Unit - 1

Introduction to concept of microcontroller

1.1 Introduction to the concept of microcontroller

A microcontroller is a computer present in a single integrated circuit that is dedicated to performing one task and execute one specific application.

It contains memory, programmable input/output peripherals as well a processor. Microcontrollers are mostly designed for embedded applications and are heavily used in automatically controlled electronic devices such as cell phones, cameras, microwave ovens, washing machines, etc.

1.2  Intel 8051 Functional diagram

Figure 1. 8051 Functional diagram

8051 has 4 K Bytes of internal ROM. The address space is from 0000 to 0FFFh. If the program size is more than 4 K Bytes 8051 will fetch the code automatically from external memory.

The accumulator is an 8-bit register widely used for all arithmetic and logical operations. The accumulator is also used to transfer data between external memory. B register is used along with Accumulator for multiplication and division. A and B registers together are also called MATH registers.

PSW (Program Status Word). This is an 8-bit register that contains the arithmetic status of ALU and the bank select bits of register banks.

CY - carry flag

AC - auxiliary carry flag

F0 - available to the user for general purpose

RS1, RS0 - register bank select bits

OV - overflow

P - parity

 Stack Pointer (SP) – Contains the address of the data item on the top of the stack. The stack may reside anywhere on the internal RAM. On reset, SP is initialized to 07 so that the default stack will start from address 08 onwards.

 Data Pointer (DPTR)

 – DPH (Data pointer higher byte), DPL (Data pointer lower byte). This is a 16-bit register that is used to furnish address information for internal and external program memory and external data memory.

 Program Counter (PC) – 16 -bit PC contains the address of the next instruction to be executed. On reset, the PC will set to 0000. After fetching every instruction PC will increment by one.

Key Takeaways:

1.3 Functions of pins of 8051

Pin Diagram

Figure 2. Pin Diagram of 8051

Pinout Description:

8051 microcontrollers have 4 I/O ports each of 8-bit, which can be configured as input or output. Hence, a total of 32 input/output pins allows the microcontroller to be connected with the peripheral devices.

1.4 PSW and Flag bits

Program Status Word (PSW)

The PSW or Program Status Word Register is called Flag Register is one of the important SFRs. The PSW Register consists of Flag Bits, which help the programmer in checking the condition of the result and make decisions.

Flags are 1-bit storage elements that store and indicate the nature of the result that is generated by the execution of certain instructions. The following image shows the contents of the PSW Register.

0                   0                    0                       0               0                   0           0                     0

Figure 3. PSW Register

The following table describes the function of each flag.

Key takeaways:

8051 microcontrollers is a 40 pin Dual Inline Package (DIP). These 40 pins serve different functions like read, write, I/O operations interrupts, etc. 

1.5 Stack and Stack Pointer

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.

The register used to access the stack is called the SP (stack pointer) register. The stack pointer is 8 bits wide, which takes values of 00 to FFH.

When the 8051 is powered up, the SP register contains the value 07. This means that RAM location 08 is the first location used for the stack by the 8051. The storing of a CPU register in the stack is called a PUSH, and pulling the contents off the stack back into a CPU register is called a POP.

Pushing onto the stack

In the 8051 the stack pointer (SP) points to the last used location of the stack. When we push data onto the stack, the stack pointer (SP) is incremented by one.

The SP is decremented when data is pushed onto the stack.

For example, when PUSH is executed, the contents of the register are saved on the stack and SP is incremented by 1.

For example, the instruction “PUSH 1″ pushes register Rl onto the stack.

Show that the stack and stack pointer for the following. Assume the default stack area and register 0 is selected.

MOV R6, #25H

MOV R1,#12H

MOV R4, #0F3H

PUSH 6

PUSH 1

PUSH 4

Solution:

Start SP=07                  SP=08                        SP=09                           SP=0A

Popping from the stack

Popping the contents of the stack back into a given register is the opposite process of pushing. With every pop, the top byte of the stack is copied to the register specified by the instruction, and the stack pointer is decremented once.

Example:

Examine the stack shows the contents of the registers and SP after the execution of the following instructions.

POP 3; POP stack into R3

POP 5; POP stack into R5

POP 2; POP stack into R2

Solution:

Start SP=0B                  SP=0A                        SP=09                           SP=08

Stack Pointer (SP)

SP or Stack Pointer points to the top of the Stack and it indicates the next data to be accessed. Stack Pointer can be accessed using PUSH, POP, CALL, and RET instructions. The Stack Pointer is an 8-bit register and upon reset, the Stack Pointer is initialized with 07H.

