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Beginner’s Guide

to the

PI LCD

Part 2: Display

, W8BH

1) INTRODUCTION

In Part 1 of this series, we assembled the Pi LCD from mypishop.com, and tested the

‘wiring and the switches’. Now it’s time to concentrate on the display itself. You may have

purchased either a 16x2 or a 20x4 display with your kit. I’ll assume that you have the 16x2

display, and show you the small changes required for using the larger display.

2) THE HD44780 LCD CONTROLLER

Our display, like many backlit LCD modules available today, uses a 16-pin interface. We

should thank Hitachi for

developing this widely used,

informal standard.

Pins 1 & 2 power the

controller. Pin 3 is connected

to the potentiometer and used

to adjust display contrast.

Pins 15 & 16 power the

backlight. On our board, we

don’t need to read data from

the display, so we keep the

Read/Write line tied to

ground. This leaves 10

digital pins we can use for

data communication.

Summary of content (7 pages)

PAGE 1
Beginner’s Guide to the PI LCD Part 2: Display , W8BH 1) INTRODUCTION In Part 1 of this series, we assembled the Pi LCD from mypishop.com, and tested the ‘wiring and the switches’. Now it’s time to concentrate on the display itself. You may have purchased either a 16x2 or a 20x4 display with your kit. I’ll assume that you have the 16x2 display, and show you the small changes required for using the larger display.
PAGE 2
The LCD controller gives us two ways to send data: in one-byte (8 bit) chunks, or as two consecutive nibbles (4 bit). The second approach seems more complicated, but very frequently used. Why? It simplifies the hardware, and lets us talk to the LCD with half as many data lines. In general, I/O lines are a scarce resource! We use the 4-bit method on this board, and need only 6 I/O lines (2 control + 4 data) to communicate with the LCD.
PAGE 3
time.sleep(mSec) GPIO.output(LCD_E, GPIO.LOW) time.sleep(mSec) #return E low #wait before doing anything else Each waiting period is specified in the HD44780 datasheet. You can minimize wait times by following the specifications exactly. I chose a lazier method. I empirically picked a delay that is longer than necessary, but short enough to minimize visual distractions: one millisecond. It works. Even half a millisecond is OK on my displays.
PAGE 4
) INPUT & OUTPUT Let’s combine our string-writing ability with the switch input routine from part 1. In Part 1 we read the status of each switch and displayed it on the console. Now we will display switch status on the LCD screen instead. First, write something meaningful on the display: WriteMessage(‘Press a switch...’) And put the switch status on the second display line. Wait a sec, how do we put stuff on the second line? Send a carriage return? Sorry, that doesn’t work.
PAGE 5
6) PYTHON SCRIPT for PI LCD, PART 2: ######################################################################## # # LCD2: Learning how to control an LCD module from Pi # # # ######################################################################## import time import RPi.GPIO as GPIO #for timing delays #OUTPUTS: map GPIO to LCD lines LCD_RS = 7 #GPIO7 = Pi pin 26 LCD_E = 8 #GPIO8 = Pi pin 24 LCD_D4 = 17 #GPIO17 = Pi pin 11 LCD_D5 = 18 #GPIO18 = Pi pin 12 LCD_D6 = 27 #GPIO21 = Pi pin 13 (Use 21 on Rev.
PAGE 6
val1 = val2 = val3 = val4 = return not GPIO.input(SW1) not GPIO.input(SW2) not GPIO.input(SW3) not GPIO.input(SW4) (val4,val1,val2,val3) def PulseEnableLine(): #Pulse the LCD Enable line; used for clocking in data mSec = 0.0005 #use half-millisecond delay time.sleep(mSec) #give time for inputs to settle GPIO.output(LCD_E, GPIO.HIGH) #pulse E high time.sleep(mSec) GPIO.output(LCD_E, GPIO.LOW) #return E low time.sleep(mSec) #wait before doing anything else def SendNibble(data): #sends upper 4 bits GPIO.
PAGE 7
######################################################################## # # Main Program # print "Pi LCD2 program starting. Ctrl-C to stop." InitIO() InitLCD() ShowMessage('Press a switch!') while (True): GotoLine(1) switchValues = CheckSwitches() decimalResult = " %d %d %d %d" % switchValues ShowMessage(decimalResult) time.sleep(0.