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Lab 4 – Data Manipulation & the LCD ECE 375
ECE 375: Computer Organization and
Assembly Language Programming
Lab 4 – Data Manipulation & the LCD
SECTION OVERVIEW
Complete the following objectives:
• Understand the basics of data manipulation in AVR assembly.
• Learn the basic steps of initializing a program, such as setting up the stack
pointer, initializing registers, configuring peripheral devices, etc.
• Use the X, Y, and Z pointers to perform indirect addressing.
• Declare constant data in the program memory, and iteratively move that
data into the data memory.
• Use pre-written library functions to interact with a peripheral device.
PRELAB
To complete this prelab, you may find it useful to look at the AVR Starter Guide
and the AVR Instruction Set Manual. If you consult any online sources to help
answer the prelab questions, you must list them as references in your prelab.
1. What is the stack pointer? How is the stack pointer used, and how do you
initialize it? Provide pseudocode (not actual assembly code) that illustrates
how to initialize the stack pointer.
2. What does the AVR instruction LPM do, and how do you use it? Provide
pseudocode (not actual assembly code) that shows how to setup and use
the LPM instruction.
3. Take a look at the definition file m128def.inc (This file can be found in the
Solution Explorer → Dependencies folder in Atmel Studio, assuming
you have an Assembler project open and you have already built an assembly
program that includes this definition file. Two good examples of such a
project would be your Lab 1 and Lab 3 projects.) What is contained within
this definition file? What are some of the benefits of using a definition file
like this? Please be specific, and give a couple examples if possible.
BACKGROUND
Introduction
For this lab, you will learn to interact with the LCD included on the mega128
microcontroller board. In order to use the LCD, you will first need to learn how
to properly initialize an assembly program. Next, you will learn how to move
data from the program memory into the data memory, and then display that
data as characters on the LCD.
To help you along the way, a skeleton file (ece375-L4 skeleton.asm) has been
provided for this lab. Similar to the Lab 2 skeleton C file, this file contains some
code that is already written, and some comments indicating where you need to
add some code of your own. This skeleton file also provides a good foundation
for writing well-structured and well-commented assembly code, as defined in the
AVR Starter Guide, and as required for full credit on your lab write-up.
Initialization
A program initialization (INIT) consists of any code that is only run once, at
the beginning of the program. This code does not belong in the MAIN portion
of the program, which is why most assembly programs begin with INIT and not
MAIN. INIT is not a function (you wouldn’t ever jump to it from within the MAIN
part of your program), but rather a collection of instructions that are executed
at power on (and at reset). Traditionally, INIT finishes with a jump to MAIN.
There are several things that should be included in the INIT section of a
program. The first and foremost is the initialization of the stack pointer. The
stack in AVR operates in a higher-to-lower address fashion, meaning that the
most recent element placed on the stack is at a lower address than the previous
element placed on the stack. Therefore, the stack pointer should initially point
to the upper end of the data memory; in other words, to the location with the
highest address in the data memory space. See the AVR Starter Guide for
more information about the stack.
In addition to initializing the stack pointer, which must be done for your
program to perform function calls correctly, there are several other things that
are typically initialized within the INIT section of a program, such as:
• I/O ports
• Timer/counters
• Interrupts
• Peripheral devices
©2021 Oregon State University Winter 2021 Page 1
Lab 4 – Data Manipulation & the LCD ECE 375
The exact contents of INIT will depend on which features of the mega128
board you need to use in your program. For this lab, you will need to initialize
the LCD. To do so, you will perform a function call to the LCD initialization
subroutine, LCDInit, which is described later in this document. You will also
need to move data from the program memory to the data memory in this lab, so
you may want to plan on doing that in your INIT section as well.
Using the LCD Driver
To successfully interact with the LCD, you will need to use some pre-written
functions that are part of an LCD driver file provided on the lab webpage
(LCDDriver.asm). Make sure to download this file and include it into
your Atmel Studio project. Unlike the m128def.inc definition file, which is
purely pre-compiler directives, the LCD driver contains actual assembly instructions and thus cannot be included at the beginning of your main program file.
