8051 instruction set and addressing modes

*** Explain I/O Programming (OR) Discuss about I/O Programming

                  In 8051 there are four ports for I/O operation. In 40 pins, 32 pins are set for the 4 ports P_{0 ,} P₁, P₂ and P_3. Each port takes 8 pins. When “0” is sent to port, then it becomes an output . When “1” is sent to the port then it becomes an input.

                   Port-0 (32 to 39 pins) can be used as input or output. Port-0 must be connected externally to a 10KΩ pull-up resistor.

Port-0 as input : To make port-0 as input, by writing 1 to all the bits and then data is received from that port and sent to P₁. Get a byte from P₀ and send it to P₁.

             MOV A,#0FFH                   ; A=FFH

             MOV  P₀,A                            ;  make P₀  an i/p port by writing all 1’s to it

Back :   MOV A,P₀                              ;  Get data from P₀

              MOV P₁,A                             ; Send it to port-1

              SJMP  Back                          ; Keep doing it.

Dual role of Port-0  : Port-0 is also provides both address and data when 8051 is connecting to an external memory. The 8051 multiplexes address and data through port-0 to save pins.

To toggle all bits of P₀  continuously by 55H and AAH as

                  Toggle all bits of P_0.

   Back :         MOV A,#55H                   ; A=55H

                       MOV  P₀,A    

                       A CALL DELAY

                       MOV A,#0AAH               

                       MOV P₀,A

                        A CALL DELAY

                         SJMP  Back          

Port-1 : (1 to 8)pins can be used as input or output. This port does not need any pull-up resistors.Port-1 works as an input port by continuously send the code on  port-1. The alternating values 55H and AAH.

To toggle all bits of P₁  continuously by 55H and AAH as

                  Toggle all bits of P_1.

                      MOV A,#55H                   ; A=55H

      Back :      MOV  P₁,A    

                       A CALL DELAY

                       CPL A                          ; complement register A

                       SJMP  Back          

Port -1 as input : To make port -1 as input   port ,then must be writing 1 to all its bits, then data is received from that port and saved in R₇,R₆ & R₅ .

      MOV A,#0FFH       ; A=FFH

      MOV  P₁,A     :  make P₁  an i/p port by writing all 1’s to it

      MOV A,P₁                            ;  Get data from P₁ 

      MOV R₇, A                         ; Save it in register R₇

      A CALL DELAY                     ; wait 

      MOV A, P₁         ; get another data from port-1

      MOV R₆,A            ; save it in register R₆    

      A CALL DELAY         ; wait 

      MOV A, P₁           get another data from port-1

      MOV R₅,A         ; save it in register R₅    

     SJMP  Back     ; Keep doing it.

Port-2:(21 to28) . It can be used as I/P or O/P. Port-2 does not need any pull-up resistors.

   The following code will send out continuously to port-2 by the alternating values 55H and AAH

  i.e all the bits of P₂  toggle continuously.

                      MOV A,#55H                   ; A=55H

      Back :      MOV  P₂,A    

                       A CALL DELAY

                       CPL A                           ; complement register A

                       SJMP  Back 

Port -2 as input:

                      To make port-2 as an input, it must programmed by writing 1 to all its bits. Then data is received from that port and is sent to P₁  continuously.

Get a byte from P₂   and send it to P₁  

       MOV A,#0FFH                   ; A=FFH

       MOV  P₂,A    ;  make P₀  an i/p port by writing all 1’s to it

Back : MOV A,P₂                              ;  Get data from P₀

           MOV P₁,A                             ; Send it to port-1

          SJMP  Back                          ; Keep doing it.

Dual role of port -2: In many systems P₂ is used as simple I/O. When 8051/31 is connected to external memory P₂  is used for higher order address bus (A₈ –A₁₅).

  When P₀ provides lower 8-bit A₀-A₇  Address then P₂ must be provide upper 8-bit address lines A₈ –A₁₅  to access  64 Kbytes of external memory.

So normally P₂  cannot be used for I/O.

Port-3 : (10 to 17). It can be used as input or output. P₃  does not need any pull-up resistors. Port-3 is used as some extremely important control signals as alternate functions.

P_3.0  & P_3.1  is used for the (RXD & TXD) Serial communication signals.

P_3.2  & P_3.3  are used for external interrupts (INT0 bar,INT1bar).

P_3.4  & P_3.5  (T₀ ,T₁ ) are used for Timer 0 & Timer 1.

P_3.6  & P_3.7  (WR bar, RD bar) are used to provide Read and Write operations.

            BY applying reset all ports have value FFH. (1111 1111) on them. This makes  them input ports upon reset.

*** Discuss  about I/O Bit Manipulation Programming

I/O ports and bit addressability:

 I/O bit Manipulation is a powerful and widely used feature of the 8051 family.

When we need to access only 1 or 2 bits of the port instead of the entire 8-bits.

Then 4 ports (P₀, P₁,P₂, P₃ ) can access either the entire 8-bits or any single bit without altering the reset.

