Short Note On 8085 Microprocessor

Microprocessor

Some basic definitions

    1- Bit-

A digit of a binary number or code is a bit 

    2-Nibble-

A 4-bit or 4 digit binary number or code is a nibble 

    3-Byte-

A 8-bit or 8 digit binary number or code is a byte. 

    4-Word-

A 16-bit or 16 digit binary number or code is a word.

Microprocessor 8085

1. Introduction-
       Microprocessor is a component that perform instruction and tasks involved in a computer system processing. In a computer system microprocessor is a central processing unit (C.P.U.) that executes and manages the logical instruction passed to it.

1. History of Microprocessor-
       The invention of micro-programmable computer on microprocessor chip. First 4-bit microprocessor INTEL-4004 developed by Intel Corporation of America in 1971.Intel introduce in 1972 an 8-bit microprocessor 8008 and in 1973 another microprocessor 8085.
        The 8080 microprocessor was the most popular microprocessor in the early 70s. The 8080 microprocessor was not in fact a complete CPU on a chip because the clock and controller were on separate chip. Further, it utilizes two separate power supply.       In the year 1974 Intel developed a 40-pin and 8-bit microprocessor 8085 which was the enhanced version of 8080 microprocessor. The 8085 microprocessor has advantages over 8080 microprocessor that it has on chip clock and control circuit. It needs to only one power supply of +5 volt.

Basic functional blocks of Microprocessor

• A Microprocessor is a programmable IC which is capable of performing arithmetic and logical operation. 
• The basic functional blocks of microprocessor are Arithmetic and Logic unit, Flag register,Register array,program counter(PC)/Instruction pointer(IP),Instruction decoding unit and Timing and control unit.The blocks of microprocessor are given below.

Figure-(i)-Basic Blocks of Microprocessor by SHUBHAM MAURYA

Pin diagram of 8085A Microprocessor 

• The pin configuration of an 8085 microprocessor is shown in figure-(ii).
• The 8085 microprocessor has 8-pin AD₀-AD₇, for data transfer, which are multiplexed with low byte of address.

Figure-(ii) 40-Pin of 8085 Microprocessor

Pin description of 8085 Microprocessor

Figure-(iii)  8085 Pin Description 

Address Bus

1. The pins Ao – A15 denote the address bus.

2. They are used for most significant bit

Address / Data Bus

1. AD0 – AD7 constitutes the address / Data bus

2. These pins are used for least significant bit

ALE : (Address Latch Enable)

1. The signal goes high during the first clock cycle and enables the lower order address bits.

IO / M

1. This distinguishes whether the address is for memory or input.

2. When this pins go high, the address is for an I/O device.

S0 – S1

S0 and S1 are status signal which provides different status and functions.

RD

1. This is an active low signal

2. This signal is used to control READ operation of the microprocessor.

WR

1. WR is also an active low signal

2. Controls the write operation of the microprocessor.

HOLD

1. This indicates if any other device is requesting the use of address and data bus.

HLDA

1. HLDA is the acknowledgement signal for HOLD

2. It indicates whether the hold signal is received or not.

INTR

1. INTE is an interrupt request signal

2. IT can be enabled or disabled by using software

INTA

1. Whenever the microprocessor receives interrupt signal

2. It has to be acknowledged.

RST 5.5, 6.5, 7.5

1. These are nothing but the restart interrupts

2. They insert an internal restart junction automatically.

TRAP1.  Trap is the only non-maskable interrupt
2.  It cannot be enabled (or) disabled using program

RESET IN1.  This pin resets the program counter to 0 to 1 and results interrupt enable and HLDA flip flops.

X1, X2 1.  These are the terminals which are connected to external oscillator to produce the necessary and suitable clock operation.

SOD1.  This pin provides serial input data

SOD

1.     This pin provides serial output data

VCC and VSS

1. VCC is +5V supply pin

2. VSS is ground pin

ARCHITECTURE OF 8085 MICROPROCESSOR

Intel 8085, an 8-bit NMOS microprocessor is available in the form of 40 Pin dual in line IC package. It is fabricated on a single LSI chip. It operates on +5 V d. c. supply.

The clock speed used in this microprocessor is about 3MHZ. General Purpose 8-bit microprocessor is capable of addressing up to 64 K bytes (i.e. 216 = 65536 bytes) of memory. The functional block diagram is shown in figure-(iv).

The main functional components of 8085A microprocessor are as given below:

(i) Register Section

(ii)  Arithmetic and Logic Unit

(iii) Timing and Control Section

(iv)  Interrupt Control

(v)  Serial Input / Output Control

Figure-(iv)-Architecture of Microprocessor

 

Register Section

The 8085 microprocessor contains eight addressable 8-bit registers namely:

A   (Accumulator) register

F   Flag register (Flag flip-flops)

B   register

C   register

D   register

E   register

H   register

L   register

Out of these registers B, C, D, E, H and L registers are 8-bit general purpose registers. These registers can either be used as single register or a combination of two registers as 16 bit register pair.

        the valid register pairs are B-C, D-E or H-L register pairs. The higher order byte of 16 bit data is stored in first register (B in B-C register pair), and low order byte in the second register (C in B-C register pair).

