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Introduction to Embedded Microcomputer Systems
Lecture 3.1
Appdx 1. Embedded system development using TExaS
Assembler
Source code
PORTA equ
DDRA equ
org
cnt
rmb
org
main lds
movb
off
bclr
look ldd
std
loop ldaa
anda
cmpa
bne
ldx
dex
stx
bne
bset
bra
key
fcb
org
fdb
$0000
$0002
$0800
2
$F000
#$0C00
#$80,DDRA
PORTA,#$80
#4444
cnt
PORTA
#$7F
key
off
cnt
cnt
loop
PORTA,#$80
look
%00100011
$FFFE
main
Loader Microcomputer
processor
Object code
$F000
$F003
$F008
$F00B
$F00E
$F011
$F013
$F015
$F018
$F01A
$F01D
$F01E
$F021
$F023
$F026
$F028
$FFFE
CF0C00
180B800002
4D0080
CC115C
7C0800
9600
847F
B1F028
26EE
FE0800
09
7E0800
26EE
4C0080
20E3
23
F000
RAM
EPROM
I/O
External circuits
and devices
Figure 4.1. Assembly language development process.
Editor
Source code
PORTA equ
DDRA equ
org
cnt
rmb
org
main lds
movb
off
bclr
look ldd
std
loop ldaa
anda
cmpa
bne
ldx
dex
stx
bne
bset
bra
key
fcb
org
fdb
Assembler
$0000
$0002
$0800
2
$F000
#$0C00
#$80,DDRA
PORTA,#$80
#4444
cnt
PORTA
#$7F
key
off
cnt
cnt
loop
PORTA,#$80
look
%00100011
$FFFE
main
TExaS
Simulated
Loader Microcomputer
processor
Object code
$F000
$F003
$F008
$F00B
$F00E
$F011
$F013
$F015
$F018
$F01A
$F01D
$F01E
$F021
$F023
$F026
$F028
$FFFE
CF0C00
180B800002
4D0080
CC115C
7C0800
9600
847F
B1F028
26EE
FE0800
09
7E0800
26EE
4C0080
20E3
23
F000
RAM
ROM
I/O
Simulated
External circuits
and devices
Figure 4.2. Assembly language development using TExaS.
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.2
A1.5. Tutorial A1. Getting started
The purpose of this tutorial is to introduce the first time user to TExaS.
1) how to launch the simulator,
2) how to modify input switches,
3) how to edit, assemble, and run a 6812 program,
4) how to modify display format in the ViewBox,
5) how to move and resize windows,
6) how to get on-line help.
visualize four places information can be stored on a computer.
1) external switches are input devices that hold information,
2) registers are high-speed temporary storage inside the processor,
3) global variables can hold information that is easy to access,
4) external LEDs are output devices that hold information.
Action: Install and upgrade the TExaS application.
www.ece.utexas.edu/~valvano
Action: Copy the tutorial programs onto a hard drive.
copy the Mc6812T subdirectory from the CD onto a hard drive.
Action: How to launch the TExaS application.
Figure A1.9. TExaS application icon.
Question A1.1. List the names of the six files that are now open.
Action: How to assemble a program.
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.3
Execute the command Assemble->Assemble,
Question A1.2. What happened to the colors of the Start, ExecuteOne, and StepOver
tools in the tool bar after the program was assembled? Why?
Action: How to modify input switches.
Question A1.3. Describe the relationship between the switch position (open or closed)
and
1) the voltage across the switch (probe near the switch)
2) the voltage at the output of the 74HC14 not gate
3) the digital value of the input port
Action: Observing the details of execution.
TheList.rtf will highlight the current instruction
details of the simulation are dumped into TheLog.rtf.
Question A1.4. Explain the behavior of each of the following modes.
Mode->FollowPC
Mode->CycleView
Mode->InstructionView
Mode->LogRecord
Observation: The simulation speed (real human time to run your program) is
greatly improved by turning these four modes off.
Question A1.5. Use the help system to answer this question, “Into which file are the
parameters specified by the Mode menu commands (e.g., FollowPC CycleView
InstructionView LogRecord) saved?” In particular, which file do you save when you
want to remember these settings? To answer this question, execute Help->HelpTopics,
double-click Menus, then double-click Mode menu.
