1. Hardware Summary
ECE473/573
Microprocessor System Design, Dr. Shiue

2. Introduction
The MCS-51 is a family of microcontroller ICs developed, manufactured, and marketed by Intel. Other IC manufacturers are
Siemens, Advanced Micro Devices (AMD), Fujitsu, Philips are licensed ‘second source’ suppliers of devices in the MCS-51 family.
The generic MCS-51 IC is the 8051, the first device in the family offered commercially. Its features are summarized
4KB ROM (64KB external code memory space)
128B RAM (64KB external data memory space)
8-bit I/O ports (4EA)
16-bit timers (2EA)
Serial interface
Boolean processor
210 bit-addressable locations
4 s multiply/divide
ECE473/573
Microprocessor System Design, Dr. Shiue

3. Comparison of MCS-51 ICs Part No. ROM/EPROM RAM Timers 8051 4K ROM
128B 2
8031
8751
4K EPROM
8052
8K ROM
256B 3
8032
8752
8K EPROM
ECE473/573
Microprocessor System Design, Dr. Shiue

4. 8051 Block Diagram Oscillator CPU Interrupt Control INT1 INT2 Timer 1
Serial port
Other registers
128B RAM
4KB ROM
Bus Control
I/O ports T1 T0
ALE
PSEN
RST EA
P0 P1 P2 P3
TXD
RXD
Address/data
Note: bold-faced pin assignments for P1 and P3.
ECE473/573
Microprocessor System Design, Dr. Shiue

5. 8051 Pinouts 8051 30pF Port 0 Port 1 Port 3 Port 2 VCC (40) VSS(20)
RD(17)
WR(16)
T1(15)
T0(14)
INT1(13)
INT0(12)
TXD(11)
RXD(10)
P0.7
P0.6
P0.5
P0.4
P0.3
P0.2
P0.1
P0.0
AD7(32)
AD6(33)
AD5(34)
AD4(35)
AD3(36)
AD2(37)
AD1(38)
AD0(39)
P1.7
P1.6
P1.5
P1.4
P1.3
P1.2
P1.1
P1.0
(8)
(7)
(6)
(5)
(4)
(3)
(2)
(1)
P2.7
P2.6
P2.5
P2.4
P2.3
P2.2
P2.1
P2.0
A15(28)
A14(27)
A13(26)
A12(25)
A11(24)
A10(23)
A9(22)
A8(21)
PSEN
ALE EA
RST
(29)
(30)
(31)
(9)
XTL1 (19)
XTL2 (18)
30pF
8051
Port 0
Port 1
Port 3
Port 2
ECE473/573
Microprocessor System Design, Dr. Shiue

6. I/O Ports
32 of the 8051’s 40 pins function as I/O port lines. However, 24 of these 32 lines are dual-purpose.
Dual purpose: can operate as I/O, control line, or part of address/data bus.
The 8-line in a port can be treated as a unit in interfacing to parallel devices such as printers, A/D converters, and so on.
Or, each line can operate independently in interfacing to single-bit devices such as switches, LEDs, transistors, motors, and loudspeakers.
ECE473/573
Microprocessor System Design, Dr. Shiue

7. I/O Ports Port 0 Port 1 Port 2
Is a dual-purpose port on pins of the 8051 IC.
In minimum-component designs,
It is used as a general purpose I/O port.
For larger designs with external memory,
It becomes a multiplexed address and data bus.
Port 1
Is a dedicated I/O port on pins 1-8.
The pins are available for interfacing to external devices as required.
Port 2
Is a dual-purpose port on pins of the 8051 IC.
As a general purpose I/O port
Or as the high-byte of the address bus for designs with external ROM or more than 256B of RAM.
ECE473/573
Microprocessor System Design, Dr. Shiue

