8051

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Latest update: June 15, 2013,http://www.wavedigitech.com/

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Presentation on 8051 and Addressing

Modes

By :Faizan Ali KhanMail @ [email protected]


MuP and MuC

Microcontroller

It itself is a small computer It has a CPU, RAM,ROM and other

peripherals embedded on a single chip It has I/O ports and Serial Com port

Has separate m/ry to store program and data

Designed to perform specific tasks. Ex – mouse, mobile….

Has its own inbuilt memory Lower clock frequency and low

performance simple CPU arch

Microprocessor

• Its just a CPU of a computer• It just has a CPU

• None. Requires extra devices.(8155 for I/O, 8250 for Serial Com port)

• Same memory• Unspecific tasks.

Ex – gaming, websites….• Doesn’t have• higher clock frequency and a high

performance pipelined parallel CPU arch


Harvard and VON Neumann

Harvard architecture

• Has physically separate memory and s/g s for code and data memory

• Program memory and data memory can be accessed simultaneously

• It is impossible for program contents to be modified by the program itself

• Faster , as Program memory and data memory are accessed simultaneously

• Pipelining is possible• Processor can complete an instruction

in one cycle

Von Neumann architecture

• Has shared s/g s and memory

• Not possible

• Program can be easily modified by itself since it is stored in read-write memory

• Slower

• Not possible• Processor requires 2 clock cycles to

complete an instruction

programmemory processor Data

memoryprocessor

Program memoryAnd

Data memory


8051 Internal Block Diagram


Register Banks

Register Banks with Addresses Selecting the Register Banks


Stack

• The stack is a section of RAM used by the CPU to store information temporarily. This information could be data or an address. The CPU needs this storage area since there are only a limited number of registers

• Stack is LIFO• Register used to access the stack is called a Stack

Pointer (SP). Default value of a SP is 07H• PUSH and POP


Special Function Registers


SFRs

• Accumulator, A – Used in logical and arithmetic operations

• Register B – Used only in MUL and DIV• Program Counter – It points to the address of the next

instruction to be executed.• Stack Pointer - Used to access the stack • Data Pointer – Address of higher bytes from external

RAM


Program Status Word


PCON

SMOD is used to double the effective baud rate of the 8051's serial port.

The Power Control SFR is used to control the 8051's power control modes


.

Timer Control (TCON) Special Function

Register


BIT SYMBOL FUNCTION

7 TF1 Timer 1 Overflow flag. Set when timer rolls from all 1's to 0,i.e overflows. Cleared when processor vectors to execute interrupt service routine located at program address 001Bh.

6 TR1 Timer 1 run control bit. Set to 1 by program to enable timer to count; cleared to 0 by program to off timer.

5 TF0 Timer 0 Overflow flag. Set when timer rolls from all 1's to 0. Cleared when processor vectors to execute interrupt service routine located at program address 000Bh.

4 TR0 Timer 0 run control bit. Set to 1 by program to enable timer to count; cleared to 0 by program to halt timer.

3 IE1 External interrupt 1 Edge flag. Set to 1 when a high-to-low edge signal is received. Cleared when processor vectors to interrupt service routine.

2 IT1 External interrupt 1 signal type control bit. Set to 1 by program to enable external interrupt 1 to be triggered by a falling edge signal. Set to 0 by program to enable a low-level signal on external interrupt 1 to generate an interrupt.

1 IE0 External interrupt 0 Edge flag. Set to 1 when a high-to-low edge signal is received. Cleared when processor vectors to interrupt service routine.

0 IT0 External interrupt 0 signal type control bit. Set to 1 by program to enable external interrupt 1 to be triggered by a falling edge signal. Set to 0 by program to enable a low-level signal on external interrupt 0 to generate an interrupt.


.

Timer Mode Control (TMOD) Special

Function Register


BIT SYMBOL FUNCTION

7 Gate To control start and stop of timer 1.If GATE=0, SETB TR1 starts timer and CLR TR1 stops timer

6 C/T Set to 1 by program to act as a counter by counting pulses from external input pins 3.5. Cleared to zero by program to act as a timer by counting internal frequency.

5 M1 Timer/counter operating mode select bit 1. Set/cleared by program to select mode.


3 GATE To control start and stop of timer 0.If GATE=0, SETB TR1 starts timer and CLR TR1 stops timer

2 C/T Set to 1 by program to make timer 0 act as a counter by counting pulses from external input pin 3.4. Cleared to zero by program to make timer act as a timer by counting internal frequency.




.

SCON Serial Control Register


.

SM0 SM1 Mode Baud Rate

-----------------------

0 0 0 f osc/12

0 1 1 variable

1 0 2 f osc/32 or f osc/64

SM2 Multiprocessor Mode Control Bit

1 = Multi-processor mode

0 = Normal mode

REN Receiver Enable Bit

1 = Receive Enable

0 = Receive Disabled

TB8 Transmit Bit 8

Enabled only for modes 2 and 3. Made 0 as its not used

RB8 Receive Bit 8

Used in modes 2 and 3. it gets a copy of Stop Bit when an 8 bit data is received

RI, TI Serial Interrupts

RI is set to indicate receipt of a serial word and TI is set to indicate completion of a serial transmission and ready to transfer another byte.