As indicated in the AVR Starter Guide, any included code files are included at
the end of the main program, i.e. in the last line(s).
After including the LCD driver into your program, you will still need to properly setup and call the functions defined in the LCD driver to interact with the
LCD. (Remember, for any function calls to work correctly, the stack pointer must
have already been initialized.) Detailed descriptions of the LCD driver functions
are provided in Appendix A at the end of this handout, but here is an overview:
• LCDInit – Initialize the LCD (call once from within INIT)
• LCDWrite – Update all characters of both lines
• LCDWrLn1 – Update all characters of line 1 only
• LCDWrLn2 – Update all characters of line 2 only
• LCDClr – Clear (write “space” to) all characters of both lines
• LCDClrLn1 – Clear all characters of line 1 only
• LCDClrLn2 – Clear all characters of line 2 only
• LCDWriteByte – Write directly to a single character of the LCD
• Bin2ASCII – Convert an 8-bit unsigned value into an ASCII string
There are other functions within the LCD driver, but they simply support the
functions listed above, so they should not be called directly from your program.
Data Manipulation
To move data from one memory type to another, you first must understand how
the available memory is organized. The ATmega128 microcontroller has an 8-
bit AVR architecture. This means that all registers and data memory locations
are 8 bits wide, and all data is handled 1 byte at a time (disregarding certain
instructions that can treat certain pairs of registers as one 16-bit “word”).
However, the AVR instructions supported by the ATmega128 are 16 bits wide
(or 32 bits wide, for a small number of instructions), and so for efficiency reasons the ATmega128’s program memory is 16 bits (2 bytes) wide. This has an
important consequence: Unlike pointers to data memory, pointers to program
memory initially point to a 16-bit memory location, and you will have to
take certain steps to read just the low byte or just the high byte of that location.
When writing (and especially when testing) your program, it is often useful
to include some data directly into program memory. For example, imagine you
are simulating your program, and you are testing a function which uses several
data memory locations as input. Instead of manually entering test input values
into the data memory via the Memory window in Atmel Studio, you can use the
.DB (Data Byte) directive to have the compiler place your test values into the
program memory during compilation, and then write a simple loop or function
that copies your test values from program memory into data memory at runtime.
The following example shows how to place data into the program memory:
DATA:
.DB $05, $F4
The hexadecimal values $05 and $F4 can then be accessed using the label DATA,
which the compiler converts to a program memory address. To read this data,
use the LPM (Load Program Memory) instruction. You can also enter strings (i.e.,
a sequence of ASCII characters) into program memory using the .DB directive;
just take a look at the Lab 4 skeleton file to see how.
Note: Since the program memory is 16 bits wide, the total amount of data
you specify using a single instance of the .DB directive should be an integer
multiple of 16 bits. Otherwise, the compiler will pad the program memory with
an extra byte of data, usually $FF.
Movement within data memory is accomplished using variations of load and
store instructions, and with two main addressing modes: direct addressing and
indirect addressing. Direct addressing, where a memory address is provided directly as an operand, is useful when you want to move a single byte to/from a
single memory location (such as an extended I/O register). Indirect addressing,
©2021 Oregon State University Winter 2021 Page 2
Lab 4 – Data Manipulation & the LCD ECE 375
where the X, Y, and Z-pointers are an operand and they contain an effective
address, is useful for moving blocks of data into contiguous locations in the data
memory. By using indirect addressing in conjunction with a loop, you can efficiently move lots of data around in memory. The AVR Starter Guide has more
information on pointers and indirect addressing. Also, Figure 1 shows some
pseudocode you can use to properly read a bunch of data from program memory.
PROCEDURE
You will use three buttons; PD0, PD1, and PD7 to write and clear text on the
LCD display. Write a program that does the following:
1. When you press PD0: Displays your name on the first line of the LCD, and
a phrase like ”Hello World!” (or your partners name) on the second line of the
screen.