When accessing a Port in Single-bit manner, we use the syntax SET B X.Y

Where X represents port number 0,1,2,3 and Y represents for bit number from 0to7 for data bits D₀ to D₇  

Eg:        SET B P_1.5                             ; Sets bit 5 of Port-1

           To toggle bit P_1.2  continuously

Back :   CPL P_1.2                       ; Complement P_1.2  only

              A CALL DELAY

             S JMP BACK

 Single bit – instructions:  

*** Explain 8051 instruction set  (OR) Explain classification of 8051 instructions

 The 8051 instructions can be classified into five groups.

1)    Data transfer instructions

2)    Arithmetic instructions

3)    Logical instructions

4)    Branching instructions

5)    Boolean instructions

Data transfer instructions:

              In 8051 microcontroller MOV, MOVC, MOVX, PUSH, POP,  XCH and XCHD instructions are used to transfer data between registers and memory locations.

In 8051 microcontroller data transfer operations are:

·        Copy the content of a SFR to internal memory (OR) Vice versa.

·        Load an immediate operand to SFR/internal memory.

·        Exchange the content of SFR/internal memory with ACC.

·        Copy the content of program memory to Accumulator.

·        Copy the content of data memory to accumulator (OR) Vice versa.

Eg:   MOV  A,Rn

        MOV  A,direct

        MOV  A,@R_i

       MOV A,#data

 Arithmetic Instructions:  

              In 8051 Microcontroller we perform addition, subtraction, multiplication, division ,increment  and decrement operations on binary data.

              The mnemonic used for arithmetic operations are ADD, ADDC  , SUBB, INC, DEC, MUL, DIV and DA.

             The result of most of the arithmetic operation is stored in accumulator except a few decrement and increment operations.

             Except increment and decrement instructions all other arithmetic instructions are effected the flags of 8051.

Eg:     ADD A, Rn    

          INC @R_i

         SUBB A,direct

         DEC  Rn 

          MUL AB 

          DIVAB

Logical instructions: 

              In 8051 microcontroller we performing logical AND , OR, EX-OR, Complement operations and Right, left rotating operations.

             The mnemonic used for logical operations are ANL, ORL, XRL, CLR , CPL, RL,RLC,RR, RRC and SWAP.

              In logical operations only rotate through carry operation is effected the flags of 8051.

             In most of the logical instructions the result is stored in accumulator and in some instructions the result is stored in internal RAM/SFR.

Eg:            ANL   A, R_{n          ;}  logical operation is performing                                                        between A & R_n

                  ORL    A, # data

                  RRC     A

                 SWAP A

Program branching instructions:

         Normally a program is executed sequentially a PC (program counter) keeps track of the address of instructions and it is incremented appropriately after each fetch operation.

           There are two types of program branching instructions

1.     Conditional branching instruction

2.     Un Conditional branching instruction

             In conditional branching instructions the content of pc is modified only if

 the condition in the instruction is true,

            In Unconditional branching instruction the pc is always modified.

         The instructions like ACALL & L CALL will save the previous value of Pc in stack before modifying the Pc.

Eg:  JNZ offset, JZ OFFSET, LJMP 16-BIT, RET, NOP, RET1, DJNZ R_n, offset.

Boolean variable instructions:

         The Boolean variable instructions operate on a particular bit of a data.  This group include instructions which clear complement or move a particular bit of bit addressable RAM/SFR or carry flag.

         It also include jump instructions which transfer the program control to a new address if a particular bit is set or cleared.  The Boolean variable instructions of 8051 are

Eg: CLRC, SET B C,  SET B bit, CPL C, CPL bit,

         MOV C , bit, JC offset, JNC  etc

*** DISCUSS ACCESSING MEMORY USING VARIOUS ADDRESSING MODES:

         We  can use direct or register indirect addressing modes to access data stored either in RAM or registers of the 8051 and also ON-Chip ROM containing data using indexed addressing mode.

Direct addressing mode:  There are 128 bytes of RAM in the 8051(from OOH to 7FH) This RAM can be accessed using direct addressing mode.

         Mostly 30 to 7FH RAM locations are accessed by it.  Register bank locations are accessed by the register names of R₀-R₇.  But there is no such name for other RAM locations.

         In the direct addressing mode, the data is in a RAM  address is known and this address is given as a part of the instruction.

SFR REGISTERS AND THEIR ADDRESSES:

         In 8051 registers A,B,PSW &  DPTR are part of the SFR. The SFR can be accessed by their names or by their addresses.

Eg:  Reg A address is EOH and register B address is FOH.  Then we write instructions as

         MOV  OEOH,  #5H      ( OR )        MOV A, #55H

The special function register address is between 8OH to FFH

Not all the address space of 80 to FF is used by the SFR.  The unused locations in 80H to FFH are reserved and must not be used by the 8051 programmer.

         In direct address mode, the address value is limited(00H to FFH)  to access RAM locations and registers located inside the 8051.

