Internal Architecture and Pin Diagram

Internal Architecture of 8085 Microprocessor.

Source:Intel 8085 architecture

The features of INTEL 8085 are:

·         It is an 8 bit processor.

·         It is a single chip N-MOS device with 40 pins.

·         It has multiplexed address and data bus. (AD0-AD7).

·         It works on 5 Volt dc power supply.

·         The maximum clock frequency is 3 MHz while minimum frequency is 500 kHz.

·         It provides 16 address lines so it can access 2^16 =64 Kbytes of memory.

·         It generates 8 bit I/O address so it can access 2^8=256 input ports.

The internal architecture of 8085 includes the Arithmetic and logical unit (ALU), timing and control unit, instruction register and decoder, register array, interrupt control and serial I/O control. 

ALU:-The ALU performs the arithmetic and logical operations. The operations performed by ALU of 8085 are addition, subtraction, increment, decrement, logical AND, OR, EXCL U8IVE -OR, compare, complement and left / right shift. The accumulator and temporary register are used to hold the data during an arithmetic / logical operation. After an operation the result is stored in the accumulator and the flags are set or reset according to the result of the operation.

Registers: The 8085 includes six registers, one accumulator, and one flag register, as shown in Figure. In addition, it has two 16-bit registers: the stack pointer and the program counter. The 8085 has six general-purpose registers to store 8-bit data; these are identified as B, C, D, E, H, and L as shown in the figure.

They can be combined as register pairs - BC, DE, and HL - to perform some 16-bit operations. The programmer can use these registers to store or copy data into the registers by using data copy instructions.

Accumulator: The 8085 microprocessor is an accumulator (A) based microprocessor, Accumulator register must be the one of the operand in Arithmetic and logical operations. The accumulator is an 8-bit register that is a part of arithmetic/logic unit (ALU). This register is used to store 8-bit data and to perform arithmetic and logical operations. The result of an operation is stored in the accumulator. The accumulator is also identified as register A.

Temporary registers(W & Z): They are 8 bit registers not accessible to the programmer. During program execution, 8085A places the data into it for a brief period.

Stack Pointer (SP): - It is a 16 bit register used as a memory pointer. It points to a memory location in R/W memory, called the stack. The beginning of the stack is defined by loading a 16- bit address in the stack pointer.

Program Counter (PC): - Microprocessor uses the PC register to sequence the execution of the instructions. The function of PC is to point to the memory address from which the next byte is to be fetched. When a byte is being fetched, the PC is incremented by one to point to the next memory location.

Flag Register

D7	D6	D5	D4	D3	D2	D1	D0
S	Z	X	AC	X	P	X	CY

      Fig : Flag register of 8085 microprocessor 

Flag register indicates the status of result.

Register consists of five flip flops, each holding the status of different states separately is known as flag register and each flip flop are called flags. 8085A can set or reset one or more of the flags and are sign(S), Zero (Z), Auxiliary Carry (AC) and Parity (P) and Carry (CY). The state of flags indicates the result of arithmetic and logical operations, which in turn can be used for decision making processes. The different flags are described as:

·        Carry: - If the last operation generates a carry its status will 1 otherwise 0. It can handle the carry or borrow from one word to another.

·        Zero: - If the result of last operation is zero, its status will be 1 otherwise o. It is often used in loop control and in searching for particular data value.

·        Sign: - If the most significant bit (MSB) of the result of the last operation is 1 (negative), then its status will be 1 otherwise 0.

·        Parity: - If the result of the last operation has even number of 1’s (even parity), its status will be 1 otherwise 0.

·        Auxiliary carry: - If the last operation generates a carry from the lower half word (lower nibble), its status will be 1 otherwise 0. Used for performing BCD arithmetic.

Timing and Control Unit: This unit synchronizes all the microprocessor operations with the clock and generates the control signals necessary for communication between the microprocessor and peripherals. The control signals are similar to the sync pulse in an oscilloscope. The and signals are sync pulses indicating the availability of data on the data bus.

Interrupt controls: The various interrupt controls signals (INTR, RST 5.5, RST 6.5, RST 7.5 and TRAP) are used to interrupt a microprocessor.