Beside these general purpose registers, the 8085 has remaining two 8-bit registers

Accumulator (A) and Flag (F) as special purpose registers and two 16 bit registers namely Program counter (PC) and stack pointer (SP).

Arithmetic and Logic Unit

The arithmetic and logical unit (ALU) basically consists of accumulator (A), flag-register (F) and a temporary register. This unit carries out the arithmetic and logic calculations of the data stored in general purpose registers or in memory locations. The arithmetic operations are ADD, SUB, compare, increments, decrements and complements etc.; while logical operations are AND, OR, XOR and Rotate. For these operations, ALU receives the control signals from the timing and control unit.

Timing and Control Unit

This unit consists of the following sections:

1. Instruction Register and Decoder

2. Control signals

1. Instruction Register and Decoder

The CPU fetches an instruction from the memory for its execution. This instruction can be of 1-3 byte long. The first byte contains the op code of instruction which basically specifies the nature of operation to be performed indicating the length of the instruction. The decoder decodes the op code and directs the control unit to produce the necessary control signals.

2. Control Signals

Following are the control signals of 8085 microprocessor needed for the operation of CPU.

(i)         X１, X2 and CLK Out

Two pins X1 and X2 are provided to be externally connected to a quartz crystal. The clock signal of fixed frequency is generated through the internal circuitry of the processor. The frequency at which the microprocessor 8085 works is half of the crystal frequency. The quartz crystal of 6.144 MHz is used in this processor. This gives the clock frequency of 3.072 MHz. The clock period is of about 320 n sec.

(ii)            Address Latch Enable (ALE)

        The 16 bit address bus is basically divided into two sets. The most significant bits A7-A15 of the address bus are used separately and the least significant bits of the address AD0-AD7 are time multiplexed with the bits of bidirectional data bus (D0-D7).
       The AD0-AD7 bus serves the dual purpose as they can be used as low-order address bus as well as bidirectional data bus at different times. During the first clock cycle of the machine cycle ALE is high which enables the lower 8-bit of the address to be latched either into the memory or external latch.

(iii)  R_D (Read ) Signal

    This is an active low signal to be connected to memory read input (output enable signal to memories) or to input / output read signal to enable input / output buffer.

(iv)      W_R (Write) Signal

    Similar to read signal ( D R ), write signal ( R W ) is also active low. This signal is used to write to the memory or input / output devices.

(v)    IO/ M (Input Output / Memory)

     This signal IO/Mdistinguishes that the address and data is meant for either I/O devices or memory. Whenever this signal is high (1), microprocessor will communicate to the I/O devices and whenever it is low (0), microprocessor will communicate to the memory..

(vi) Status Signals (S0, S1)

   The status signals (S0, S1) along with IO/M signal indicate the type of machine cycle in progress. The type of machine cycle are op code fetch cycle, memory read cycle, memory write cycle, I/O read cycle or I/O write cycle.

(vii) Hold and HLDA

    HOLD and HLDA (Hold Acknowledge) signals are used for Direct Memory Access (DMA) operation. In a microprocessor, the data transfer between I/O devices and memory will take place through the microprocessor. The involvement of the processor slows down the data transfer between I/O devices and memory. The transfer of data directly from I/O devices to memory without involvement of microprocessor is called DMA.

    The DMA will save the time as CPU relinquishes the control of Buses. In this way DMA transfers the large amount of data in a relatively short time.

(viii) READY signal (Input)

     Some peripheral devices connected to 8085 microprocessor operate at much

slower speed than the processor. To synchronize the speed of CPU and peripheral devices or to slow down the speed of 8085, the READY signal is used. If the READY signal is high the peripheral device is ready and the processor can complete the data transfer.

(ix) RESET IN and RESET OUT

     The signal may be low from the operator Reset button or from the processor. When the signal is low, the CPU will reset the program counter, instruction register and other circuits. It also sends a high RESET OUT. The RESET OUT signal goes to peripheral devices to reset or initialized. When signal goes high and RESET OUT goes low, the data processing may begin.

Interrupt Control

     Sometimes it is necessary to interrupt the execution of the main program. For this an interrupt request is obtained from the I/O devices. After receiving the interrupt request (INTR), processor temporarily stops what it was doing and attends to the I/O device.

INTA is an interrupt acknowledge signal which is sent by the microprocessor after INTR signal is received. After the work of the I/O device is complete it returns to what it was doing earlier.

Basically 8085A has five hardware interrupts namely:

INTR

RST 5.5

RST 6.5

RST 7.5

and     TRAP

Serial I/O Control

    Serial input / output control circuit incorporated in this microprocessor is used for the data transmission. For this purpose two pins SID and SOD are provided in the serial input/output control unit. The SID (Serial Input Data) terminal receives the serial data stream from an input device. Similarly, SOD (Serial Out Data) terminal outputs the 8-bit parallel available with the accumulator into serial form to the peripheral device connected with the computer.