8. I/O Ports Port 3 Is a dual-purpose port on pins 10-17 of the 8051 IC.
As a general purpose I/O port
These pins are multifunctional, with each having an alternate purpose related to special features of the 8051.
BIT
NAME
BIT address
functions
P3.0
RXD
B0H
Receive data for serial port
P3.1
TXD
B1H
Transmit data for serial port
P3.2
INT0
B2H
External interrupt 0
P3.3
INT1
B3H
External interrupt 1
P3.4 T0
B4H
Timer/counter 0 external input
P3.5 T1
B5H
Timer/counter 1 external input
P3.6 WR
B6H
External data memory write strobe
P3.7 RD
B7H
External data memory read strobe
P1.0 T2
90H
Timer/counter 2 external input
P1.1
T2EX
91H
Timer/counter 2 capture/reload
ECE473/573
Microprocessor System Design, Dr. Shiue

9. Control Signals PSEN (Program Store Enable)
Is an output signal on pin 29.
It is a control signal that enables external program (code) memory (ROM).
It usually connects to an EPROM’s output enable (OE) pin to permit reading of program bytes.
The PSEN signal pulses low (active stage) during the fetch stage of an instruction, which is stored in external program memory.
The binary codes of a program (opcode) are read from EPROM, travel across the data bus, and are latched into the 8051’s instruction register (IR) for decoding.
ECE473/573
Microprocessor System Design, Dr. Shiue

10. Control Signals ALE (Address Latch Enable)
Is an output signal on pin 30.
It is used for demultiplexing the address and data bus.
When port 0 is used in its alternate mode – as the data bus and low-byte of the address bus – ALE is the signal that latches the address into an external register during the first-half of a memory cycle. This done, the port 0 lines are then available for data input or output during the second-half of the memory cycle, when the data transfer takes place.
The ALE signal pulses at 1/6th the on-chip oscillator frequency and can be used as a general-purpose clock for the rest of the system.
ECE473/573
Microprocessor System Design, Dr. Shiue

11. Control Signals EA (External Access) RST (Reset)
Is an input signal on pin 31.
Is generally tied high (5V) or low (ground).
If high
The 8051 executes programs from internal ROM when executing in the lower 4K/8K of memory.
If low
Programs execute from external memory only (and PSEN pulses low)
RST (Reset)
Is an input signal on pin 9.
When this signal is brought high for at least 2 machine cycles, the 8051 internal registers are loaded with appropriate values for an orderly system start-up.
For normal operation, RST is low.
ECE473/573
Microprocessor System Design, Dr. Shiue

12. Memory Organization
Most microprocessors implement a shared memory apace for data and programs. Both the data and programs reside in the system RAM. Microcontroller, on the other hand, the control program must reside in ROM.
The internal memories consist of ROM and RAM. The RAM contains a rich arrangement of general-purpose storage, bit addressable storage, register banks, and special function registers.
ECE473/573
Microprocessor System Design, Dr. Shiue

13. RAM 7F FF General-purpose RAM (80 bytes) 30
Special Function Registers (SFR) 2F
Bit-addressable locations (16 bytes) 20 1F
Bank registers
(32 bytes) 00 80
ECE473/573
Microprocessor System Design, Dr. Shiue

14. General-Purpose RAM 7F
Ex: To read the contents of internal RAM address 5FH into the accumulator.
Solution1:(direct address mode)
MOV A, 5FH
Solution2:(immediate addressing & indirect address mode)
MOV R0, #5FH
MOV
General-purpose RAM (80 bytes) 30 2F 20 1F
Bank registers
(32 bytes) 00
ECE473/573
Microprocessor System Design, Dr. Shiue

15. 210 (128+82) Bit-Addressable RAM7F FF
The idea of individually accessing bits through software is a powerful feature of most microcontroller. 30
Special Function Registers (SFR)
(82 bits) 2F
Bit-addressable locations (16 bytes)
(128 bits) 20 1F 00 80
ECE473/573
Microprocessor System Design, Dr. Shiue