Serial I/O controls: Two serial I/O control signals (Serial Input Data (SID) and Serial Output Data(SOD)) are used to implement the serial data transmission.

Pin Diagram of 8085 Microprocessor

The pin diagram of 8085 microprocessor is shown in above fig. From the figure it is clear that it is 40 pin DIP chip. The various pins of 8085 microprocessor can be grouped in the following categories

·         Power Supply and Clock pins

·         Data bus and Address bus

·         Control and Status signals

·         Interrupt signals

·         DMA signals

·         Serial I/O signals

The description of various pins is given below.

Power supply and clock pins:

Vcc: +5V power supply

Vss: Ground reference.

X₁ and X₂: A Crystal (or RC, LC Network) is connected at these two pins. The internal clock generator divides oscillator frequency by 2, therefore to operate a system at 3MHZ, the crystal of the tuned circuit should have a frequency of 6MHZ.

CLK (OUT): This signal is used as a system clock for other devices. Its frequency is half the oscillator frequency

Data bus and Address bus:

AD₀-AD₇: These lines are Address/Data lines, which are bidirectional with dual purpose. They are used as the low-order address bus as well as the data bus. During the first part of the machine cycle (T₁), lower 8 bits of memory address or I/O address appear on the bus. During the remaining part of the machine cycle (T₂,T₃) these lines are used as a bi-directional data bus

A₈-A₁₅: These are the upper half of the 16 bit address lines. These lines are exclusively used for the most significant 8 bits of the 16 bits of the16 bit address bus.

Control and Status Signals:

ALE (Address Latch Enable): This is a positive going pulse generated every time the 8085 begins an operation. The ALE=High indicates that the bits on AD₇-AD₀ are address bits.

This signal is mainly used to latch the low order address from the multiplexed bus and generate a separate set of eight address lines (A7-A₀)

RD(Read): This is an active low read control pin. This signal indicates that the selected I/O or memory device is to be read and data are available on data bus.

WR (Write):  This is an active low write control pin. It indicates that the data on the data  on the data bus are to be are to be written into a selected memory or I/O location

IO/M: This is a status signal used to differentiate between IO and memory operations. When it is high, it indicates an I/O operation and when it is low, it indicates  a memory operation. This signal is combined with RD and WR signals to generate I/O and memory control signals.

S₁ and S₀: These are status signals and they indicate the type of machine cycle in progress during execution of an instruction.

READY (Input): Through this pin, the microprocessor will know whether peripheral device is ready or not for data transfer. If the device is not ready the processor waits. So, this pin helps to synchronize slow devices to the microprocessor.

Interrupt signals:

TRAP, RST 7.5, RST 6.5, RST5.5 and INTR: These are the interrupt signals which are externally initiated.

INTR (Interrupt Request): This is used as a general purpose interrupt. It has a lowest priority and it is the only non-vectored interrupt.

RST 7.5: It is a restart interrupt pin. It has higher priority than RST 6.5, RST5.5 and INTR. It is a maskable vectored interrupt.

RST 6.5 and RST5.5: These two are maskable vectored interrupt with higher priority than INTR.

TRAP: It is a non-maskable vectored interrupt. It has higher priority.

INTA (Output): It is an active low interrupt acknowledge pin. This will acknowledge the receive of interrupt request to the peripheral device.

DMA Signals:

Hold: This pin is used during the Direct Memory Access. A high on this pin indicates that, a peripheral like DMA controller is requesting the use of address and data buses.

HLDA (Output): A high on this p in acknowledges the hold request from peripheral.

RESET IN : It is an active low signal. When the signal on this pin goes low, the system is in reset i.e. the program counter is set to zero, the address & data buses are tristated.

RESET OUT: This signal is used to Reset other devices in microprocessor system.

Serial input/ Output signals:

SID: Serial input Data is a pin through which serial data are brought into the micro processor accumulator after the RIM instruction is executed.

SOD: Serial output Data pin is used by the microprocessor to output data serially to the external devices. Serial data is sent out of the microprocessor by executing SIM instruction. The most significant bit of accumulator should have the serial bit and D₆ bit of the accumulator must be made high to enable the serial data transfer.

For Architecture and pin diagram of 8085  Microprocessor  click here for the pdf download  Ref: www.phy.davidson.edu