Protocol for writing data to 16x2 LCD via I2C

Question

I am new to electronics and has completed a tutorial on how to operate a 16x2 Character LCD via I2C in Arduino using liquidCrystal_I2C. Everything works fine but I have a question about the low level interaction between the I2C and the LCD. Looking at the library's source code, I notice that when writing a 4 bits nibble (LiquidCrystal_I2C::write4bits), the code writes the nibble to the I2C expander first (LiquidCrystal_I2C::expanderWrite), and then writes again when pulsing the Enable bit. Why is the first expanderWrite necessary? Why can't write4bits just call pulseEnable (with the blacklight bit set)?

I am sure there is a reason as I checked other library like RPLCD and see a similar pattern. Can anyone enlighten me? Thank you.

Answer 1

From the datasheet I found the LCD requires specific timing in the communication protocol. On the rising edge of the enable line the Register Select and Read/Write lines must have already settled for tsu1 (100ns). On the falling edge of the enable line the data must have already settled for tsu2 (60ns). By writing _data they are also writing the RS and R/W lines as they are the lower nibble of _data.

This article covers the topic very thoroughly.

//**** From LiquidCrystal_I2C.h
// flags for backlight control
#define LCD_BACKLIGHT 0x08
#define LCD_NOBACKLIGHT 0x00

#define En B00000100  // Enable bit
#define Rw B00000010  // Read/Write bit
#define Rs B00000001  // Register select bit
//              ^--------Backlight bit defined above
//          ^^^^---------Data bits

//**** From LiquidCrystal_I2C.cpp
void LiquidCrystal_I2C::write4bits(uint8_t value) {
    expanderWrite(value);
    pulseEnable(value);
}

void LiquidCrystal_I2C::expanderWrite(uint8_t _data){
    Wire.beginTransmission(_addr);
    Wire.write((int)(_data) | _backlightval);
    Wire.endTransmission();
}

void LiquidCrystal_I2C::pulseEnable(uint8_t _data){
    expanderWrite(_data | En);  // En high
    delayMicroseconds(1);       // enable pulse must be >450ns

    expanderWrite(_data & ~En); // En low
    delayMicroseconds(50);      // commands need > 37us to settle
}