16. 128 General-Purpose Bit-Addressable Locations2F 7F 7E 7D 7C 7B 7A 79 78 2E 77 76 75 74 73 72 71 70 2D 6F 6E 6D 6C 6B 6A 69 68 2C 67 66 65 64 63 62 61 60 2B 5F 5E 5D 5C 5B 5A 59 58 2A 57 56 55 54 53 52 51 50 29 4F 4E 4D 4C 4B 4A 49 48 28 47 46 45 44 43 42 41 40 27 3F 3E 3D 3C 3B 3A 39 38 26 37 36 35 34 33 32 31 30 25 24 23 22 21 20 1F 1E 1D 1C 1B 1A 19 18 17 16 15 14 13 12 11 10 0F 0E 0D 0C 0B 0A 09 08 07 06 05 04 03 02 01 00
Ex: to set bit 67H
Solution: (in Microcontroller)
SETB 67H
Solution: (in Microprocessor)
MOV A, 2CH
ORL A, # B
MOV 2CH, A
ECE473/573
Microprocessor System Design, Dr. Shiue

17. 128 General-Purpose Bit-Addressable Locations2F 7F 7E 7D 7C 7B 7A 79 78 2E 77 76 75 74 73 72 71 70 2D 6F 6E 6D 6C 6B 6A 69 68 2C 67 66 65 64 63 62 61 60 2B 5F 5E 5D 5C 5B 5A 59 58 2A 57 56 55 54 53 52 51 50 29 4F 4E 4D 4C 4B 4A 49 48 28 47 46 45 44 43 42 41 40 27 3F 3E 3D 3C 3B 3A 39 38 26 37 36 35 34 33 32 31 30 25 24 23 22 21 20 1F 1E 1D 1C 1B 1A 19 18 17 16 15 14 13 12 11 10 0F 0E 0D 0C 0B 0A 09 08 07 06 05 04 03 02 01 00
Ex: What instruction would be used to set bit 3 in byte address 25H?
Solution: (in Microcontroller)
SETB 2BH
ECE473/573
Microprocessor System Design, Dr. Shiue

18. Register Banks 1F 18 R7
~R0 17 10 0F 08 07 00
Bank 3 (8 bytes)
Bank 2 (8 bytes)
Bank 1 (8 bytes)
Bank 0 (8 bytes)
Ex: Read the contents of address 05H into the accumulator.
Solution: (Register address mode)
MOV A, R5 (only 1-byte)
Solution: (direct address mode)
MOV A, 05H (2-byte instruction)
Data values used frequently should use one of these registers.
ECE473/573
Microprocessor System Design, Dr. Shiue

19. Register Banks 1F 18 R7
~R0 17 10 0F 08 07 00
Bank 3 (8 bytes)
Bank 2 (8 bytes)
Bank 1 (8 bytes)
Bank 0 (8 bytes)
Ex: What location of the following instruction writes the contents of accumulator into?
SETB RS1
SETB RS0
MOV R0, A
Solution: (Register address mode): 18H
Select the register bank 3
ECE473/573
Microprocessor System Design, Dr. Shiue

20. Register Banks 1F 18 R7
~R0 17 10 0F 08 07 00
Bank 3 (8 bytes)
Bank 2 (8 bytes)
Bank 1 (8 bytes)
Bank 0 (8 bytes)
Ex: What is the address of register 5 in register bank 3?
Solution: (Register address mode): 1DH
A 1B 1C 1D 1E 1F
R0 R1 R2 R3 R4 R5 R6 R7
ECE473/573
Microprocessor System Design, Dr. Shiue

21. 21 Special Function Registers (SFR)B E0 E7 E6 E5 E4 E3 E2 E1
ACC D0 D7 D6 D5 D4 D3 D2 -
PSW B8 BC BB BA B9 IP B0 B7 B6 B5 B4 B3 B2 B1 P3 A8 AF AC AB AA A9 IE A0 A7 A6 A5 A4 A3 A2 A1 P2 99
Not bit addressable
SBUF 98 9F 9E 9D 9C 9B 9A
SCON 90 97 96 95 94 93 92 91 P1 8D
TH1 8C
TH0 8B
TL1 8A
TL0 89
TMOD 88 8F 8E
TCON 87
PCON 83
DPH 82
DPL 81 SP 80 86 85 84 P0
ECE473/573
Microprocessor System Design, Dr. Shiue

