Speed up transfers

An I2C transmission has a significant overhead of ~10+1 I2C clock cycles.
Reducing the number of beginTransmission()/endTransmission() pairs
from twice every nibble to once per _send() hence speeds up transmission
times significantly.

The specified I2C clock rate speed of the PCF8574 chip is 100KHz.
Transmitting a single byte over I2C takes 9 clock ticks or 90us.
Even if the I2C bus is operated beyond the chip's spec in fast mode
at 400 kHz, any additional delays during transmission shorter than this
are unnecessary.

Commands are only executed after the second nibble is transmitted.
The 37us delay after the first nibble is hence unnecessary.

We now omit any additional delays and perform a transmission only once
per _send() command. This reduces the time from 4x(11+9) = 80 cycles to
4x9 + 11 = 47 cycles. Taking the delays into account, this changes the
time per command from 800us + 4*38us = 952us to 470us at the default
100kHz bus speed.

Author: markisch

src/LiquidCrystal_PCF8574.cpp

@@ -267,7 +267,7 @@ void LiquidCrystal_PCF8574::createChar_P(uint8_t location, const byte *charmap)
     byte c = pgm_read_byte(p++);
     write(c);
   }
-} // createCharPgm()
+} // createChar_P()
 #endif
 
 
@@ -282,33 +282,58 @@ inline size_t LiquidCrystal_PCF8574::write(uint8_t ch)
 // write either command or data
 void LiquidCrystal_PCF8574::_send(uint8_t value, bool isData)
 {
+  // An I2C transmission has a significant overhead of ~10+1 I2C clock
+  // cycles. We consequently only perform it only once per _send().
+
+  Wire.beginTransmission(_i2cAddr);
   // write high 4 bits
-  _sendNibble((value >> 4 & 0x0F), isData);
+  _writeNibble((value >> 4 & 0x0F), isData);
   // write low 4 bits
-  _sendNibble((value & 0x0F), isData);
+  _writeNibble((value & 0x0F), isData);
+  Wire.endTransmission();
 } // _send()
 
 
 // write a nibble / halfByte with handshake
-void LiquidCrystal_PCF8574::_sendNibble(uint8_t halfByte, bool isData)
+void LiquidCrystal_PCF8574::_writeNibble(uint8_t halfByte, bool isData)
 {
   // map the data to the given pin connections
-  uint8_t data = 0;
+  uint8_t data = isData ? _rs_mask : 0;
+  // _rw_mask is not used here.
+  if (_backlight > 0)
+    data |= _backlight_mask;
 
   // allow for arbitrary pin configuration
   if (halfByte & 0x01) data |= _data_mask[0];
   if (halfByte & 0x02) data |= _data_mask[1];
   if (halfByte & 0x04) data |= _data_mask[2];
   if (halfByte & 0x08) data |= _data_mask[3];
 
-  _write2Wire(data, isData, true);
-  delayMicroseconds(1); // enable pulse must be >450ns
-  _write2Wire(data, isData, false);
-  delayMicroseconds(37); // commands need > 37us to settle
+  // Note that the specified speed of the PCF8574 chip is 100KHz.
+  // Transmitting a single byte takes 9 clock ticks at 100kHz -> 90us.
+  // The 37us delay is only necessary after sending the second nibble.
+  // But in that case we have to restart the transfer using additional 
+  // >10 clock cycles. Hence, no additional delays are necessary even
+  // when the I2C bus is operated beyond the chip's spec in fast mode
+  // at 400 kHz.
+
+  Wire.write(data | _enable_mask);
+  // delayMicroseconds(1); // enable pulse must be >450ns
+  Wire.write(data);
+  // delayMicroseconds(37); // commands need > 37us to settle
+} // _writeNibble
+
+
+// write a nibble / halfByte with handshake
+void LiquidCrystal_PCF8574::_sendNibble(uint8_t halfByte, bool isData)
+{
+  Wire.beginTransmission(_i2cAddr);
+  _writeNibble(halfByte, isData);
+  Wire.endTransmission();
 } // _sendNibble
 
 
-// private function to change the PCF8674 pins to the given value
+// private function to change the PCF8574 pins to the given value
 void LiquidCrystal_PCF8574::_write2Wire(uint8_t data, bool isData, bool enable)
 {
   if (isData)

src/LiquidCrystal_PCF8574.h

@@ -98,6 +98,7 @@ class LiquidCrystal_PCF8574 : public Print
   // low level functions
   void _send(uint8_t value, bool isData = false);
   void _sendNibble(uint8_t halfByte, bool isData = false);
+  void _writeNibble(uint8_t halfByte, bool isData);
   void _write2Wire(uint8_t data, bool isData, bool enable);
 
   void init(uint8_t i2cAddr, uint8_t rs, uint8_t rw, uint8_t enable,