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1. Chapter2 The Hardware of the 8051
Microcontroller
Section2.1 Internal structure of the 8051
CPU
RAM 4K ROMP0
P2
P1
P3
Serial
port
Timer/
counters
Interrupt
system
SFR
ALE
XTAL1
XTAL2
8 8
8
8
PSEN RESETEA
Figure2.1 Internal structure of the 8051

2. The important features,most of which are common
to all microcontrollers,are:
1.CPU:Central Processing Unit
Playing the role as a brain and heart of computer(including normal and
special purpose computers),having very strong and multidimensional
functions surrounding the mathematical and logical operation,but less
independency and connection ability unless getting in a combination
consisting of a lot of assistant elements.
2.Internal RAM
RAM:Random Access Memory
Up to 128/256 bytes or more data memory on chip for different chips
-51 sub-serial 128 bytes (They are located from 00H to 7FH.)
-52 sub-serial 256 bytes

3. 3. Internal ROM
ROM:Read Only Memory
Up to 4KB/ 8KB/16KB/32KB/64KB
8031: no
8051:4K(They are located from 0000H to 0FFFH.)
8751:4K EPROM(electrically programmable ROM)
4.I/O port
There are four I/O ports named as P0, P1, P2 and P3
respectively.
And there are thirty-two I/O pins arranged as four 8-bit ports.
5.Full Duplex UART Serial Port
UART:Universal Asynchronous Receiver/Transmitter

4. 6. Two or three 16-bit timer/counters for different subserials
-51 sub-serial two
-52 sub-serial three
As timers its pulse source is internal machine cycle.
As counters its pulse source from I/O pins(P3.4 and P3.5).
(P3.4 is the input of counter 0 and P3.5 is the input of counter 1.)
7.Five or six interrupt sources with 2 priority levels
-51 sub-serial five
-52 sub-serial six
a five vector 2-level interrupt architecture

5. 8.SFR(21):Special Function Registers
Including a mix of operational registers,I/O registers and control
registers.
They are located from 80H to FFH.

6. Section2.2 Pin connections and functions
Figure 2.2 The pinout of the 8051 family
The 8051 is available in several types of
package to suit a variety of conditions and
layout designs.Figure 2.2 shows the pinout
for the popular 40-pin DIP package with the
abbreviated names of the signals for each
pin.
It is important to note that many of the pins
are used for more than one function,the
alternate functions being shown in
slash( 斜线 ).Because of this,not all of the
possible 8051 features may be used at the
same time.Programming instructions or
physical pin connections determine the use
of any muti-function pins.

7. For example,port 3 bit 0(abbreviated P3.0) may be used as a
general purpose I/O pin,or as an input(RXD) to the serial port
data receiver.
The system designer decides which of these two functions is to be
used,and designs the hardware and software affecting that pin
accordingly.

8. There are 2 special power supply pins,2 external crystal pins,4
control pins and 32 I/O pins.
1.Main power supply pins
Pin40:Vcc +5V
Pin20:Vss ground connection
2.External crystal pins
XTAL1:Input to the inverting oscillator amplifier and input to the internal
clock operating circuit.
XTAL2:output from the inverting oscillator amplifier.
Oscillator characteristics:
XTAL1 and XTAL2 are the input and output,respectively,of an inverting
amplifier which can be configured for use as an on-chip oscillator,as
shown in figure2.3.

9. C1,C2:30PF
The range of the crystal oscillator
frequency is usually from 1.2MHz to
12MHz.
Either a quartz crystal or ceramic
resonator may be used.
Figure2.3 Internal Clock Drive Configuration
8051

10. Figure2.4 External Clock Drive Configuration
To drive the device from an external
clock source,XTAL2 should be left
unconnected while XTAL1 is driven as
shown in this figure.
NC
External oscillator
signal

11. 3.Four control pins
1)pin9:RST(Reset input)
A high level on this pin for two machine cycles while the oscillator
is running resets the device.
2)pin30:ALE/ PROG
Address Latch Enable output pulse for latching the low byte of the
address during accesses to external memory.
This pin is also the program pulse input( PROG) during EPROM
programming.
In normal operation ALE is emitted at a constant rate of 1/6 the
oscillator frequency and may be used for external timing or
clocking purposes.