Action: Backdump.
Debugging Tip: Execute the Backdump command after your software crashes.
Action: Setting breakpoints.
type the absolute address, and push the Add button.
type the symbolic name, and push the Add button.
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.4
left click in the TheList.rtf, then right click and hold.
Add a breakpoint on PORTC, and run the program.
Action: Rearranging the windows.
TheList.rtf ChapA1.uc and ChapA1.io.
Question A1.6. What is the program counter, PC? How is its initial value (right after
reset) established?
Question A1.7. What does the value PORTC=$0A in the ViewBox window mean?
Action: Changing the format of the ViewBox data.
Single step (tool bar or F10) the program until the value in PORTB just changes to
$0A.
To change the format of a ViewBox entry
1) Select the entry Click on the PORTC=$0A line in the ViewBox
2) Give the new format Type d in the Format box
3) Enter the change
Type <Enter> or click on the Enter button
Question A1.8. Using this procedure, click on the PC and give the values for the
following formats
H
16-bit unsigned hexadecimal ($F00F on the 6812)
+H 16-bit signed hexadecimal (+H and -H are the same format)
D
16-bit unsigned decimal
+D 16-bit signed decimal (+D and -D are the same format)
B
16-bit unsigned binary
+B 16-bit signed binary (+B and -B are the same format)
Using this procedure, click on the Data=$0A and give the values for the following
formats
h
8-bit unsigned hexadecimal (should be $0A)
+h 8-bit signed hexadecimal (+h and -h are the same format)
d
8-bit unsigned decimal
+d 8-bit signed decimal (+d and -d are the same format)
b
8-bit unsigned binary
+b 8-bit signed binary (+b and -b are the same format)
2d
two unsigned decimal numbers
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.5
Action: Changing the value in the ViewBox data.
The current state of the microcomputer can be changed using this window. To
change the value of a ViewBox entry
1) Select the entry
Click on the A=$0A line in the ViewBox
2) Give the new value Type 100 in the Data box
3) Enter the change
Type <Enter> or click on the Enter button
To add new entries in the ViewBox (step 2 is optional)
1) Give the Address
For the 6812 type the address $0900
2) Give the new value Type "TExaS" in the Data box (including the " ")
3) Enter the new format Type 6d in the Format box (try also s and 6c)
Debugging Tip: Choose the formats in the ViewBox to simplify debugging.
Debugging Tip: While debugging software adjust the parameters in the ViewBox
to include important information and exclude unimportant information.
Action: Simulating the microcomputer hardware/software.
ChapA1.io and ChapA1.scp
Observation: The simulation speed (real human time to run your program) is
greatly improved by closing (not hide) scope windows.
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.6
2.5. Programming numbers in assembly language
w is signed 8-bit -128 to +127
or unsigned 8-bit 0 to 255
n is signed 8-bit -128 to +127
u is unsigned 8-bit 0 to 255
W is signed 16-bit -32787 to +32767
or unsigned 16-bit 0 to 65535
N is signed 16-bit -32787 to +32767
U is unsigned 16-bit 0 to 65535
=[addr] 8-bit read from addr
={addr} 16-bit read from addr
[addr]= 8-bit write to addr
{addr}= 16-bit write to addr
ldaa #w
ldaa u
ldaa U
staa u
staa U
bra
RegA=w
RegA=[u]
RegA=[U]
[u]=RegA
[U]=RegA
U
PC=U
6811/6812
RAM
processor
Reg A
ROM
Reg PC
I/O port
Figure 2.10. The ldaa Data instruction loads
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.7
6811/6812
RAM
processor
Reg A
ROM
Reg PC
I/O port
Figure 2.11. The staa PTT instruction stores
7
0
S XH I NZ V C
8
15
Register A
Register B
CC 8 bit condition code
D
two 8 bit accumulators
X
16 bit index register
Y
16 bit index register
SP 16 bit stack pointer
PC 16 bit program counter
ldaa #w
ldaa u
ldaa U
staa u
staa U
bra
RegA=w
RegA=[u]
RegA=[U]
[u]=RegA
[U]=RegA
U
PC=U
DDRAD, DDRS, DDRM, DDRT, specify if corresponding pin
0 means input
1 means output
*****start TExaS show Lab1.rtf****
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.8
2.6. Logical operations
A
B
A&B
A|B
0
0
0
0
0
1
0
1
1
0
0
1
1
1
1
1
Table 2.14. Logical operations.