When writing a new data byte into the stack, the SP (Stack Pointer) is automatically incremented by 1 and the new data is written at an address SP+1. When reading data from the stack, the data is retrieved from the Address in SP and after that, the SP is decremented by 1 (SP-1).

1.6 Overview of special function registers

There are 21 Special function registers (SFR) in the 8051 microcontrollers, and this includes Register A, Register B, Processor Status Word (PSW), PCON, etc. 

There are 21 unique locations for these 21 special function registers and each of these registers is of 1-byte size.

Some of these special function registers are bit addressable which means you can access 8 individual bits inside a single byte, while some others are only byte-addressable.

Register A/Accumulator

The Accumulator referred to as Register A holds the result of most arithmetic and logic operations. ACC is usually accessed by direct addressing and its physical address is E0H. The accumulator is both bytes and bit addressable.

Figure 4. Register A Accumulator

Register B

The major purpose of this register is in executing multiplication and division. The 8051 microcontroller has a single instruction for multiplication (MUL) and division (DIV).

MUL A, B  – When this instruction is executed, data inside A and data inside B is multiplied and the answer is stored in A.

Figure 5. Accumulator

1.7 Data transfer functions and programs in assembly language

In this group, the instructions perform data transfer operations of the following types.

a. Move the contents of a register Rn to A

i. MOV A, R2

ii. MOV A, R7

b. Move the contents of register A to Rn

i. MOV R4, A

ii. MOV R1, A

c. Move an immediate 8-bit data to register A or to Rn or a memory location(direct or indirect)

i. MOV A, #45H

ii. MOV R6, #51H i

iii.MOV 30H, #44H

iv. MOV @R0, #0E8H

v. MOV DPTR, #0F5A2H

vi. MOV DPTR, #5467H

d. Move the contents of a memory location to A or A to a memory location using direct and indirect addressing

i. MOV A, 65H

ii. MOV A, @R0

iii.MOV 45H, A

iv. MOV @R1, A

e. Move the contents of a memory location to Rn or Rn to a memory location using direct addressing

i. MOV R3, 65H

ii. MOV 45H, R2

f. Move the contents of the memory location to another memory location using direct and indirect addressing

i. MOV 47H, 65H

ii. MOV 45H, @R0

g. Move the contents of external memory to A or A to an external memory

i. MOVX A,@R1

ii. MOVX @R0, A

iii. MOVX A,@DPTR iv. MOVX@DPTR, A

h. Move the contents of program memory to A

i. MOVC A, @A+PC

ii. MOVC A, @A+DPTR

Program

Write 8085 Assembly language program to swap two 8-bit numbers stored at location 8000H and 8001H using direct addressing mode.

MOV B, M; Load the first number to B

INX H; Point to the next number

MOV A, M; Load the second number into A

MOV M, B; Store first number to the second position

DCX H; Point to the previous location

MOV M, A; Store second number to the first position.

HLT.

Write a program to transfer the block of data from 20h to 30h to an external location 1020h to 1030h. 

Mov r7, #0Ah             ; initialize counter by 10d

Mov r0, #20h               ; get initial source location

Mov dptr, #1020h       ; get initial destination location

Nxt:     Mov a, @r0                 ; get first content in acc

Movx @dptr, a           ; move it to external location

Inc r0                            ; increment source location

Inc dptr                        ; increase destination location

Djnz r7, nxt                ; decrease r7. if zero then over otherwise move next

Key Takeaways:

Data transfer instructions are responsible for transferring data between various memory storing elements like registers, RAM, and ROM.

References:

1. The 8051 Microcontroller and Embedded Systems using Assembly and C by Muhammad Ali Mazidi.

2. The 8051 Microcontroller by I. Scott Mackenzie, Raphael C.W Phan

8051 Microcontrollers: Internals, Instructions, Programming, and Interfacing Book by Subrata Ghoshal

3. 8051 Microcontroller Based Embedded Systems Textbook by MANISH K PATEL

4. 8051 Microcontrollers: An Applications Based Introduction Book by David Calcutt, Frederick Cowan, and G. Hassan

5. Advanced PIC Microcontroller Projects in C Book by Dogan Ibrahim