SMO SM1 SM2 REN TB8 RB8 TI RI


. Interrupt Enable Register


Interrupt Enable Register

• EA - disable all the interrupts. EA=0, no interrupt is acknowledged. EA=1, interrupt source is enabled or disabled

• ----- for future use• ET2 – enables or disables timer 2 overflow interrupt• ES - enables or disables the serial port interrupt• ET1 - enables or disables timer 1 overflow interrupt• EX1 - enables or disables external interrupt 1• ET0 - enables or disables timer 0 overflow interrupt• EX0 - enables or disables external interrupt 0

EA ---- ET2 ES ET1 EX1 ET0 EX0


. Interrupt Priority Register


.

• ---- reserved• PT2 – timer2 interrupt priority bit• PS – serial port interrupt priority bit• PT1 – timer1 interrupt priority bit• PX1 – external interrupt 1 priority bit• PT0 – timer0 interrupt priority bit• PX0 – external interrupt 0 priority bit

---- ---- PT2 PS PT1 PX1 PT0 PX0


Interrupt Priority Register

Addressing Modes


Addressing modes

• The 8051 instructions use eight addressing modes. These are:

1.Register

2. Direct

3. Indirect

4. Immediate

5. Relative

6. Absolute

7. Long

8. Indexed


Register Addressing

Register Addressing

• In this mode the data, which the instruction operates on, is in one of eight registers labeled R0 to R7 (Rn, in general). These registers are to be found in one of four register banks, only one of which can be active at any one time. The active bank may be selected by using bit 3 and bit 4 of the PSW (rs0 & rs1). On power-up or reset, the default register bank is bank 0.

• For example, to logically OR the contents of accumulator A with that of register R3, the following instruction is used:

ORL A, R3• and the op-code is 01001011B. The upper five bits, 01001, indicate the

instruction, and the lower three bits, 011, the register.


Direct Addressing• Instructions using direct addressing consists of two bytes: op-code and address.

• Such instructions can access any on-chip variable or hardware register. Note that the most significant bit of the direct address determines which area in the on-chip is to be accessed. An address between 00H and 7FH accesses a location in the low-order on-chip RAM.

• Any address with bit 7 = 1 refers to one of the special function registers. It is not necessary to remember the addresses of these special function registers. The assembler usually understands and converts the mnemonic of a special function register, e.g. P2 for Port 2, into the appropriate address. An example of a direct addressing instruction is

MOV P1, A• which transfer the content of the accumulator to Port 1. The direct address of Port 1

(90H) is determined by the assembler and inserted as byte 2 of the instruction. The source of the data, the accumulator, is specified implicitly in the op-code.


Indirect Addressing

• In this mode of addressing the instruction performs an operation on the data whose address is contained in register R0 or R1. Instructions using indirect addressing are single byte instructions. In 8051 assembly language the symbol @ before R0 or R1 denotes indirect addressing. An example of an indirect addressing instruction is

SUBB A, @R0• This instruction performs the operation:

(A) (A) – (C) – ((R0)).


Immediate Addressing

• In an instruction that uses immediate addressing, the operand of the instruction is given as the byte that follows the op-code. The operand may be a numeric constant, a symbolic variable, or an arithmetic expression using constants, symbols, and operators.

• In assembly language we use the symbol # before an operand to denote immediate addressing. An example of an instruction using immediate addressing is

ANL A, #77

which performs the operation: (A) (A) · #77.


Relative Addressing

• Sometimes this is also called program counter relative addressing. This addressing mode is used only with certain jump instructions. A relative address (or offset) is an 8-bit signed value, which is added to the program counter to form the address of the next instruction executed.

• The range for such a jump instruction is –128 to +127 locations. Although the range is rather limited, relative addressing does offers the advantage of providing position-independent code (since absolute addresses are not used). For example, the instruction

JZ rel

performs the following operations:

(PC) ¬ (PC) + 2

IF (A) = 0

THEN (PC) ¬ (PC) + rel

ELSE continue• The branch destination is computed by adding the signed relative-displacement in the

second instruction byte to the PC, after incrementing the PC twice.


Absolute Addressing

• There are only two instructions that use this addressing: ACALL (absolute call) and AJMP (absolute jump).

• These instructions perform branching within the current 2K page of program memory. The branch address is obtained by successively concatenating the five high-order bits of the program counter, bits 5 – 7 of the op-code, and the second byte of the instruction.

• The diagram illustrate how this is done:


Absolute Addressing


Long Addressing

• Only two instructions use this addressing mode. These instructions are LCALL addr16 and LJMP addr16.

• Both of these are three byte instructions with the op-code being the first byte and the following two bytes are the address high-byte and address low-byte respectively.

• These instructions enable the program to branch to anywhere within the full 64 K-bytes of program memory address space.


Indexed Addressing

• In this mode the 16-bit address in a base register is added to a positive offset to form an effective address for the jump indirect instruction JMP @A+DPTR, and the two move code byte instructions MOVC A,@A+DPTR and MOVC A,@A+PC.

• The base register in the jump instruction is the data pointer and the positive offset is held in the accumulator. For the move instructions the base register can either be the data pointer or the program counter, and again the positive offset is in the accumulator. The operations of these three instructions are as follows:

JMP @A+DPTR (PC) (A) +(DPTR)

MOVC A,@A+DPTR (A) ((A) + (DPTR))

MOVC A,@A+PC (PC) (PC) + 1

(A) ((A) + (PC))


© Copyright 2013, wavedigitech.com.

Latest update: Jan 30, 2013,http://www.wavedigitech.com/

Call us on : 91-9632839173E-Mail : [email protected]

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