2. When you press PD1: The contents should swap so that a phrase like ”Hello
World” is shown on line 1 and your name is shown on line 2.
3. When you press PD7: The content should be cleared.
The display should be initially blank. To receive full credit, you must:
(1) properly initialize your program, (2) place your two strings into program
memory using the .DB directive. (3) Use two unique strings, for example one
string could contain your name, and another your partners. (4) copy the strings
from program memory to the appropriate data memory locations using a loop,
and (5) there must not be any unintended trailing characters on the LCD.
When you are finished, demonstrate to the TA that you have displayed both
strings on the LCD and met these requirements.
Button PD0: Line 1: Your name
Line 2: Hello, World (or partner’s name)
Button PD1: Line 1: Hello, World (or partner’s name)
Line 2: Your name
Button PD7: Line 1:
Line 2:
STUDY QUESTIONS / REPORT
A full lab write-up is required for this lab. When writing your report, be sure to
explain in detail what you did and why, indicate any problems you may
• Z <- program memory address of first character
• Y <- data memory address of character destination
• do { mpr <- ProgramMemory[Z], Z++,
DataMemory[Y] <- mpr, Y++ }
while (Z != program memory address after last character)
Figure 1: Pseudocode for String Copy from Program Memory to Data Memory
have encountered, and answer the study questions given below. Your
write-up and code must be submitted online by the beginning of next week’s lab.
Remember, NO LATE WORK IS ACCEPTED.
Study Questions
1. In this lab, you were required to move data between two memory types:
program memory and data memory. Explain the intended uses and key
differences of these two memory types.
2. You also learned how to make function calls. Explain how making a function
call works (including its connection to the stack), and explain why a RET
instruction must be used to return from a function.
3. To help you understand why the stack pointer is important, comment out
the stack pointer initialization at the beginning of your program, and then
try running the program on your mega128 board and also in the simulator.
What behavior do you observe when the stack pointer is never initialized?
In detail, explain what happens (or no longer happens) and why it happens.
CHALLENGE
Not being content with displaying a static message, you would like to add some
flair by displaying a scrolling, marquee-style message. Modify your program so
that the text scrolls across the two lines of the LCD, from left to right and right
to left for PD5 and PD6, respectively. Include some wait time (delay) between
each “scroll” event, so that there is enough time to read the characters in their
current position before they move again.
©2021 Oregon State University Winter 2021 Page 3
Lab 4 – Data Manipulation & the LCD ECE 375
When a character reaches the end of a line, display it next at the beginning of
the opposite line. In other words, a character at index 15 of line 1 will move
to index 0 of line 2, and a character at index 15 of line 2 will move to slot 0
of line 1. Here is an example of the required display and motion, where the ‘ ’
character signifies a space:
1: Line 1: _____My_Name_is_
Line 2: _______Jane_Doe_
(wait for .25 seconds)
2: Line 1: ______My_Name_is
Line 2: ________Jane_Doe
(wait for .25 seconds)
3: Line 1: e______My_Name_i
Line 2: s________Jane_Do
(wait for .25 seconds)
4: Line 1: oe______My_Name_
Line 2: is________Jane_D
etc.
Use the same two strings from the required part of the lab as the strings that
you make scroll. If you complete this challenge, demonstrate it to the TA, and
submit your challenge code online with your regular code and your lab write-up.
APPENDIX A – LCD FUNCTIONS
LCDInit
This subroutine initializes the serial interface that is used to communicate
with the LCD, and also initializes the display itself, configuring it to display
2 lines/rows of 16 characters (2×16). This function can be called directly, e.g.:
rcall LCDInit ; Initialize LCD peripheral interface
LCDWrite
This function writes data to both lines of the LCD. First, line data is retrieved
from the following data memory addresses:
Line 1: $0100 – $010F
Line 2: $0110 – $011F
Next, that data is actually written out to the LCD. So, for this function to work
properly, you must first move the data (e.g., an ASCII string) that you want
displayed on the LCD into the appropriate locations in data memory. Then, call
the function as follows:
rcall LCDWrite ; Write to both lines of the LCD
LCDWrLn1
This function writes data to the first/top line of the LCD. First, the data is
retrieved from data memory addresses $0100 – $010F, and then the data is
written out to the first line of the LCD.
