Implement sequential read/write for 16-bit operations (#43)

* Sequential read/write for 16-bit operations ==> ~30% performance improvement

MCP23S17.cpp

@@ -66,10 +66,16 @@ bool MCP23S17::begin(bool pullup)
   if (! isConnected()) return false;
 
   //  disable address increment - datasheet P20
+  //  note that address increment must be enabled for readReg16() and writeReg16() to work.
   //    SEQOP: Sequential Operation mode bit
   //    1 = Sequential operation disabled, address pointer does not increment.
   //    0 = Sequential operation enabled, address pointer increments.
-  //  if (! writeReg(MCP23017_IOCR, MCP23017_IOCR_SEQOP)) return false;
+  uint8_t reg = readReg(MCP23x17_IOCR);
+  if (reg & MCP23x17_IOCR_SEQOP)  //  check if already zero
+  {
+    reg &= ~MCP23x17_IOCR_SEQOP;  //  clear SEQOP bit for sequential read/write
+    if (! writeReg(MCP23x17_IOCR, reg)) return false;
+  }
 
   if (pullup)
   {
@@ -833,11 +839,6 @@ bool MCP23S17::writeReg16(uint8_t reg, uint16_t value)
     _mySPI->transfer(MCP23S17_WRITE_REG | _address );
     _mySPI->transfer(reg);
     _mySPI->transfer(value >> 8);
-    ::digitalWrite(_select, HIGH);
-
-    ::digitalWrite(_select, LOW);
-    _mySPI->transfer(MCP23S17_WRITE_REG | _address );
-    _mySPI->transfer(reg + 1);
     _mySPI->transfer(value & 0xFF);
     _mySPI->endTransaction();
   }
@@ -847,11 +848,6 @@ bool MCP23S17::writeReg16(uint8_t reg, uint16_t value)
     swSPI_transfer(MCP23S17_WRITE_REG | _address );
     swSPI_transfer(reg);
     swSPI_transfer(value >> 8);
-    ::digitalWrite(_select, HIGH);
-
-    ::digitalWrite(_select, LOW);
-    swSPI_transfer(MCP23S17_WRITE_REG | _address );
-    swSPI_transfer(reg + 1);
     swSPI_transfer(value & 0xFF);
   }
   ::digitalWrite(_select, HIGH);
@@ -880,11 +876,6 @@ uint16_t MCP23S17::readReg16(uint8_t reg)
     _mySPI->transfer(MCP23S17_READ_REG | _address );
     _mySPI->transfer(reg);
     rv = _mySPI->transfer(0xFF) << 8;
-    ::digitalWrite(_select, HIGH);
-
-    ::digitalWrite(_select, LOW);
-    _mySPI->transfer(MCP23S17_READ_REG | _address );
-    _mySPI->transfer(reg + 1);
     rv += _mySPI->transfer(0xFF);
     _mySPI->endTransaction();
   }
@@ -894,11 +885,6 @@ uint16_t MCP23S17::readReg16(uint8_t reg)
     swSPI_transfer(MCP23S17_READ_REG | _address );
     swSPI_transfer(reg);
     rv = swSPI_transfer(0xFF) << 8;
-    ::digitalWrite(_select, HIGH);
-
-    ::digitalWrite(_select, LOW);
-    swSPI_transfer(MCP23S17_READ_REG | _address );
-    swSPI_transfer(reg + 1);
     rv += swSPI_transfer(0xFF);
   }
   ::digitalWrite(_select, HIGH);