The 2 Line LCD Display and its HD44780 Controller

Uli Raich

UCC semester 2017/2018

Small devices in the field may nevertheless need to display some values to the user

Small and cheap LCD displays can take over this task

Our display can be used for many purposes (see the open day):

• Name display
• Weather station
• Voltmeter

How to program the PCF8574

Understand the HD44780 display controller

Understand the interface between the two

Since the display can be used for many different purposes,

writing a library is the thing to do!

We will therefore need a great deal of bit fiddling

to create this data byte and to modify it to

• Set the back light on or off
• To send a pulse on the E (strobe) line

• bitwise or operator: |
• bitwise and operator: &
• bit inversion: ~

is a shift register. It can actually do both:

• serial to parallel conversion (write)
  
  
• and parallel to serial conversion (read)

This means that when reading, we will read back

what has been written before, if the signals are pure output signals.

Since the device will put all pins high with a low pull-up current,

external signals can easily pull the pins low

and thus you can read the state of these signals

writing a data byte to the hd44780 controller via the

pcf8574 I/O extender is actually pretty simple:

After this instruction the 8 data bits of bVal lie at the input of the hd44780 controller.

LCD display and it must provide instructions to

• Reset the display
• Clear it
• Have cursor functions
  • Enable/disable the cursor
  • Move it, “Home” it ..
  • Make it blink.
• Have character generators in ROM or
  provide RAM memory to download character matrices (or both)
• Allow to send characters and convert the ASCII code to
  dot matrices understood by the display
  
  
• Switch from one line to the next

• The binary code: libhd44780.so
• The include file hd44780.h

and you should be able to program the display.

Of course you have to include

-I/opt/ucc/include in your CFLAGS

-L/opt/ucc/lib in your LDFLAGS

and -lhd4780 in your LDLIBS

However, we want to understand what is happening behind the scene!

• We can initialize the device ?
• We can write something to it and read it back ?
• Or can we find any other function showing us that the device responds?

It does test the PCF8574 though!

Switching bit 7 in the data word does the trick.

Later we will read a byte from the display and and/or bit seven

into the byte read before writing it back.

indicating that there must be writable 2 registers:

• IR: the instruction register
  
  
• DR: the data register

indicating that it performs an internal operation

and cannot take another command a this very moment.

Then there is an address counter which increments each time

the DR is written, thus pointing to the display data RAM (DDRAM)

location where the next character will be written.

It has a size of 80 characters which in our case corresponds

to 2 lines of 40 chars (of which only 2*16 will be used.

8 bit character code (mainly ascii code) to

• a 5*8 dot matrix with 208 predefined characters
  (this is what we will use)
• a 5*10 dot matrix with 32 characters

create up to 8 5*8 dot matrices yourself

connected to d4-d7. The hd44780 data lines d0 – d3 are disabled

The lower four bits are used to drive

• RS (register select)
• RW (read / write)
• Strobe (E=enable)
• and back light

• Prepare an 8 bit value and put the highest significant
• 4 bits of the data to be written into its lower 4 bits
• Set the R/W bit to low
• Setup the RS bit (0 for IR, 1 for DR)
• Strobe the data into the controller
• Repeat the same thing for the lower significant
  8 bits of the data we want to write
• Keep the BL bit constant

• hd44780WriteCmd (unsigned char cmd)
• hd44780PutC (char c)

while the second one writes a single character to the DR

Of course these two calls use the strobe function explained before.

In these functions I hide all the details of the 4-bit interface

All subsequent functions will pass through these two basic functions.

First we define the function set command

(0x20) with bit 4 set to zero (4 bit interface)

This is written as an 8 bit command (the lower 4 bits are not seen)






Now the controller switches to 4 bit format and

from now on we have to write twice for each

8 bit word sent to the controller.

We re-write the same command in 4 bit mode specifying

the font (5*8 dots) and the no of lines = 2 in addition

These bits were lost in the first (8 bit) transfer