Microprocessing Systems - Chapter 3: Instruction Set. How to write a program - Lê Chí Thông

t until R2=0 (10 times)
MOV R5,A
Lê Chí Thông 102
DJNZ – another example
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Write a program to copy a block of 10 bytes from RAM 
location starting at 37h to RAM location starting at 59h.
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DJNZ – Your Turn!
Write a program to copy a block of 10 bytes from RAM 
location starting at 37h to RAM location starting at 59h.
Solution:
MOV R0,#37h ; source pointer
MOV R1,#59h ; dest pointer 
MOV R2,#10 ; counter
L1: MOV A,@R0
MOV @R1,A
INC R0
INC R1
DJNZ R2,L1 
Lê Chí Thông 104
Solutions
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Blinky Program
P1.0  1
Delay
P1.0  0
Delay
schematic
ORG 0
LOOP: SETB P1.0
ACALL DELAY
CLR P1.0
ACALL DELAY
SJMP LOOP
DELAY: MOV R6,#200
DL1: MOV R7,#250
DJNZ R7,$
DJNZ R6,DL1
RET
END
(source)

Waveform? Period? Frequency?
Lê Chí Thông 106
Blinky Program
P1.0  1
Delay
P1.0  0
Delay
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ORG 0
LOOP: SETB P1.0
ACALL DELAY
CLR P1.0
ACALL DELAY
SJMP LOOP
DELAY: MOV R6,#200
DL1: MOV R7,#250 1MC x 1
DJNZ R7,$ 2 MC x 250
DJNZ R6,DL1
RET
END
Lê Chí Thông 107
Blinky Program
501 MC
ORG 0
LOOP: SETB P1.0
ACALL DELAY
CLR P1.0
ACALL DELAY
SJMP LOOP
DELAY: MOV R6,#200 1 MC x 1
DL1: MOV R7,#250
DJNZ R7,$
DJNZ R6,DL1 2 MC x 200
RET
END
Lê Chí Thông 108
Blinky Program
501 MC x 200 100,601 MC
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ORG 0
LOOP: SETB P1.0
ACALL DELAY
CLR P1.0
ACALL DELAY
SJMP LOOP
DELAY: MOV R6,#200
DL1: MOV R7,#250
DJNZ R7,$
DJNZ R6,DL1
RET 2 MC
END
Lê Chí Thông 109
Blinky Program
100,601 MC
100,603 MC
ORG 0
LOOP: SETB P1.0 1 MC
ACALL DELAY 100,063 + 2 MC
CLR P1.0 1 MC
ACALL DELAY 100,063 + 2 MC
SJMP LOOP 2 MC
DELAY: MOV R6,#200
DL1: MOV R7,#250
DJNZ R7,$
DJNZ R6,DL1
RET
END

tH = 100,063 + 2 + 1 = 100,066 MC 

tL = 100,063 + 2 + 2 + 1 = 100,068 MC
Lê Chí Thông 110
Blinky Program
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If using 12 MHz crystal, 1 MC = 1 μs

 tH = 100,066 MC = 100,066 μs

tL = 100,068 MC = 100,068 μs

 T = tH + tL = 200,134 μs

 f = 1/T = 4.99 Hz
Lê Chí Thông 111
Blinky Program
tH tL
T
P1.0  1
Delay tH
P1.0  0
Delay tL
ORG 0
LOOP: SETB P1.0
ACALL DELAY
CLR P1.0
ACALL DELAY
SJMP LOOP
DELAY: MOV R6,#200
DL1: MOV R7,#250
DJNZ R7,$
DJNZ R6,DL1
RET
END

