Once you’ve played with LEDs, switches and stepper motors the next natural step is 16×2 alphanumeric LCD modules. These modules are cheap (less than $10) and easy to interface to the Raspberry Pi. They have 16 connections but you only need to use 6 GPIO pins on your Pi.
Most of the 16×2 modules available are compatible with the Hitachi HD44780 LCD controller. This allows you to buy almost any device and be sure it is going to work in much the same way as any other. There are loads to choose from on eBay with different coloured backlights. The one I purchased had a blue backlight.
LCD Module Hardware
- VCC (Usually +5V)
- Contrast adjustment (VO)
- Register Select (RS).
RS=0: Command, RS=1: Data
- Read/Write (R/W).
R/W=0: Write, R/W=1: Read
- Bit 0 (Not required in 4-bit operation)
- Bit 1 (Not required in 4-bit operation)
- Bit 2 (Not required in 4-bit operation)
- Bit 3 (Not required in 4-bit operation)
- Bit 4
- Bit 5
- Bit 6
- Bit 7
- LED Backlight Anode (+)
- LED Backlight Cathode (-)
Usually the device requires 8 data lines to provide data to Bits 0-7. However the device can be set to a “4 bit” mode which allows you to send data in two chunks (or nibbles) of 4 bits. This is great as it reduces the number of GPIO connections you require when interfacing with your Pi.
Here is how I wired up my LCD :
|LCD Pin||Function||Pi Function||Pi Pin|
|15||+5V via 560ohm|
NOTE : The RW pin allows the device to be be put into read or write mode. I wanted to send data to the device but did not want it to send data to the Pi so I tied this pin to ground. The Pi can not tolerate 5V inputs on its GPIO header. Tying RW to ground makes sure the device does not attempt to pull the data lines to 5V which would damage the Pi.
In order to control the contrast you can adjust the voltage presented to Pin 3. This must be between 0 and 5V. I tied this pin to ground.
Pin 15 provides 5V to the backlight LED. It wasn’t clear on my device if this could be connected direct to 5V so I played safe and placed a 560ohm resistor in line with this pin.
Here are some sanity checks before you power up your circuit for the first time :
- Pin 1 (GND), 3 (Contrast), 5 (RW) and 16 (LED -) ( should be tied to ground.
- Pin 2 should be tied to 5V. Pin 15 should have a resistor inline to 5V to protect the backlight.
- Pin 7-10 are unconnected
- Pin 11-14 are connected to GPIO pins on the Pi
You can control a HD44780 style display using any programmming environment you like but my weapon of choice is Python. I use the RPi.GPIO library to provide access to the GPIO.
Here is my code :
#!/usr/bin/python # # HD44780 LCD Test Script for # Raspberry Pi # # Author : Matt Hawkins # Site : http://www.raspberrypi-spy.co.uk # # Date : 26/07/2012 # # The wiring for the LCD is as follows: # 1 : GND # 2 : 5V # 3 : Contrast (0-5V)* # 4 : RS (Register Select) # 5 : R/W (Read Write) - GROUND THIS PIN # 6 : Enable or Strobe # 7 : Data Bit 0 - NOT USED # 8 : Data Bit 1 - NOT USED # 9 : Data Bit 2 - NOT USED # 10: Data Bit 3 - NOT USED # 11: Data Bit 4 # 12: Data Bit 5 # 13: Data Bit 6 # 14: Data Bit 7 # 15: LCD Backlight +5V** # 16: LCD Backlight GND #import import RPi.GPIO as GPIO import time # Define GPIO to LCD mapping LCD_RS = 7 LCD_E = 8 LCD_D4 = 25 LCD_D5 = 24 LCD_D6 = 23 LCD_D7 = 18 # Define some device constants LCD_WIDTH = 16 # Maximum characters per line LCD_CHR = True LCD_CMD = False LCD_LINE_1 = 0x80 # LCD RAM address for the 1st line LCD_LINE_2 = 0xC0 # LCD RAM address for the 2nd line # Timing constants E_PULSE = 0.00005 E_DELAY = 0.00005 def main(): # Main program block GPIO.setmode(GPIO.BCM) # Use BCM GPIO numbers GPIO.setup(LCD_E, GPIO.OUT) # E GPIO.setup(LCD_RS, GPIO.OUT) # RS GPIO.setup(LCD_D4, GPIO.OUT) # DB4 GPIO.setup(LCD_D5, GPIO.OUT) # DB5 GPIO.setup(LCD_D6, GPIO.OUT) # DB6 GPIO.setup(LCD_D7, GPIO.OUT) # DB7 # Initialise display lcd_init() # Send some test lcd_byte(LCD_LINE_1, LCD_CMD) lcd_string("Rasbperry Pi") lcd_byte(LCD_LINE_2, LCD_CMD) lcd_string("Model B") time.sleep(3) # 3 second delay # Send some text lcd_byte(LCD_LINE_1, LCD_CMD) lcd_string("Raspberrypi-spy") lcd_byte(LCD_LINE_2, LCD_CMD) lcd_string(".co.uk") time.sleep(20) def lcd_init(): # Initialise display lcd_byte(0x33,LCD_CMD) lcd_byte(0x32,LCD_CMD) lcd_byte(0x28,LCD_CMD) lcd_byte(0x0C,LCD_CMD) lcd_byte(0x06,LCD_CMD) lcd_byte(0x01,LCD_CMD) def lcd_string(message): # Send string to display message = message.ljust(LCD_WIDTH," ") for i in range(LCD_WIDTH): lcd_byte(ord(message[i]),LCD_CHR) def lcd_byte(bits, mode): # Send byte to data pins # bits = data # mode = True for character # False for command GPIO.output(LCD_RS, mode) # RS # High bits GPIO.output(LCD_D4, False) GPIO.output(LCD_D5, False) GPIO.output(LCD_D6, False) GPIO.output(LCD_D7, False) if bits&0x10==0x10: GPIO.output(LCD_D4, True) if bits&0x20==0x20: GPIO.output(LCD_D5, True) if bits&0x40==0x40: GPIO.output(LCD_D6, True) if bits&0x80==0x80: GPIO.output(LCD_D7, True) # Toggle 'Enable' pin time.sleep(E_DELAY) GPIO.output(LCD_E, True) time.sleep(E_PULSE) GPIO.output(LCD_E, False) time.sleep(E_DELAY) # Low bits GPIO.output(LCD_D4, False) GPIO.output(LCD_D5, False) GPIO.output(LCD_D6, False) GPIO.output(LCD_D7, False) if bits&0x01==0x01: GPIO.output(LCD_D4, True) if bits&0x02==0x02: GPIO.output(LCD_D5, True) if bits&0x04==0x04: GPIO.output(LCD_D6, True) if bits&0x08==0x08: GPIO.output(LCD_D7, True) # Toggle 'Enable' pin time.sleep(E_DELAY) GPIO.output(LCD_E, True) time.sleep(E_PULSE) GPIO.output(LCD_E, False) time.sleep(E_DELAY) if __name__ == '__main__': main()
If you use this code the only thing you will need to change is the GPIO pin mapping depending on what pins you use on your Pi GPIO header. Here are some photos :
Additional Notes : RS is low when sending a command to the LCD and high when sending a character. RW is always low to ensure we only ever input data into the module. 8 bit bytes are sent 4 bits at a time. Top 4 bits first and the last 4 bits second. Delays are added between certain steps to ensure the module can react to the signal before it changes.
The code above was inspired by code submitted by ‘texy’ on the RaspberryPi.org forum. I changed the way the bytes are broken down to bits as this significantly increased the response time of the display.