1. Introduction to Assembly language
Using the AVR microprocessor
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2. Outline Introduction to Assembly Code The AVR Microprocessor
Binary/Hex Numbers
Breaking down an example microprocessor program
AVR instructions overview
Compiling and downloading assembly code
Running and debugging assembly code
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3. A really simple program
Some variables (i,j)
Set variables to initial values
A loop
Check a condition to see if we are finished
Do some arithmetic
Output some information
Increment the loop counter
int main(void) {
char i;
char j;
i=0;
j=0;
while (i<10) {
j = j + i;
PORTB = j;
PORTB = i;
i++; }
return 0;
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4. Running a program on a microprocessor
When you want to turn your program into something which runs on a microprocessor you typically compile the program
This creates a program in the “machine language” of the microprocessor
A limited set of low level instructions which can be executed directly by the microprocessor
We can also program in this language using an assembler
We can have a look at the assembly language the c compiler generates for our simple program to understand how this works
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5. Translating our program into assembly language
int main(void) { char i; char j; i=0; j=0; while (i<10) { j = j + i; PORTB = j; PORTB = i; i++; } return 0;
000000be <main>:
be: 80 e ldi r24, 0x00 ; 0
c0: 90 e ldi r25, 0x00 ; 0
c2: 98 0f add r25, r24
c4: 92 bb out 0x18, r25 ; 18
c6: 82 bb out 0x18, r24 ; 18
c8: 8f 5f subi r24, 0xFF ; 255
ca: 8a cpi r24, 0x0A ; 10
cc: d1 f brne ; 0xc2 <main+0x4>
ce: 80 e ldi r24, 0x00 ; 0
d0: 90 e ldi r25, 0x00 ; 0
d2: ret
Location in program memory (Address)
Opcodes – the program code
The Assembly language equivalent of the opcodes
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6. AVR Microprocessor architecture
Program Memory
Your code goes here
Registers
Storage for numbers the processer will perform arithmetic on
Arithmetic Logic Unit (ALU)
The heart of the processor which handles logic/math
Input/Output
Interface to the outside world
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7. Numbers on a microprocessor
We’ve seen that our program is converted into a series of numbers for execution on the microprocessor
Numbers are stored in a microprocessor in memory
They can be moved in an out of registers and memory
Calculations can be done
Numbers can be sent to Output devices and read from Input devices
The numbers are stored internally in binary representations.
We will study precisely how this is done shortly, and even build some simple memory devices.
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8. Binary/Hexadecimal Numbers
A “Bit” can be a 0 or 1
The value is set using transistors in the hardware
Bits are organized into groups to represent numbers
4 Bits is a “Nybble”
Can store numbers 0-15
8 Bits is a “Byte”
Can store numbers 0-255
16 Bits is a word
Can store numbers
Hexadecimal is a very handy representation for binary numbers
Each 4 bits maps onto a HEX number
Can quickly convert from HEX to binary
Binary
Hex
Decimal
0000
0001 1
0010 2
0011 3
0100 4
0101 5
0110 6
0111 7
1000 8
1001 9
1010 A 10
1011 B 11
1100 C 12
1101 D 13
1110 E 14
1111 F 15
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9. Binary Representation
This representation is based on powers of 2. Any number can be expressed as a string of 0s and 1s
Example: 5 = 1012 = 1* *21 + 1*20
Example: 9 = = 1* * *21 + 1*20
Exercise: Convert the numbers
19, 38, 58 from decimal to binary.
(use an envelope, a calculator or C program)
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10. Hexadecimal Representation
This representation is based on powers of 16. Any number can be expressed in terms of:
0,1,2,…,9,A,B,C,D,E,F (0,1,2,…,9,10,11,12,13,14,15)
Example: 256 = = 1* * *160
Example: 1002 = 3EA16 = 3* * *160
Exercise: Convert the numbers
1492, 3481, 558 from decimal to hex.
(use calculator or C program)
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11. HEX/Binary Conversion
Converting HEX/Binary to Decimal is a bit painful, but converting HEX/Binary is trivial
We often use the notations 0x or $ to represent a HEX number
Example 0x15AB for the HEX number 15AB
In assember language we see the notation $A9 for the HEX number A9
Exercise: Convert the numbers
0xDEAD 0xBEEF from Hex to binary.