AND Gate
A
B
A&B
A^B
0
1
1
0
OR Gate
A
B
74HC08
A|B
74HC32
EOR Gate
A^B
A
B
NOT Gate
A
74HC86
Figure 2.12. implemented with discrete digital gates.
A
~A
0
1
1
0
Table 2.15. Logical complement.
anda #w
anda u
anda U
oraa #w
oraa u
oraa U
eora #w
eora u
eora U
coma
RegA=RegA&w
RegA=RegA&[u]
RegA=RegA&[U]
RegA=RegA|w
RegA=RegA|[u]
RegA=RegA|[U]
RegA=RegA^w
RegA=RegA^[u]
RegA=RegA^[U]
RegA=~RegA
Mark W. Welker (From Jonathan W. Valvano)
A
74HC04
Introduction to Embedded Microcomputer Systems
Lecture 3.9
The and operation to extract, or mask, individual bits
Pressed = PTS&0x01;
+5V
+5V
C
q
10k 
Input port
PS 0
10k
C
p
74H C14
q
2 2
Input port
PS 0
5F
Figure 2.13. Interface of a switch.
ldaa PTS
read input Port S
anda #$01
clear bits except bit 0
staa Pressed true iff PS0 is high
a7 a6 a5 a4 a3 a2 a1 a0
0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 a0
value of PTS
$01 constant
result of the anda instruction
The or operation to set bits 4 and 5 of the register DDRT.
The other six bits of DDRT remain constant.
Friendly software modifies just the bits that need to be.
DDRT |= 0x30; /*PT4,PT5 outputs */
ldaa DDRT
oraa #$30
staa DDRT
read previous value
set bits 4 and 5
update
c7 c6 c5 c4 c3 c2 c1 c0
0 0 1 1 0 0 0 0
c7 c6 1 1 c3 c2 c1 c0
value of DDRT
$30 constant
result of the oraa instruction
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.10
Maintenance Tip: When interacting with just some of the bits of an I/O register it
is better to modify just the bits of interest, leaving the other bits unchanged. In this
way, the action of one piece of software does not undo the action of another piece.
The exclusive or operation can also be used to toggle bits.
PTM ^= 0x08; /* toggle PM3 */
ldaa PTM
eora #$08
staa PTM
read output Port M
toggle bit 3
update
b7 b6 b5 b4 b3 b2 b1 b0
0 0 0 0 1
0 0 0
b7 b6 b5 b4 ~b3 b2 b1 b0
value of PTM
$08 constant
result of the eora instruction
The output of an open collector gate,
drawn with the ‘x’,
has two states low (0V) and HiZ (floating.)
+5V
C
2 20 
O u t p u t po r t p
P M3
q
LE D
7 40 6
Figure 2.16. LED interface.
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.11
The and operation can be used to clear bits.
PTM &= 0xF7; /* PM3 becomes 0 */
ldaa PTM
anda #$F7
staa PTM
read output Port M
clear just bit 3
update
b7 b6 b5 b4 b3 b2 b1 b0
1 1 1 1 0 1 1 1
b7 b6 b5 b4 0 b2 b1 b0
value of PTM
$F7 constant
result of the anda instruction
Checkpoint 2.33: Write assembly code that clears bit 1 of Port M.
Checkpoint 2.34: Write assembly code that sets bit 7 of Port M.
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.12
2.7. Shift operations
LSR
0
C
Figure 2.19. 8-bit logical shift right.
ASR
C
Figure 2.21. 8-bit arithmetic shift right.
LSL/ASL C
0
Figure 2.22. 8-bit shift left.
ROR
C
ROL
C
Figure 2.23. 8-bit roll right and 8-bit roll left.
asla
lsla
asra
lsra
rola
rora
RegA=RegA*2
RegA=RegA*2
RegA=RegA/2
RegA=RegA/2
roll left RegA
roll right RegA
Maintenance Tip: Use the asla instruction when manipulating signed numbers,
and use the lsla instruction when shifting unsigned numbers.