22. 21 Special Function Registers (SFR)B E0 E7 E6 E5 E4 E3 E2 E1
ACC
Some SFRs are both bit-addressable and byte-addressable.
SETB 0E0H (bit-addressable) 98 9F 9E 9D 9C 9B 9A 99
SCON 90 97 96 95 94 93 92 91 P1
The bits within Port 1 have addresses 90H to 97H xxxB
ECE473/573
Microprocessor System Design, Dr. Shiue

23. Program Status Word (PSW)-
PSW CY AC F0
RS1
RS0 OV P
Carry flag (CY)
Is a dual-purpose. Carry out of bit 7 during add, or borrow into bit 7 during a subtract.
EX: MOV A, #FFH
ADD A, #1
What is the state of the carry flag and the content of the accumulator after execution of the following instruction sequence? MOV R5, #55H; MOV A, #0AAH; ADD A, R5;
Solution:
A=FFH and CY=0 (No Carry)
A=00H and sets the carry flag in the PSW (ie CY=1).
ECE473/573
Microprocessor System Design, Dr. Shiue

24. Program Status Word (PSW)-
PSW CY AC F0
RS1
RS0 OV P
Auxiliary Carry flag (AC)
When adding a BCD values, the AC is set if a carry was generated out of bit 3 into bit 4.
What is the state of the AC and the content of the accumulator after execution of the instruction sequence below?
MOV R5, #1; MOV A, #9; ADD A, R5;
Solution:
A=10H (BCD) =1010 = B  AC=1
ECE473/573
Microprocessor System Design, Dr. Shiue

25. Program Status Word (PSW)-
PSW CY AC F0
RS1
RS0 OV P
Register bank select bit (RS1 and RS0)
RS1 RS0 Register Bank #
0 2
SETB RS1; CLR RS0;  Register Bank 2
SETB RS1 (= SETB 0D4H)
SETB RS0 (=SETB 0D3H)
ECE473/573
Microprocessor System Design, Dr. Shiue

26. Program Status Word (PSW)-
PSW CY AC F0
RS1
RS0 OV P
Overflow Flag (OV)
When signed numbers are added or subtracted, software can examine this bit to determine if the result is in the proper range ( -128 < X < 127).
If X >=127 and X <=-128  OV=1
MOV R7, #0FFH; MOV A, #0FH; ADD A, R7;
Solution
R7= ( = B=-1)
A= (15)
A=-1+15=14 =0EH< 127  OV=0 (No overflow)
ECE473/573
Microprocessor System Design, Dr. Shiue

27. Program Status Word (PSW)-
PSW CY AC F0
RS1
RS0 OV P
Parity bit (P)
The P is set or cleared each machine cycle to establish even parity accumulator.
MOV A, #55H
A= B  numbers of 1-bit = 4  P=0
ECE473/573
Microprocessor System Design, Dr. Shiue

28. B Register and Stack PointerF0 F7 F6 F5 F4 F3 F2 F1 B 81
Not bit addressable SP 80 87 86 85 84 83 82 P0
B register or B accumulator
MUL AB
Results of low-byte in A and high-byte in B.
DIV AB
Integer results in A and remainder in B.
It is bit-addressable thru bit addresses F0H to F7H.
Stack Pointer (SP)
The SP is an 8-bit register at address 81H.
ECE473/573
Microprocessor System Design, Dr. Shiue

29. Data Pointer (DPL, DPH) 83
Not bit addressable
DPH 82
DPL
Data pointer (DPTR) is used to access external code or data memory (16-bit register)
DPH: Data Pointer High-byte) at 83H
DPL: Data Pointer Low-byte) at 82H EX
MOV A, #55H  A=55H
MOV DPTR, #1000H  DPTR=1000H (16-bit)
A  Move the content of A to the external RAM location whose address is in DPTR (1000H)
ECE473/573
Microprocessor System Design, Dr. Shiue