Now convert them to Decimal
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12. 8 Bit Microprocessors
The AVR microprocessor we will be using is an “8 bit” processor
The operations the processor performs work on 8 bit numbers
Data is copied from memory into the processors internal “registers” 8 bits at a time
Operations (addition/subtraction/etc..) can be performed on the 8 bit registers
We can of course do calculation with bigger numbers, but we will have to do this as a sequence of operations on 8 bit numbers
We will see how to do this later – using a “carry” bit
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13. Operations The processor can perform several operations
Including “Boolean” algebra
Operation
Register A
Register B
Result
Add A,B
Not A
OR A,B
ShiftL A
ShiftR A
AND A,B
Exercise: (1) Find NOT(AAA)
(2) Find OR(AAA; 555)
(3) Find AND (AEB123; FFF000)
Why is shift important ?
Try SHIFT R(011) SHIFT L(011)
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14. What About Subtraction: Negative Numbers
With 4 bits, you can represent
0 to +15
-8 to + 7
There are three ways to represent these negative numbers
Integer
Sign Magnitude
1’s complement
2’s complement +7
0111 +6
0110 +5
0101 +4
0100 +3
0011 +2
0010 +1
0001
0000 -1
1001
1110
1111 -2
1010
1101 -3
1011
1100 -4 -5 -6 -7
1000 -8
1000 (-0)
11111
Sign/Magnitude: Set the top bit to 1
1’s complement : Take the complement of the number
2’s complement : Take the complement of the number and then add 1
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15. What About Subtraction: Negative Numbers
There is a good reason to use a 2’s complement representation
Binary addition of a two’s complement numbers “just works” whether the numbers are positive or negative
3 + 4
3 + -4
-3 + 4
0011
1101
+ 0100
+ 1100
= 0111
= 1111
= 0001
= 1001
(7)
(-1)
(+1)
(-7)
Exercise: Fill out the same table using sign magnitude numbers. Do you understand now why 2’s complement is a useful representation!
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16. 8 Bit representations 8 bits can be used for The numbers 0-255
The numbers -128 to + 127
Part of a longer number
A “Character”
Hence “char” in c
This is a bit out of date
Unicode uses 16 bits
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17. Back to our program The program is stored in program memory
There are 64Kilobytes of program memory (2^16 bytes)
Each location stores an 8 bit number
The location is specified with a 16 bit Address
(0x0000-0xFFFF)
000000be <main>:
be: 80 e ldi r24, 0x00 ; 0
c0: 90 e ldi r25, 0x00 ; 0
c2: 98 0f add r25, r24
c4: 92 bb out 0x18, r25 ; 18
c6: 82 bb out 0x18, r24 ; 18
c8: 8f 5f subi r24, 0xFF ; 255
ca: 8a cpi r24, 0x0A ; 10
cc: d1 f brne ; 0xc2 <main+0x4>
ce: 80 e ldi r24, 0x00 ; 0
d0: 90 e ldi r25, 0x00 ; 0
d2: ret
Address
Value
00BE 80
00BF E0
00C0 90
00C1
00C2 98
00C3 0F
00C4 92
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18. AVR Registers – where the numerical work is done in a program
Registers on the AVR
There are 32 General purpose registers on the AVR microprocessor (R0-R31)
Each of these can hold an 8 bit number
R26:R27 is also a 16 bit register called X
R28:R29 is Y
R30:R31 is Z
You can perform calculations on these registers very quickly
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19. Back to our program: Loading a register
The first instruction is loading a value of 0x00 into register r24
This instruction is encoded in the 16 bit opcode stored at address 00BE
The Assember code is
ldi r24,0x00
LoaD Immediate r24 with 0x00
000000be <main>:
be: 80 e ldi r24, 0x00 ; 0
c0: 90 e ldi r25, 0x00 ; 0
c2: 98 0f add r25, r24
c4: 92 bb out 0x18, r25 ; 18
c6: 82 bb out 0x18, r24 ; 18
c8: 8f 5f subi r24, 0xFF ; 255
ca: 8a cpi r24, 0x0A ; 10
cc: d1 f brne ; 0xc2 <main+0x4>
ce: 80 e ldi r24, 0x00 ; 0
d0: 90 e ldi r25, 0x00 ; 0
d2: ret
Address
Value
00BE 80
00BF E0
00C0 90
00C1
00C2 98
00C3 0F
00C4 92
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20. Decoding an Opcode Words are stored “little endian”
Read into the processor as E080
E080=
dddd: 1000=8 -> 16+8 = r24
KKKKKKKK=0x00
be: 80 e ldi r24, 0x00
Exercise: what is the opcode for
ldi r19, 0x3F
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21. Back to our program: Loading a register
The second instruction is loading a value of 0x00 into register r25
000000be <main>:
be: 80 e ldi r24, 0x00 ; 0