To use this function, make sure the data you want to display is in the appropriate locations in data memory, and then call the function as follows:
rcall LCDWrLn1 ; Write to first line (Line 1) of the LCD
LCDWrLn2
This function writes data to the second/bottom line of the LCD. First, the data
is retrieved from data memory addresses $0110 – $011F, and then the data is
written out to the second line of the LCD.
To use this function, make sure the data you want to display is in the appropriate locations in data memory, and then call the function as follows:
rcall LCDWrLn2 ; Write to second line (Line 2) of the LCD
LCDClr
This subroutine clears both lines of the LCD. Specifically, it writes the “space”
character (ASCII value $20) to data memory locations $0100 – $010F and $0110
– $011F, and then writes to both lines of the LCD. To call it, use:
rcall LCDClr ; Clear both lines of the LCD
LCDClrLn1
This subroutine works similarly to LCDClr, except it only clears the first/top line
of the LCD.
rcall LCDClrLn1 ; Clear first line (Line 1) of the LCD
©2021 Oregon State University Winter 2021 Page 4
Lab 4 – Data Manipulation & the LCD ECE 375
LCDClrLn2
This subroutine works similarly to LCDClr, except it only clears the second/bottom line of the LCD.
rcall LCDClrLn2 ; Clear second line (Line 2) of the LCD
LCDWriteByte
This function allows you to write a single ASCII character (or byte) to anywhere
on the LCD. It allows direct control over where data is displayed on the LCD,
and does not require anything to be stored in data memory first (unlike many
of the previous functions). There are three registers that must be loaded with
input parameters before this function is called:
• mpr – Contains the byte/character that you want to display on the LCD
(must be a value between 0 and 255)
• line – Contains the line number where you want to display the byte (must
be either 1 for first line, or 2 for second line))
• count – Contains the index number where you want to display the byte
(must be a number between 0 and 15, with 0 specifying the leftmost position). Note: The LCD actually accepts index values up to 39, but only
indexes 0 through 15 are actually visible on the display. Any index value
between 16 and 39 will result in the character being display “off screen”
The following example uses the LCDWriteByte function to write the character
‘D’ to Line 2, index 7 of the LCD:
; load parameters for LCDWriteByte
ldi mpr, ‘D’ ; load character ‘D’ into mpr
ldi line, 2 ; load line number 2 into line register
ldi count, 7 ; load index 7 into count register
; call LCDWriteByte function
rcall LCDWriteByte ; write character to LCD
Bin2ASCII
This function converts an unsigned 8-bit value into its numerically equivalent
ASCII string, i.e. from 13810 → “138”. The resulting string (with length between
1 and 3 bytes) is stored in the data memory. This function is useful for displaying
a value that changes throughout the course of a program (such as a counter).
Two registers must be loaded with input parameters before this function is
called, and a third register must be available to store a returned value:
• mpr – Contains the 8-bit value that will be converted
• XH:XL – Contains the beginning 16-bit data memory address where the
ASCII string will be stored
• count – Will contain the return value (the length of the created string)
The following example uses the Bin2ASCII function to convert the value 13810
into “138”, and then store it in data memory beginning at location $0112:
; load parameters for Bin2ASCII
ldi mpr, 138 ; load to-be-converted value into mpr
ldi XL, low($0112) ; load X with beginning address
ldi XH, high($0112) ; of where result will be stored
; call Bin2ASCII function
rcall Bin2ASCII ; convert value in ASCII
After the above example is executed, the data memory locations $0112, $0113,
and $0114 will contain the characters ‘1’, ‘3’, and ‘8’, respectively, and count
will contain the value 3.
If a smaller value like 7510 had instead been converted, only locations $0112
and $0113 would contain characters, and count would have a value of 2.
©2021 Oregon State University Winter 2021 Page 5

Lab 4 – Data Manipulation & the LCD
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