Lecture11:The2LineLCDDisplay < Embedded

The 2 Line LCD Display and its HD44780 Controller

Uli Raich

UCC semester 2017/2018

Slide 1 of 15

Why a simple 2 line display?

Often embedded systems do not have a screen, keyboard and mouse associated with them.

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

and there are many more applications

Slide 2 of 15

Our Display

Slide 3 of 15

Interfacing the display

Slide 4 of 15

The PCF8574 I2C interface

The PCF8574 I2C I/O expander allows to limit the number of connection wires to just four.
It is a kind of a shift register: serial in, parallel out

Slide 5 of 15

Interfacing signals

The display needs a large number of interface signals to be able to work: Min. 4 data lines R/W line register select back light back light power power gnd

The display needs a large number of interface signals
to be able to work:

Min. 4 data lines

R/W line

back light

back light power

power

Slide 6 of 15

Writing the to HD4780 controller via the PCF8574

Slide 7 of 15

Writing to PCF8574 via pigpio

Even though it looks complicated at first sight,

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.

Slide 8 of 15

A data byte to be sent

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

Hope you remember the

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

Slide 9 of 15

The HD44780 library

As already explained several times, a library must supply:

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

Slide 10 of 15

The function prototypes

Slide 11 of 15

Using the library

In addition to the prototypes, have a look at the doxygen

documentation

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!

Slide 12 of 15

Testing access

Before starting to write the library we must first test if

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?

Slide 13 of 15

Back light

To give us some confidence, let’s try to switch the back light on and off:

This one is easy as it does not access the hd44780.

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.

Slide 14 of 15

Timing diagrams 4 bit mode

Slide 15 of 15

-- Uli Raich - 2017-11-06

Comments

I	Attachment	History	Action	Size	Date	Who
png	2linedisplay.png	r1	manage	873.1 K	2017-11-06 - 15:33	UnknownUser
png	4bitTransfer.png	r1	manage	24.0 K	2017-11-06 - 15:33	UnknownUser
png	4bitdataTransfer.png	r1	manage	28.0 K	2017-11-06 - 15:33	UnknownUser
png	displaySignals.png	r1	manage	16.0 K	2017-11-06 - 15:33	UnknownUser
png	i2c_pcf8574.png	r1	manage	1014.0 K	2017-11-06 - 15:33	UnknownUser
png	i2c_write_byte.png	r1	manage	14.9 K	2017-11-06 - 16:10	UnknownUser
png	initProc1.png	r1	manage	29.9 K	2017-11-06 - 15:33	UnknownUser
png	initProc2.png	r1	manage	27.7 K	2017-11-06 - 15:33	UnknownUser
png	libFunctions.png	r1	manage	28.5 K	2017-11-06 - 15:33	UnknownUser
png	pcf8574ifce.png	r1	manage	63.7 K	2017-11-06 - 15:33	UnknownUser
png	pcf8574write.png	r1	manage	33.8 K	2017-11-06 - 16:08	UnknownUser
png	strobe.png	r1	manage	68.3 K	2017-11-06 - 15:33	UnknownUser

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Topic revision: r3 - 2017-11-12 - uli