c0: 90 e ldi r25, 0x00 ; 0
c2: 98 0f add r25, r24
c4: 92 bb out 0x18, r25 ; 18
c6: 82 bb out 0x18, r24 ; 18
c8: 8f 5f subi r24, 0xFF ; 255
ca: 8a cpi r24, 0x0A ; 10
cc: d1 f brne ; 0xc2 <main+0x4>
ce: 80 e ldi r24, 0x00 ; 0
d0: 90 e ldi r25, 0x00 ; 0
d2: ret
This adds r24 to r25 and stores the result into register r24
The next instructions output r24 and r25 (we’ll learn where later)
This is subtracting a value of 0xFF from register r24
This is the compiler cleverly adding 1
This instruction is comparing the value 0x0A to register r24
If they are not equal, the next instruction will branch back to location 0xC2 that is loop back if they are equal it continues on (and returns from main)
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22. Compare assembly language to c
int main(void) { char i; char j; i=0; j=0; while (i<10) { j = j + i; PORTB = j; PORTB = i; i++; } return 0;
000000be <main>:
be: 80 e ldi r24, 0x00 ; 0
c0: 90 e ldi r25, 0x00 ; 0
c2: 98 0f add r25, r24
c4: 92 bb out 0x18, r25 ; 18
c6: 82 bb out 0x18, r24 ; 18
c8: 8f 5f subi r24, 0xFF ; 255
ca: 8a cpi r24, 0x0A ; 10
cc: d1 f brne ; 0xc2 <main+0x4>
ce: 80 e ldi r24, 0x00 ; 0
d0: 90 e ldi r25, 0x00 ; 0
d2: ret
Here the variables are stored in registers r24, r25
Pretty easy to see how this program is translated
More complex programs quickly become very complicated to understand in assembly language
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23. Programming in this course
We will be programming exclusively in assembler
Allows us to understand precisely what the microprocessor is going to do and how long it will take to do so
important for time critical applications
Full access to all functions of the microprocessor
With care can make very efficient use of resources
Sometimes very important for small microprocessors
Programming in assembler requires some discipline
Code can be very difficult to understand
The code is very low level
Line by line comments very important
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24. The AVR instruction set
We’ve seen a few sample instructions which cover most of the basic type of operations
Arithmetic and Logic instructions
(ADD, SUB, AND, OR, EOR, COM, INC, DEC, …)
Branch Instructions
Jump to a different location depending on a test
Data transfer instructions
Move data to/from Registers and memory
Bit setting and testing operations
Manipulate and test bits in registers
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25. Arithmetic and Logic Instructions
Addition
add r20,r21
R20  r20+r21
Increment
inc r20
R20  r20+1
Subtraction
subi r20,$22
R20  r20-$22
sub r20,r21
R20  r20-r21
Logic
and r20,r24
R20  AND(r20,r24)
Many instructions work either with two registers, or with “immediate” data values (stored in the opcode)
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26. Arithmetic and Logic Instructions – The full set
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27. The Status Register
Every time the processor performs an operation it sets bits in the Status Register (SREG)
Example cpi r01,$77
This register is in the I/O region
SREG can be examined/set with the in and out instructions
in r17,SREG
out SREG,r22
The status bits are also tested by branch instructions to decide whether or not to jump to a different location
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28. Branch Instructions Jump Branch Call, Return jmp label3
breq label1
Branch if equal to location label1
brlo label2
Branch if lower to location label2
If the Branch test fails – the next line of code is executed
If the Branch test is successful, the program jumps to the location specified
Jump
jmp label3
Jump to label3 – no information about where we jumped from is saved. This is a one way trip
Call, Return
rcall mysub
ret
Call subroutine mysub. The program saves information about where it currently is executing and then jumps to the code at mysub. When the subroutine is finished it calls ret, which then returns to where the rcall was made.
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29. All Branch Instructions
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30. Data Transfer Instructions
Load
ldi r20,$73
R20  $73
ld r20,X
R20  (X)
Copy Register
mov r20,r21
R20  r21
Input
in r20,PIND
R20  PIND
Output
out PORTD,r24
PORTD  r24
There are a few quirks about loading and storing to memory. We will cover this in detail soon.
