In my previous post I had used an 8 bit i2c port expander to drive the 16x2 LCD. It saved precious GPIO pins but added complexity and cost. In this post I will be using the RPi.GPIO library and Python to control the LCD.The LCD used in this post is based on Hitachi HD44780 LCD controller.Although the LCD has 16 pins available for interfacing, using the 4 bit mode only 6 GPIO pins are required ( RS,E,D4,D5,D6,D7).                                                                               


  LCD Pin    Pi Pin
     01  <------>  GPIO-06
     02  <------>  GPIO-02
     03  <------>  GPIO-06
     04  <------>  GPIO-26
     05  <------>  GPIO-06
     06  <------>  GPIO-24
     07
     08
     09
     10
     11  <------>  GPIO-22
     12  <------>  GPIO-18
     13  <------>  GPIO-16
     14  <------>  GPIO-12
     15  +5V    
     16  <------>  GPIO-06

NOTE : With the help of  RW pin the device can be set to read/write mode.Setting [R/W=0] will write to the register and setting [R/W=1] will read from the register.To display data on LCD read access is not required,so the RW in connected to GND. This ensures that there is no outbound data from HD44780 as Pi cannot tolerate 5V.

You can check the pinout of Pi from here.

Code:-

HD44780 based display can be controlled using any programming environment.Here I have used Python & RPi.GPIO library to provide access to the GPIO.

#!/usr/bin/python
import RPi.GPIO as GPIO
from time import sleep
class HD44780:
    def __init__(self, pin_rs=7, pin_e=8, pins_db=[25, 24, 23, 18]):
        self.pin_rs=pin_rs
        self.pin_e=pin_e
        self.pins_db=pins_db
        GPIO.setmode(GPIO.BCM)
        GPIO.setup(self.pin_e, GPIO.OUT)
        GPIO.setup(self.pin_rs, GPIO.OUT)
        for pin in self.pins_db:
            GPIO.setup(pin, GPIO.OUT)
        self.clear()
    def clear(self):
        """ Blank / Reset LCD """
        self.cmd(0x33) # $33 8-bit mode
        self.cmd(0x32) # $32 8-bit mode
        self.cmd(0x28) # $28 8-bit mode
        self.cmd(0x0C) # $0C 8-bit mode
        self.cmd(0x06) # $06 8-bit mode
        self.cmd(0x01) # $01 8-bit mode
    def cmd(self, bits, char_mode=False):
        """ Send command to LCD """
        sleep(0.001)
        bits=bin(bits)[2:].zfill(8)
        GPIO.output(self.pin_rs, char_mode)
        for pin in self.pins_db:
            GPIO.output(pin, False)
        for i in range(4):
            if bits[i] == "1":
                GPIO.output(self.pins_db[::-1][i], True)
        GPIO.output(self.pin_e, True)
        GPIO.output(self.pin_e, False)
        for pin in self.pins_db:
            GPIO.output(pin, False)
        for i in range(4,8):
            if bits[i] == "1":
                GPIO.output(self.pins_db[::-1][i-4], True)
        GPIO.output(self.pin_e, True)
        GPIO.output(self.pin_e, False)
    def message(self, text):
        """ Send string to LCD. Newline wraps to second line"""
        for char in text:
            if char == '\n':
                self.cmd(0xC0) # next line
            else:
                self.cmd(ord(char),True)
if __name__ == '__main__':
    lcd = HD44780()
    lcd.message("Raspberry Pi\n  Take a byte!")

One of the few things that separates the Pi from other SBC (Single Board Computer)  is the ability to use the GPIO (General Purpose Input/Output) pins which can be set as HIGH or LOW to control any external devices. All you need is a female to male jumper wire to get started. Here I have used a HDD IDE connector to get the job done.

In this post pin 9 is used for GND and pin 11 for GPIO17. The LED was connected using a 470 ohm register in series with pin 9 and 11 to limit the current.

Software Implementation:-

The fastest way to get started is to use python which comes pre-installed with all images. Download the RPi.GPIO library and copy the gz tar ball to the RPi wheezy raspbian. Open the terminal and navigate to the extracted folder containing the RPi.GPIO library. Then type:  $ sudo python setup.py install to install the module. Imp: As the OS is multitasking  and not Real-time unlike Arduino there may be jitters depending on CPU priority.

Based on the library I have written a simple code to turn ON and turn OFF the LED after a delay of 1 sec (1000ms) each.The LED blinks 50 times.

import RPi.GPIO as GPIO
import time
# blinking function
def blink(pin):
        GPIO.output(pin,GPIO.HIGH)
        time.sleep(1)
        GPIO.output(pin,GPIO.LOW)
        time.sleep(1)
        return
# to use Raspberry Pi board pin numbers
GPIO.setmode(GPIO.BOARD)
# set up GPIO output channel
GPIO.setup(11, GPIO.OUT)
# blink GPIO17 50 times
for i in range(0,50):
        blink(11)
GPIO.cleanup()