tH = tL ≈ tDELAY ≈ 200 x 250 x 2 MC = 100,000 MC = 100,000 μs

 T ≈ 200,000 μs

 f ≈ 5 Hz
Lê Chí Thông 112
Blinky Program - Estimating
P1.0  1
Delay tH
P1.0  0
Delay tL
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ORG 0
LOOP: CPL P1.0
ACALL DELAY
SJMP LOOP
DELAY: MOV R6,#200
DL1: MOV R7,#250
DJNZ R7,$
DJNZ R6,DL1
RET
END
Lê Chí Thông 113
Blinky Program – Alternative Method
P1.0  NOT (P1.0)
Delay
P1.0  1
Delay
P1.0  0
Delay
Write a program that creates a 10-KHz square wave at pin P1.3. 
Assume that crystal is 24 MHz.
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10-kHz square wave
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Write a program that creates a 10-KHz square wave at pin P1.3. 
Assume that crystal is 24 MHz.
Lê Chí Thông 115
10-kHz square wave
tH tL
T
P1.3  1
Delay tH
P1.3  0
Delay tL
P1.3  NOT (P1.3)
Delay
If tH = tL
Write a program that creates a 10-KHz square wave at pin P1.3. 
Assume that crystal is 24 MHz.
ORG 0000H
lap: CPL P1.3
ACALL delay
SJMP lap
delay:
MOV R4,#50
DJNZ R4,$
RET
END
Lê Chí Thông 116
10-kHz square wave
P1.3  NOT (P1.3)
Delay
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Write a program that creates a 10-KHz square wave with duty cycle 
30% at pin P1.3. Assume that crystal is 24 MHz.
Lê Chí Thông 117
10-kHz square wave, duty cycle 30%
tH tL
T
P1.3  1
Delay tH
P1.3  0
Delay tL
Write a program that creates a 10-KHz square wave with duty cycle 
30% at pin P1.3. Assume that crystal is 24 MHz.
ORG 0000H
lap: SETB P1.3
ACALL delay1
CLR P1.3
ACALL delay2
SJMP lap
Delay1: MOV R4,#30
DJNZ R4,$
RET
Delay2: MOV R4,#70
DJNZ R4,$
RET
END
Lê Chí Thông 118
10-kHz square wave, duty cycle 30%
P1.3  1
Delay tH
P1.3  0
Delay tL
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Write a program that creates a 100-kHz square wave at pin P1.1. 
Assume that crystal is 12 MHz.
Lê Chí Thông 119
100-kHz square wave
Write a program that creates a 100-kHz square wave at pin P1.1. 
Assume that crystal is 12 MHz.
ORG 0000H
lap: CPL P1.1
NOP
NOP
SJMP lap
END
Lê Chí Thông 120
100-kHz square wave
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Write a program that creates a 100-kHz square wave with duty cycle 
40% at pin P1.2. Assume that crystal is 12 MHz.
Lê Chí Thông 121
100-kHz square wave, duty cycle 40%
Write a program that creates a 100-kHz square wave with duty cycle 
40% at pin P1.2. Assume that crystal is 12 MHz.
ORG 0000H
lap: SETB P1.2
NOP
NOP
NOP
CLR P1.2
NOP
NOP
NOP
SJMP lap
END
Lê Chí Thông 122
100-kHz square wave, duty cycle 40%
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CJNE A,#05H,Skip
(Statement 1)
Skip: (Continue)
Lê Chí Thông 123
CJNE – Equal/Not Equal (1)
A = 05H?
N
Y
Statement 1
Ex: Write a program to write 40H to internal RAM from location 
30H to location 36H.
(source)
ORG 0000H
MOV R1,#30H;Address=30H
Again: MOV @R1,#40H
INC R1
CJNE R1,#37H,Again 
END
Lê Chí Thông 124
An Example
Addr 30H
(Addr)  40H
AddrAddr+1
Addr=37H?
N
Y
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CJNE A,#05H,Not_Eq
(Statement 1)
SJMP Next
Not_Eq: (Statement 2)
Next: (Continue)
Lê Chí Thông 125
CJNE – Equal/Not Equal (2)
A = 05H?
N
Y
Statement 1Statement 2
CJNE A,#05H,Next
Next: JC LessThan
(Statement 1)
LessThan: (Continue)
Lê Chí Thông 126
CJNE – Greater Than or Equal/Less Than (1)
A ≥ 05H?
N
Y
Statement 1
CJNE A,#05H,$+3
JC LessThan
(Statement 1)
LessThan: (Continue)
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CJNE A,#05H,Next
Next: JNC GT_Eq
SJMP Continue
GT_Eq: (Statement 1)
Continue: (Continue)
Lê Chí Thông 127
