_t*, uint8_t);
-  uint8_t twi_writeTo(uint8_t, uint8_t*, uint8_t, uint8_t);
+  uint8_t twi_readFrom(uint8_t, uint8_t*, uint8_t, uint8_t);
+  uint8_t twi_writeTo(uint8_t, uint8_t*, uint8_t, uint8_t, uint8_t);
   uint8_t twi_transmit(const uint8_t*, uint8_t);
   void twi_attachSlaveRxEvent( void (*)(uint8_t*, int) );
   void twi_attachSlaveTxEvent( void (*)(void) );