30. Port Registers B0 B7 B6 B5 B4 B3 B2 B1 P3 A0 A7 A6 A5 A4 A3 A2 A1 P2 90 97 96 95 94 93 92 91 P1 80 87 86 85 84 83 82 81 P0
Ports 0, 2, and 3 may not be available for I/O if external memory is used or if some of the 8051 special features are used (interrupt. Serial port etc.)
Nevertheless, P1.2 to P1.7 are always available as general purpose I/O lines.
SETB P1.7 (=SETB 97H) might turn the motor ON
CLR P1.7 (=CLR 97H) might turn the motor OFF.
ECE473/573
Microprocessor System Design, Dr. Shiue

31. Timer Registers 8D
Not bit addressable
TH1 8C
TH0 8B
TL1 8A
TL0 89
TMOD 88 8F 8E
TCON
The 8051 contains two 16-bit timer/counters for timing intervals or counting events. Timer 0 is TH0 and TL0, timer 1 is TH1 and TL1.
Timer operation is set by the timer mode register (TMOD) and timer control register (TCON).
The TCON is bit-addressable.
ECE473/573
Microprocessor System Design, Dr. Shiue

32. Serial Port Registers 99
Not bit addressable
SBUF 98 9F 9E 9D 9C 9B 9A
SCON
The 8051 contains an on-chip serial port for communicating with serial devices such as terminals or modem, or interfaces with other ICs with a serial interface (A/D converters, shift registers, nonvolatile RAM, etc.)
The SBUF (serial data buffer) at address 99H holds both the transmit data and receive data.
SCON (serial port control register) is used for various modes of operation.
SCON is a bit-addressable.
ECE473/573
Microprocessor System Design, Dr. Shiue

33. Interrupt Registers B8 - BC BB BA B9 IP A8 AF AC AB AA A9 IE
Interrupts are disabled after a system reset and then enabled by writing to the interrupt enable register (IE) at address A8H.
The priority level is set through the interrupt priority register (IP) at address B8H.
Both registers are bit-addressable.
ECE473/573
Microprocessor System Design, Dr. Shiue

34. Power Control Register87
Not bit addressable
PCON
SMOD -
GF1
GF0 PD
IDL
SMOD: Double baud rate bit; when set, baud rate is doubled in serial port modes 1, 2, and 3.
GF1 & GF0: General-purpose flag bits 1 and 0.
PD: Power down; set to activate power down mode; only exit is reset.
The oscillator is stopped, all functions are stopped, all RAM contents are retained, port pins retain their logic levels, and ALE and PSEN are held low. VCC is 2V.
IDL: Idle mode; set to activate idle mode; only exit is an interrupt or system reset.
The internal clock signal is gated off to the CPU. The CPU status is preserved and all register contents are maintained. Port pins also retain their logic levels. ALE and PSEN are held high.
ECE473/573
Microprocessor System Design, Dr. Shiue

35. External Memory
Address
Data
A0-A15
D0-D7
Non-multiplexed (24 lines)
A8-A15
AD0-AD7
Multiplexed (16 lines)
Memory Cycle
When external memory is used, Port 0 is not available as an I/O port. It becomes a multiplexed address (A0-A7) and data (D0-D7) bus, with ALE latching the low-byte of the address at the beginning of each external memory cycle. Port 2 is usually employed for the high-byte of the address bus.
ECE473/573
Microprocessor System Design, Dr. Shiue

36. Accessing External Code Memory
8051
D Q G
Port 0
ALE EA
Port 2
PSEN
D0-D7
A0-A7
A8-A15 OE
EPROM
74HC373
Here’s how the multiplexed arrangement works: during the first half of each memory cycle, the low-byte of the address is provided on Port 0 and is latched using ALE.
A 74HC373 latch holds the low-byte of the address stable for the duration of the memory cycle.
During the second half of the memory cycle, Port 0 is used as the data bus, and data are read or written depending on the operation.
ECE473/573
Microprocessor System Design, Dr. Shiue