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Beginner’s Guide

to the

PI LCD

Part 3: Graphics Intro

, W8BH

1) INTRODUCTION

In Part 1 and Part 2 of this series, we learned how to check the switches and display text on

the LCD board (available from mypishop.com). Now it’s time to learn how to display more

complex data, including graphics. You may have purchased either a 16x2 or a 20x4 display

with your kit. In this write-up I’ll be using the 20x4 display, but a 16x2 will work just as well.

2) THE HD44780 CHARACTER SETS

Here is a list of characters that our LCD can

display. Numbers, uppercase letters, and

lowercase letters are in their standard ASCII

positions. There is a block of unused character

positions in the middle at addresses 0x80 to

0x9F. On the right is a set of Japanese katakana

characters. This ‘A0’ character set is the set

available on my display.

Some controller chips are burned with a different

character set, one that contains European and

Cyrillic characters instead of the Japanese ones.

Interesting! Which one do you have? Let’s find

out.

The easiest way is to send a single character to

the screen, and see what it looks like. For

example, character 0xE0 on my display is the

Greek letter alpha (α). On a chip with the A2

Summary of content (11 pages)

PAGE 1
Beginner’s Guide to the PI LCD Part 3: Graphics Intro , W8BH 1) INTRODUCTION In Part 1 and Part 2 of this series, we learned how to check the switches and display text on the LCD board (available from mypishop.com). Now it’s time to learn how to display more complex data, including graphics. You may have purchased either a 16x2 or a 20x4 display with your kit. In this write-up I’ll be using the 20x4 display, but a 16x2 will work just as well.
PAGE 2
character set, 0xE0 is an a-grave (à). #determine the character set SendByte(0x01) #clear the display SendByte(0xE0,True) #Either alpha (A0) or a-grave (A2) Does your controller chip have A0 or A2? Now, just for fun, let’s fill up the display with characters. I set up this routine for 4 rows of 15 characters each, giving me extra space for a text label.
PAGE 3
SendByte(code,True) code = GetNextCharacter(code) time.sleep(delay) def CharTest(numCycles): for count in range(numCycles): rand = random.randint(0,255) firstChar = GetNextCharacter(rand) FillScreen(firstChar,0.03) CharTest(5) #Five screens of characters 3) TIMING IS EVERYTHING If you are like me, you’ll watch some slow character screens for a while, then remove all delays to see how zippy it is.
PAGE 4
) FASTER, FASTER The functions in Part 2 included low-level routines for sending data to the LCD. One of those routines included timing delays required by the LCD. The routines worked, and I was happy. But now I wonder: can we shorten any of the delays? And if so, will it speed up our data transfer to the LCD? We now have a tool, TimeTest, that will measure time and give us a visual indicator of success. Let’s look at the routine with the most delays: PulseEnableLine.
PAGE 5
) CUSTOM CHARACTERS The LCD controller is great for ready-made alphanumeric characters: send the code for a character in ROM, and it is displayed on the LCD. But there is also a very small amount of RAM reserved for custom characters. The controller can hold up to 8 characters of your own design.
PAGE 6
) LOADING THE CG-RAM Now that we have our artwork encoded, it’s time to load it into the character-generator RAM. Remember that we have 8 slots (of 8 bytes each), so our list of 7 symbols will fit. To load a single character, we send a ‘LOADSYMBOL’ (0x40) command which includes the start address in its lower 6 bits. For example, if we want to store an 8 bit character for position 0x02, the command will be 0x40 + 0x10 = 0x50.
PAGE 7
) PYTHON SCRIPT for PI LCD, PART 3: # ######################################################################## # # LCD2: Learning how to control an LCD module from Pi # # # # This code assumes 20x4 display, but will run on 16x2 display. # See w8bh.net for more information. # ######################################################################## import time import RPi.
PAGE 8
] ######################################################################## # # Low-level routines for configuring the LCD module. # These routines contain GPIO read/write calls. # def InitIO(): #Sets GPIO pins to input & output, as required by LCD board GPIO.setmode(GPIO.BCM) GPIO.setwarnings(False) for lcdLine in OUTPUTS: GPIO.setup(lcdLine, GPIO.OUT) for switch in INPUTS: GPIO.setup(switch, GPIO.IN, pull_up_down=GPIO.
PAGE 9
SendByte(CURSOROFF) def CursorBlink(): SendByte(CURSORBLINK) def CursorLeft(): SendByte(CURSORLEFT) def CursorRight(): SendByte(CURSORRIGHT) def InitLCD(): #initialize the LCD controller SendByte(0x33) SendByte(0x32) SendByte(0x28) SendByte(LEFTTORIGHT) CursorOff() ClearDisplay() & clear display #initialize #initialize/4-bit #4-bit, 2 lines, 5x8 font #rightward moving cursor def SendChar(ch): SendByte(ord(ch),True) def ShowMessage(string): #Send string of characters to display at current cursor position f
PAGE 10
######################################################################## # # Basic HD44780/LCD Test Routines # # def LabelTest(label): #Label the current Test ClearDisplay() GotoXY(1,20-len(label)); ShowMessage(label) GotoXY(2,16); ShowMessage('test') def CommandTest(): LabelTest('Command') while (True): st = raw_input("Enter a string or command: ") if len(st)==2: SendByte(int(st,16)) else: ShowMessage(st) def AnimateCharTest(numCycles=8,delay=0.
PAGE 11
def FillChar(code): #fill the LCD display with a single ASCII character #assumes a width of 15 characters x 4 lines = 60 chars total for count in range(60): UpdateCursor(count) SendByte(code,True) def CharTest(numCycles=4, delay=0.03): #show screenfull of sequential symbols from character set #starting with a random symbol #delay = time between characters LabelTest('Char') for count in range(numCycles): rand = random.