STACK IN DIRECT ADDRESSING MODE:

         The major use of direct addressing mode is stack.  In 8051 family only direct addressing mode is used to PUSH and POP the stack by their register addresses.

REGISTER INDIRECT ADDRESSING MODE:

         In register indirect addressing mode a register is used as a pointer of the data.  Registers R_O & R₁ are used for this purpose. 

           R₂ to R₇ cannot be used to locate the RAM. They must be represented by the “@” sign.

          One of the advantage of register indirect addressing mode is that it makes accessing data dynamic rather than static.

         Looping is not possible in direct addressing mode, it is possible only in Indirect addressing mode.  This is the difference between the direct& register indirect addressing mode.

         In register indirect addressing mode, to accessing any information in internal RAM is limited because only two registers R_O&R₁ are used in  it.

         To accessing externally connected RAM or ON-chip ROM we need a 16-bit pointer.  In such cases the DPTR register is used.

INDE XED ADDRESSING MODE AND ON-CHIP ROM ACCESS:

         Indexed addressing mode is widely used in accessing data elements of look up table is located in 8051 program ROM.

         MOVC  A, @A+DPTR

Instruction  is used for this purpose.  The 16-bit register DPTR and register A are used to form the address of the data element stored in ON-CHIP ROM.

           The instruction MOV C is used instead of MOV. The “c” means code.  In this instruction the contents of A are added with DPTR  to form the 16-bit address of the needed data.

         Program counter is another register used in Indexed addressing mode.  The 8051 has another 64k bytes of memory space set for data storage i.e., data memory.  This external memory is accessed only by the MOV X instruction.

*** Explain 8051 ADDRESSING MODES:

         The CPU can access data in various ways i.e., in a register(or) be provided as an immediate value.  These various ways of accessing data are called addressing modes.

         The 8051 provides a total of five addressing modes.  They are

1                   Immediate addressing mode

2                   Register addressing mode

3                   Direct addressing mode

4                   Register indirect addressing mode

5                   Indexed addressing mode

1.     IMMEDIATE ADDRESSING MODE:

         In immediate addressing mode an 8/16 bit immediate data/constant is specified in the instruction itself.

         The immediate data must be expressed as pound sign i.e., #

         This addressing mode is used to load information into any of the register including the DPTR register.

         We can also use immediate addressing mode to send data to 8051 ports.

         When 8051 executes an immediate data.  Program counter is automatically incremented.

Eg: MOV A,#25H

MOV R₄,#56H

MOV DPTR,#5467H

MOV DPL,#48H

MOV DPH,#87H

2.REGISTER ADDRESSING MODE:

         In register addressing mode, the instruction will specify the name of register as a operand.  In register addressing mode.  Register A, DPTR and R₀ to R₇ are used in the part of a mnemonic as source or destination.

Eg: MOV A,R₀

MOV A,R₇

ADD A,R₅

         We can move the data between accumulator and R_n registers.  But data will not move in between R_n registers.

Eg: MOV R₄,R₇   is invalid.

3.DIRECT ADDRESSING MODE:
         In direct addressing mode, the address of the data is directly specified in the instruction.  The direct address can be the address of an internal RAM location(00H T07FH) or address of a special function register(80H to FFH).

Eg: MOV R₀, 40H

MOV OE0H, #55H

MOV A, #55H

MOV OE0H, R₂

MOV 56H,A

         The SFR can be accessed by the names or by their addresses.

         The major use of direct addressing mode is the stack.  In 8051 family, only direct addressing mode is allowed for pushing onto the stack.

 Therefore “PUSH A” instruction is invalid.

         Pushing the accumulator onto the stack must be coded as “PUSH OEOH”.  Similarly pushing R₃ of bank0  is coded as “PUSH 03”.

         For the pop instruction to pop the top of the stack into R₄ of bank 0 we use “POP 04” instructions.

4.     REGISTER INDIRECT ADDRESSING MODE:

         In the register indirect addressing mode, a register is used as a pointer to the data.  If the data is inside the CPU, only registers R₀ and R₁ are used for this purpose.

         When R₀ and R₁ are used as pointers then  “@” sign must be used, to address only for the internal RAM locations (00H TO 7FH).  The external RAM can be addressed indirectly through DPTR.

         Looping is not possible in direct addressing mode and it is possible in Indirect addressing mode.

Eg: MOV A,@R₀    

MOV @R_1, B

5.INDEXED ADDRESSING MODE:

         Indexed addressing mode is widely used in accessing data elements of look up table entries located in the program ROM space of the 8051.

         The instruction used for this purpose is MOVC  A, @A+DPTR.

          The 16-bit register DPTR and register A are used to form the address of the data element stored in on-chip ROM.

         Because the data elements are stored in the program(code) space ROM of the 8051.  The instruction MOV C is used instead of MOV.  The “c” means code.

         In this instruction the contents of A are added to the 16-bit register DPTR to form the 16-bit address of the needed data.