High and Low are unsigned 4-bit components, which will be combined into a single
unsigned 8-bit Result.
Result = (High<<4)|Low;
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.13
The assembly code for this operation is
ldaa High
read value of High
lsla
shift into position
lsla
lsla
lsla
oraa Low
combine the two parts
staa Result save answer
Lab 1. Logic Function
74LS14
Microcontroller
+5 V 5k 
S
22
switch
PS0
CPU
10 
Input
Port
74LS14 +5 V 5k 
RAM
PM0
ROM
22
switch
10 
Output
Port
M
+5 V
7405
LED
T
PT0
Simulated hardware circuit for Lab 1.
The specific function you will implement is
T  M &S
This means the LED will be on if and only if the M switch is not pressed and the S switch
is pressed, as shown belwo
TExaS IO window showing the door is unlocked.
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
void main(void){
DDRM = 0x00;
// make Port M an input,
DDRS = 0x00;
// make Port S an input,
DDRT = 0xFF;
// make Port T an output,
while(1){
PTT = (~PTS)&PTM; // LED on iff PS0=0 and
}
The first C program to illustrate Lab 1.
Lecture 3.14
PM0 is M
PS0 is S
PT0 is T
PM0=1
Approach -> start with template
;****************** Lab1.RTF ***************
; The overall objective of this system is a digital lock
; Hardware connections
; PM0 is switch input M
; PS0 is switch input S
; PT0 is LED output T (on means unlocked)
; The specific operation of this system
;
unlock if M is pressed and S is not pressed
; Your name and date
;I/O port definitions
PTM
equ $0250
;Port M I/O Register
PTS
equ $0248
;Port S I/O Register
PTT
equ $0240
;Port T I/O Register
DDRM equ $0252
;Port M Data Direction Register
DDRS equ $024A
;Port S Data Direction Register
DDRT equ $0242
;Port T Data Direction Register
org $3800
;2k RAM ($3800-$3FFF)
;Global variables (none required for
this lab)
org $4000
;32k EEPROM ($4000-$7FFF,$C000-$FFFF)
main
;Software performed once at the
beginning
loop
;Software repeated over and over
bra loop
org $FFFE
fdb main
;Starting address
end
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.15
2.8. Arithmetic operations
Checkpoint 2.37: How many bits does it take to store the result of two unsigned 8bit numbers added together?
Checkpoint 2.39: How many bits does it take to store the result of two unsigned 8bit numbers multiplied together?
adda #w
adda u
adda U
suba #w
suba u
suba U
RegA=RegA+w
RegA=RegA+[u]
RegA=RegA+[U]
RegA=RegA-w
RegA=RegA-[u]
RegA=RegA-[U]
condition code register (CC or CCR)
C set after an unsigned add if the answer is wrong
V set a signed add if the answer is wrong
bit
name
N
negative
Z
zero
V
overflow
C
carry
Table 2.18. Condition code bits.
meaning after add or sub
result is negative
result is zero
signed overflow
unsigned overflow
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.16
96+64
224+64
+64
255 0
255 0
224
32
64
192
160
64
192
96
128
128
+64
Figure 2.26. Unsigned number wheel.
160-64
32-64
255 0
255 0
224
64
192
160
-64
32
64
192
96
128
128
-64
Figure 2.28. Unsigned number wheel.
Observation: The carry bit, C, is set after an unsigned addition or subtraction
when the result is incorrect.
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.17
-32+64
96+64
+64
-1 0
-1 0
-32
32
64
-64
64
-64
-96
-128 127
96
-128 127
+64
Figure 2.29. Signed number wheel.
32-64
-96-64
-64
-1 0
-1 0
-32
32
64
-64
64
-64
-96
-128 127
-64
96
-128 127
Figure 2.30. Signed number wheel
Observation: The overflow bit, V, is set after a signed addition or subtraction when
the result is incorrect.
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.18
Let the result R be the result of the addition A+B.
N bit is set
if unsigned result is above 127 or
if signed result is negative.
N = R7
Z bit is set if result is zero.