12. 3)pin29:PSEN
Program Strobe Enable is the read strobe( 闸门 ) to external program
memory.
When the 8051 is executing code from external program memory, PSEN
is activated twice each machine cycle,except that （不同之处在
于） two PSEN activations are skipped during each access to external
data memory.
4)pin31:EA/Vpp(External Access Enable/Voltage Pulse of Programming)
EA must be strapped to GND in order to enable the device to fetch code
from external program memory locations starting at 0000H to
FFFFH.EA should be strpped to Vcc for internal program executions.
This pin also receives the 21-volt programming enable voltage during
EPROM programming ,for parts that require 21-volt Vpp.

13. 4.Thirty-two I/O pins:
1)pin39~pin32:P0.0~P0.7 or named as port 0,which is often used
as multiplexed low order address/data bus during accesses to
external program and data memory.
P0 is often used as ADB.
2) pin1~pin8:P1.0~P1.7 or named as port 1,which is a true I/O
port.
3) pin21~pin28:P2.0~P2.7 or named as port 2,which is often used
to output the high order address when accessing external
memory.
4) pin10~pin17:P3.0~P3.7 or named as port 3,which is often used
as alternate functions.

14. All the pins on port 3 can be used
for other purposes than simple
input/output.Some are used for
extended functions of the
microcontroller,which others
serve as inputs for particular
operations.The functions are
shown in Table 2.1.
Port
pin
Alternate
function
Description
P3.0 RXD Serial port input
P3.1 TXD Serial port output
P3.2 INT0 External Interrupt 0
P3.3 INT1 External Interrupt 1
P3.4 T0 Counter 0 external input
P3.5 T1 Counter 1 external input
P3.6 WR External data memory write
strobe
P3.7 RD External data memory read
strobe

15. 5.Bus structure
1)AB:16-bit provided by P0 and P2
2)DB: 8-bit provided by P0
3)CB:P3 alternate functions
RESET,ALE,PSEN,EA

16. Section2.3 Memory Organization in
8051 Microcontroller
Physically the 8051 has four memory spaces.
Internal/external program memory
Internal/external data memory
Logically it has three memory spaces,the whole program memory.
FFFFH

17. 1.Program memory
There are five special entrance units which are
as following respectively.
0003H external interrupt 0
000BH timer 0 interrupt
0013H external interrupt 1
001BH timer 1 interrupt
0023H serial port interrupt

18. EA:strapped to Vss -----external program memory
(64KB,0000H~FFFFH)
strapped to Vcc -----internal program memory
PC:0FFFH
The address of internal and external program memory is
continuous.

19. 2.External data memory
It is possible to interface an external RAM chip to the 8051,in
which case locations from 0000H to FFFFH can be accessed
for both reading and writing.
External RAM can be up to 64KB.(0000H~FFFFH)
We can use MOVX instruction to access.
Indirect addressing mode.

20. 3.Internal data memory
This 256-byte area of RAM is divided into two sections:
1)The lower section 128 bytes
Direct or indirect addressing mode.
(They are located from 00H to 7FH.)
2)SFRs(They are located from 80H to FFH.)
The lower section 128 bytes is composed of :
 Working register bank 00H~1FH(32 bytes)
 Bit-addressable bank 20H~2FH(16 bytes)
 General RAM bank 30H~7FH(80 bytes)

21. Working register bank:
RS1 RS0
0 0 0 R0~R7 00H~07H
1 0 1 R0~R7 08H~0FH
2 1 0 R0~R7 10H~17H
3 1 1 R0~R7 18H~1FH
A number of instructions refer to these RAM locations as R0
through R7.The selection of which of the four banks is being
referred to is made on the basis of the bits RS0 and RS1 at
execution time.