CJNE – Greater Than or Equal/Less Than (1)
A ≥ 05H?
N
Y
Statement 1
CJNE A,#05H,$+3
JNC GT_Eq
SJMP Continue
GT_Eq: (Statement 1)
Continue: (Continue)
Examine the content of A, if 5 ≤ A ≤ 10 then output A to Port 1; 
if not, output A to Port 2.
ORG 0
CJNE A,#5,$+3
JC PORT2
CJNE A,#11,$+3
JNC PORT2
MOV P1,A
SJMP DONE
PORT2:MOV P2,A
DONE: NOP
END
128Lê Chí Thông
An Example
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CJNE A,#05H,$+3
JNC GT_Eq
(Statement 2)
SJMP Next
GT_Eq: (Statement 1)
Next: (Continue)
Lê Chí Thông 129
CJNE – Greater Than or Equal/Less Than (2)
A ≥ 05H?
N
Y
Statement 1Statement 2
Given a 20-byte string in internal RAM, starting at address 40H. 
Write a program that output even numbers to Port 2.
130Lê Chí Thông
Bit Testing
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Given a 20-byte string in internal RAM, starting at address 40H. 
Write a program that output even numbers to Port 2.
ORG 0
MOV R4,#20 ; Number of loops
MOV R0,#40H ; Address pointer
LOOP:
MOV A,@R0 ; Read data from internal RAM to A
JB ACC.0,NEXT ; skip if odd number
MOV P2,A ; Output to Port 2 if even number
NEXT:
INC R0
DJNZ R4,LOOP
END 131Lê Chí Thông
Bit Testing
Given a 20-byte string in external RAM, starting at address 
4000H. Write a program that output odd numbers to Port 2.
132Lê Chí Thông
Your Turn!
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Given a 20-byte string in external RAM, starting at address 
4000H. Write a program that output odd numbers to Port 2.
ORG 0
MOV R4,#20 ; Number of loops
MOV DPTR,#4000H ; Address of external RAM
LOOP:
MOVX A,@DPTR ; Read data from external RAM to A
JNB ACC.0,NEXT; skip if even number
MOV P2,A ; Output to Port 2 if odd number
NEXT:
INC DPTR
DJNZ R4,LOOP
END 133Lê Chí Thông
Solutions
Given a 100-byte unsigned number string in external RAM at 
address starting from 0100H. Write a program that sends positive 
numbers to Port 1 and negative numbers to Port 2.
Hint:
- A positive number has MSB = 0.
- A negative number has MSB = 1.
- Use JB / JNB instruction
--
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Bit Testing
134
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Given a 100-byte unsigned number string in external RAM at 
address starting from 0100H. Write a program that sends positive 
numbers to Port 1 and negative numbers to Port 2.
ORG 0000H
MOV DPTR,#0100H
MOV R4,#100
loop: MOVX A,@DPTR
JNB ACC.7,positive
MOV P2,A
SJMP next
positive: MOV P1,A
next: INC DPTR
DJNZ R4,loop
END
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Bit Testing
135
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Bit Testing
LOOP: MOV C,P1.0
JNB P1.1,SKIP
CPL C
SKIP: MOV P1.2,C
SJMP LOOP
XOR
P1.0
P1.1
P1.2
136
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Lê Chí Thông
Example Problem
A 4-bit DIP switch and a common-anode 7-segment LED are 
connected to an 8051 as shown in the following figure. Write a 
program that continually reads a 4-bit code from the DIP switch and 
updates the LEDs to display the appropriate hexadecimal character. 
For example, if the code 1100B is read, the hexadecimal character 
“C” should appear, thus, segments a through g respectively should 
be ON, OFF, OFF, ON, ON, ON, and OFF. Note that setting an 8051 
port pin to “1” turns the corresponding segment “ON”.
137
138
References 
Lê Chí Thông
• I. Scott MacKenzie , The 8051 Microcontroller, 2nd 
Edition, Prentice-Hall, 1995
• Kenneth J. Ayala, The 8051 Microcontroller: 
Architecture, Programming, and Applications, West 
Publishing Company
• hsabaghianb@kashanu.ac.jr , Lecture notes