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31. Data transfer reference
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32. The Atmega128
The microprocessor you will be using has several input and output ports
Some of these are already connected to switches or LEDs on the boards we are using
Others will be available to you to connect to various devices.
It is time to make some lights blink!
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33. Setting up the Input/Output ports:
For the ports we are using we set the Data Direction Register (DDR) which has a bit for each bit of I/O (1 for input, 0 for output)
We can then set the initial value of the data bits at the I/O port
PortB is connected to LEDs on our board
PortD is connected to the blue switches
; ******* Port B Setup Code ****
ldi r16, $FF ; all bits out
out DDRB , r16 ; Port B Direction Register
ldi r16, $FF ; Init value
out PORTB, r16 ; Port B value
; ******* Port D Setup Code ****
ldi r16, $00 ; all bits in
out DDRD, r16 ; Port D Direction Register
ldi r16, $FF ; Init value
out PORTD, r16 ; Port D value
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34. The ATmega128 Microprocessor
In this course you will be using the ATmega128 processor mounted on an ATMEL programming board (STK300)
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35. The ATMEL BOARD Connecting the board with your PC
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36. Where the I/O ports are connected
PORTD Switches
PORTB LEDs
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37. Setting up a code directory
Create a directory where you will store your code
Download the file Simple.ASM into your code directory
from the course web page (Lecture 2b):
DO NOT DOWNLOAD m128def.inc
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38. Getting Started with STUDIO 4:
Go to Start  Programs  ATMEL AVR Tools
 AVR Studio 4
Select New
Project
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39. Getting Started with STUDIO 4:
Create new folder
Do not create new file
You should now see the window:
Pick a name for your project
Pick Atmel AVR Assembler
At the
end
Click this and navigate to your code directory
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40. Getting Started with STUDIO 4:
You should now see the window:
Select ATmega 128
Select AVR Simulator
At the
end
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41. Entering files in STUDIO 4 (I):
(2) Navigate and find SIMPLE.ASM
(1) Right click here and select to ‘Add Files to Project’
Chose Open to load the file
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42. Entering files in STUDIO 4 (II):
Here is list of files attached to project
This window is for editing one of the files
Change the ‘editing window’ by double clicking on a file icon
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43. Select the output file format :
Change the format of the output file to Intel Hex format
Click on Project/ Assembler Options
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44. Running your Program in Studio 4 :
Click on
Build/ Build and run
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45. Running your Program in Studio 4:
The Build process generates some new files
Program is halted at this instruction
Green means ok;
Otherwise click on red and debug
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46. Open monitoring Windows:
View/ Toolbars/ I/O
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47. Open monitoring Windows:
Expand views as required
Adjust screen display using Window/…
Input
Output
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48. Watch your Program Running:
Start here:
Everything zeroed
Use this to reset
Use this to run continuously with active display
Use this to stop
Use this to run continuously without display
Click this once
and again
Step through the whole program and make sure you understand what is happening
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49. Exercising the ATmega128 commands:
Download the program simple.asm, assemble it and run it in the simulator
Step through the program and make sure you understand what is happening after each instruction
Try changing the number of times the loop is executed and make sure you understand the outputs on PORTB
Now we will download the program to the development board
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50. Downloading with AVRISP
Select from Start : AVRISP
Make sure Device Atmega128 is selected
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51. Erase the device
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52. Select the file to load Select Load/Flash…
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53. Chose your .hex file to download
Tell AVRISP which .hex file to download to the Atmega128
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54. Now download the program into the Flash
Select Program/Flash -- This writes the machine code to be downloaded into the ATmega128 chip
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55. Running your program
Select Run – If all goes well your program should now be executing
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56. Programs to write I:
Download the program simple.asm, assemble it download it to the board and run it.
What do you see on the LEDs?
Do you know why (note that a dark LED  1)?
change the code so that output on the LEDs has a lit LED for a 1 (think about using the neg instruction)
Modify the program so that it changes the number of times the loop is run depending on which switch button is pushed
Use the instruction in Rxx,PIND to get the state of the buttons
Make sure that the LED output makes sense when you push the different buttons
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57. Programs to write :
Make a counter from 0 – FF, output the values to PORTB and look with your scope probe at the LSB (PB0). How long does it take to make an addition? Why does the B0 bit toggle with a frequency that is twice that of B1? (check this using two scope probes; one on B0 and another on B1)
In the documentation you will find how many clock counts are required to perform an instruction in your program. The ATmega128 has an 8 MHz clock. Predict the time it takes to do an addition and compare with your measurement using the scope.
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