22. Bit-addressable
bank:20H~2FH.
Bit address
covers:00H~7FH.

23. SFR(direct addressing mode)
1)Acc(E0H):Acc is the Accumulator register.The mnemonics for
accumulator-specific instructions,however,refer to the
accumulator simply as A.
2)B register(F0H):The B register is used during multiply and divide
operations.For other instructions,it can be treated as another
scratch pad(general-purpose) register.
3)PSW(D0H):Program Status Word
The PSW register contains program status information as detailed
in Table2.2.

24. CY AC F0 RS1 RS0 OV ----- P
CY:carry flag(carry from 7th to 8th )
AC:auxiliary carry flag(carry from 3th to 4th )
F0:flag 0(available to the user for general purposes)
RS1,RS0:register bank select control bits 1&0.
Set/cleared by software to determine working register bank.
Note:The contents of (RS1,RS0)enable the working register banks
as follows:
(0,0)-bank 0 R0~R7(00H~07H)
(0,1)-bank 1 R0~R7 (08H~0FH)
Table 2.2:

25. (1,0)-bank 2 R0~R7 (10H~17H)
(1,1)-bank 3 R0~R7 (18H~1FH)
OV:overflow flag
The Overflow flag will be set when the value of a number exceeds the limits for
signed numbers.In case of 8-bit numbers,OV will be set if a number in signed
form is greater than +127 or less than -128..In case of 16-bit numbers,OV will
be set if a number in signed form is greater than +32767 or less than -32768.
---:reserved
P:Parity flag.Set/cleared by hardware each instruction cycle to indicate an odd/even
number of “one” bits in the accumulator,i.e. even parity.
an odd number of “one” bits in A P=1
an even number of “one” bits in A P=0
67 CCOV ⊕= 1415 CCOV ⊕=

26. The Program Status Word(PSW) contains several status bits that
reflect the current state of the CPU.The PSW resides in SFR
space.It contains the carry bit,the auxiliary carry(for BCD
operations),the two register bank select bits,the overflow flag,a
parity bit,and two user-definable status flags.
The carry bit,other than serving the functions of a carry bit in
arithmetic operations,also serves as the”Accumulator” for a
number of Boolean operations.

27. For example:
MOV A,#10111000B
ADD A,#11001100B
In this case both the carry and auxiliary carry will be set.
Because of the number of zeros and ones in A,the parity flag will
be cleared.

28. 4)SP(81H): The stack may reside anywhere in on-chip RAM,and
therefore uses a one-byte stack pointer (The stack pointer
register is 8 bits wide.).The stack grows upwards,with the stack
pointer being incremented before each PUSH operation and
decremented after each POP operation.The stack pointer is
initialized to 07H after a reset.This causes the stack to begin at
location 08H.
5)DPTR:The data pointer consists of a high byte(DPH)(83H) and a
low byte(DPL)(82H).Its intended function is to hold a 16-bit
address.It may be manipulated as a 16-bit register or as two
independent 8-bit registers.
DPTR=DPH+DPL
MOV DPTR,#7FFFH
MOV DPH,#7FH
MOV DPL,#0FFH

29. 6)Ports 0 to 3:
P0(80H),P1(90H),P2(A0H) and P3(B0H) are the SFR latches of ports
0,1,2 and 3,respectively.
MOV P1,#00H
MOV A,P1
The names P0, P1, P2 ,P3 may be used directly as operands in
instructions.
7)SBUF(99H):
The serial data buffer is actually two separate registers,a transmission
buffer and a reception buffer register.When data is moved to
SBUF,it goes to the transmission buffer where it is held for serial
transmission.(Moving a byte to SBUF is what initiates the
transmission.)When data is moved from SBUF,it comes from the
reception buffer.

30. SBUF
TXD
RXD
8)Timer registers:
Register pairs(TH0(8CH),TL0(8AH)),(TH1(8DH),TL1(8BH)) are the
16-bit counting registers for timer/counters 0,1 respectively.
T0=TH0+TL0
T1=TH1+TL1

31. 9)PC:Program Counter 16-bit
The address of the next instruction to be executed is held in this
16-bit register.Strictly speaking,this is not an SFR, since it does
not have an address in memory.It is included here because in
programming its use is sometimes similar to an SFR.
10)Control registers:
Special Function Registers IP,IE,TMOD,TCON,SCON and PCON
contain control and status bits for the interrupt system,the
timer/counters,and the serial port.
They are described in later sections.