Z=
R7 & R6 & R5 & R4 & R3 & R2 & R1 & R0
V bit is set after a signed addition if result is incorrect
V=
A7 & B7 & R7  A7 & B7 & R7
C bit is set after an unsigned addition if result is incorrect
C=
A7 & B7  A7 & R7  B7 & R7
Checkpoint 2.41: Assume Register A is initially -100. After executing the
instruction adda #64 what is the value in Register A, and the NZVC bits?
Checkpoint 2.42: Assume Register A is initially -100. After executing the
instruction adda #-64 what is the value in Register A, and the NZVC bits?
Let the result R be the result of the subtraction A-B.
N bit is set
if unsigned result is above 127 or
if signed result is negative.
N = R7
Z bit is set if result is zero.
Z=
R7 & R6 & R5 & R4 & R3 & R2 & R1 & R0
V bit is set after a signed addition if result is incorrect
V=
A7 & B7 & R7  A7 & B7 & R7
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.19
C bit is set after an unsigned addition if result is incorrect
C=
A7 & B7  B7 & R7  A7 & R7
Common Error: Ignoring overflow (signed or unsigned) can result in significant
errors.
Observation: Microcomputers have two sets of conditional branch instructions (if
statements) that make program decisions based on either the C or V bit.
Promotion involves increasing the precision of the input numbers, and performing the
operation at that higher precision.
decimal
8-bit
16-bit
224
1110,0000
0000,0000,1110,0000
+ 64
+0100,0000
+0000,0000,0100,0000
288
0010,0000
0000,0001,0010,0000
We can check the 16-bit intermediate result to see if the answer will fit back into the 8-bit
result.
unsigned add
unsigned sub
promote A to A16
promote B to B16
promote A to A16
promote B to B16
R16=A16+B 16
R16=A16-B 16
ok
R16 < 255
R16
overflow
R16 >255
R=255
R=R16
end
ok
R16 > 0
R16
underflow
R16 < 0
R=0
R=R16
end
Figure 2.31.Promotion to detect and correct unsigned arithmetic errors.
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.20
To promote a signed number, we duplicate the sign bit
decimal
8-bit
16-bit
-96
1010,0000
1111,1111,1010,0000
- 64
-0100,0000
-0000,0000,0100,0000
-160
0110,0000
1111,1111,0110,0000
signed add
signed sub
promote A to A16
promote B to B16
promote A to A16
promote B to B 16
R16=A16+B 16
R16=A16-B 16
underflow
R16 < -128
R16
R = -128
overflow
R16 >127
underflow
R16 < -128
R=127
R = -128
R16
overflow
R16 >127
R=127
R=R16
R=R16
end
end
Figure 2.32. Flowcharts showing how to use promotion to detect and correct signed
arithmetic errors.
Common Error: Even though most C compilers automatically promote to a higher
precision during the intermediate calculations, they do not check for overflow
when demoting the result back to the original format.
bcc
bcs
bvc
bvs
bpl
bmi
beq
bne
l1
l2
l3
l4
l5
l6
l7
l8
jump
jump
jump
jump
jump
jump
jump
jump
to
to
to
to
to
to
to
to
l1
l2
l3
l4
l5
l6
l7
l8
if
if
if
if
if
if
if
if
C=0
C=1
V=0
V=1
V=0
N=1
Z=0
Z=1
Mark W. Welker (From Jonathan W. Valvano)
Introduction to Embedded Microcomputer Systems
Lecture 3.21
ceiling and floor
unsigned add
R=A+B
unsigned sub
R=A-B
C=1
C=1
C
C=0
end
C
R=255
C=0
R=0
end
Figure 2.33. Flowcharts showing how to use overflow bits to detect and correct unsigned
arithmetic errors.
Assume A8 B8 and R8 are three 8-bit (1-byte) global variables defined in RAM.
A8
ds
1
Input
B8
ds
1
Input
R8
ds
1
Output
The following assembly language adds two unsigned 8-bit numbers, using the algorithm
presented in Figure 2.33.
ldaa A8
get first input
adda B8
A8+B8
bcc OK1 if C=0, then no error,
ldaa #255 overflow
OK1
staa R8
The following assembly language subtracts two unsigned 8-bit numbers.
ldaa A8
get first parameter
suba B8
A8-B8
bcc OK2 if C=0, then no error,
ldaa #0
underflow
OK2
staa R8
Mark W. Welker (From Jonathan W. Valvano)