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Sunday, 8 March 2020

How to connect an LCD display with I2C bus to ESP


The changes hw & sw are described to adapt an I2C LCD display to ESPs or other microcontrollers powered at 3.3V.

An LCD display, compatible with the Hitachi HD44780 controller, connected with 4 data bus bits, requires a minimum of 6 GPIO pins. Using a special module it is possible to use only two pins with I2C bus and several Arduino libraries are also available for it. I used the LiquidCrystal_I2C [1] which, although the Arduino compiler prints a warning message that is for AVRs, has also worked well for ESP.
Figure 1 shows the module I used, made specifically to be soldered directly to a common alphanumeric LCD display.
 
Figure 1
Before soldering it, check the position of the pins and compare them with those in the figure. The complete scheme of this module is shown in figure 2.
Figure 2
This module uses the PCF8574 chip, 8-bit I/O expander with I2C bus, which connects to the 4-bit bidirectional bus display. It also controls the display backlighting via PA3 and the transistor Q1, with the jumper inserted.

I preferred to use an LCD display because it consumes much less than an Oled, and my goal was to operate the ESP in deep sleep mode and power everything on battery power.
For these reasons I used a display without backlight, with a 14-pin connection and therefore I had to cut the two pins 15 and 16 of the module in order to insert it.

I/O level adapting
No problem for interfacing with Arduino Uno and similar with 5V power supply and I/O signals with the same level.
The ESP8266/32 have 3.3V power supply and GPIO levels are not 5V tolerant, so the first thing I looked at is the level of the output signals.
The characteristic of I2C is that every device on the bus must connect to both the clock signal (SCL) and the data signal (SDA) via open-drain (or open-collector) output drivers. This means that controllers can only bring the output to zero and that pull-up resistors must be used to return to high level.
So it is not necessary to use a level adapter, but to check if the pull-up resistors are mounted and at what voltage. As can be seen in the diagram, these are present and connected to the 5V.
If we want to connect this module to ESP or other 3.3V powered microcontrollers, we have to unsolder the 4.7 kW resistors R5 and R6. Since my intention is also to consume less current, I also removed R1 (1 kW) of the useless red  LED1 and R7, near the transistor Q1. Figure 3 shows the positions of the four resistors to be unsoldered. If you want to use a backlit display, R7 should not be removed.
Figure 3

Obviously two 4.7 kW resistors connected to + 3.3V must be mounted externally, as in figure 4.
Then we have to set the I2C address using the three jumpers near the trimmer that regulates the contrast. The three 10 kW resistors R2, R3, R4 are pull-ups of the three bits A0, A1, A2 of address I2C of the PCF8574. The other four bits, of the seven I2C address bits, are fixed and have the following values:
0
1
0
0
A2
A1
A0
If we do not solder the jumpers: A0 = A1 = A2 = 1, so the address will be 0x27, each jumper will produce a zero as it is connected to ground.
Figure 4 shows the connections of the LCD display complete with adapter with a NodeMCU card with ESP8266. I used pin D5 (GPIO14) for SCL and D4 (GPIO2) for SDA.
Figure 4

Figure 5 shows the two-line 16-character display to which the modified module has been soldered. Consumption is around 2 mA.
Figure 5
Figure 6 shows the display under test, made with a breadboard for the resistors and the connection wires with the NodeMCU board.

Anyone wishing to mount a display with backlighting must, in addition to not remove the R7 and not cut pins 15 and 16, also enter the instruction that lights the display LEDs:
   lcd.backlight ();
In the next realization I intend to use only the ESP-V12f module, always to reduce power consumption.
Figure 6
The test program
To test the system I wrote a simple program that prints the seconds on the second line.
To avoid overlapping the text on the previous print, I first print a row of spaces.
The library must be configured with the instruction:
LiquidCrystal_I2C lcd (0x27,16,2);
Which allows you to set the display address (0x27), the number of columns and rows.
If more than one LCD display is used, jumpers must be set to create a different address.
On the ESP8266 almost all I/O bits can be used for the I2C interface. The LiquidCrystal_I2C library allows you to assign the two pins using the function:
lcd.begin (SDA, SCL);
Below is the list of the test programs.
/*program ESP_LCD.ino, for test
 ESP8266 with a I2C LCD display
 Giovanni Carrera, rev. 22/02/2020 */

#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#define SDA 2    // D4 for NodeMCU
#define SCL 14   // D5 for NodeMCU


char bline[17] = "                ";// blank line
int sec = 0;
unsigned long cmilli, pmilli;
const int deltat = 1000;

LiquidCrystal_I2C lcd(0x27,16,2); // set the LCD address to 0x27 for a 16 chars and 2 line display

void setup(){
  Wire.begin (SDA, SCL);// pin used for I2C bus
  lcd.begin(SDA, SCL);// for ESP8266
  lcd.setCursor(0,0);// lcd.setCursor(col, row)
  lcd.print("ESPV12 LCD test"); // Print intro message
}

void loop(){
  cmilli = millis();
  if (cmilli - pmilli >= deltat) {
    pmilli = cmilli;
    lcd.setCursor(0, 1);
    lcd.print(bline);// clear the second row
    lcd.setCursor(0, 1);
    lcd.print(sec);// print seconds on the second row
    sec++;
    lcd.setCursor(6, 1);
    lcd.print("seconds");   
  }
}


References
1.       “LiquidCrystal I2C”, Marco Schwartz, https://github.com/marcoschwartz/LiquidCrystal_I2C/archive/master.zip


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