32. SFRs whose byte addresses
end in 0H or 8H can be bit-
addressable.
There are 11 SFRs that can be
bit-addressable.
21 SFRs

33. Bit addressable SFRs
There are five bits
undefined.
There are a number
of 83 bits that can be
bit-addressable.

34. Section 2.4 CPU Timing
1.Oscillator cycle:
2.Machine cycle:
A machine cycle is the time that CPU performs a basic operation.
A machine cycle consists of 6 states(12 oscillator periods).
Each state is divided into a phase 1 half,during which the phase 1 clock is
active,and a phase 2 half,during which the phase 2 clock is active. Thus a
machine cycle consists of 12 oscillator periods,numbered
S1P1(State1,Phase1),through S6P2(State6,Phase2). Each phase lasts for
one oscillator period.Each state lasts for two oscillator periods.
ALE is normally activated twice during each machine cycle:one during
S1P2 and S2P1,and again during S4P2 and S5P1.
OSC
OSC
f
T
1
=
OSCcy TT 12=

37. 3.Instruction cycle:1~4Tcy
An instruction cycle is the time that CPU executes an instruction.
Most 8051 instructions execute in one cycle.MUL(multiply) and
DIV(divide) are the only instructions that take more than two
cycles to complete.They take four cycles.

38. Exercises:
1.How many parts the 8051 microcontroller is
composed of?
2.What is the function of EA?
3.What’s the relationship between an oscillator
cycle and a machine cycle?
4.In 8051,if fosc=6MHz,what’s the value of a
machine cycle?

39. 5.In the program memory of the 8051,there are five special entrance
units.Please write out these five special entrance units and the
corresponding interrupt sources.
6.In internal data memory,the bit address is 30H,98H.What are the byte
addresses of these two bits?
7.If the byte address is 2AH in internal RAM,the bit address of the last bit
is ________.
If the byte address is 88H in internal RAM,the bit address of the last bit
is______.
8.If the content of A is 63H,then the value of parity flag is ______.
9.Determine the following statements are correct or wrong.
(A)In 8051,CPU is composed of RAM and ROM.
(B)The most reliable method to distinguish external program memory from
external data memory is to see the address area.

40. (C)PC can be regarded as the address pointer of program
memory.
10.After a reset,the address of R4 is _____.Because
(PSW)=____.
The current working register bank is _____.
11.1)What’s a machine cycle?
2)How is the timing sequence of a machine cycle divided ?
3)If fosc=12MHz,how long is a machine cycle?
12.Determine the following statements are correct or wrong.
(A)DPTR can be accessed,but PC cannot.
(B)They are 16-bit registers.
(C)They all have the function of incrementing by one

41. (D)DPTR can be used as a 16-bit register or as two independent
8-bit registers,but PC cannot.
13.In internal RAM,how many locations can be used as working
register bank?How many locations are bit addressable ?Please
write out the byte address of them.
14.How many parts the internal RAM are divided into?
15.How to select the current working register bank in 8051?
16.Judge if the following parlance is correct:
(A)PC can not be used directly for user programming ,because it
has no address.
(B)Bit-addressable bank in internal RAM can only be bit-
addressable,but can not be byte-addressable.

42. (C)The bits of 21 SFRs can be set by software,so they are bit-
addressable.
17.what’s the value of PC?
18.Please write out the alternate functions of P3.
19.The addressing range of program memory is decided by the
bits of PC.Because PC is a 16-bit register,so the addressing
range is ______KB.
20.How to get rid of the dilemma when the 8051 is running error or
the program gets into an infinte loop?
21.Judge if the following parlances are correct?
(A)PC is an unaddressable SFR.

43. (B)The frequency of 8051 is more high,the operating speed is
more fast.
(C)A machine cycle is 1us in 8051.
(D)The content of SP is the content of the stack top location.
22.During CALL execution,we should first PUSH____onto stack to
protect the breakpoint.
During RET executing,we should first POP the breakpoint into
____.