Pin Configuration of 8085 Microprocessor

The 8085 microprocessor is an eight-bit microprocessor. It is a single-chip, NMOS device implemented with 6200 transistors in a 40-pin dual-in-line package. It operates on a single +5V power supply with a maximum of 3 MHz clock frequency. Below given (a) figure shows the pin configuration and (b) shows the function pin diagram of the 8085 microprocessor.

In the above figure, the arrow shows the direction of signal flows either outgoing from the microprocessor or incoming to the microprocessor. The signals of the 8085 microprocessor can be classified into eight groups according to their functions. These are

1. Power supply and ground signals
2. Higher-Order Address Bus
3. Multiplexed Address/ Data Bus
4. Control signals
5. Status signals
6. Externally initiated signals
7. Serial input/output signals
8. Clock signals

The 8085 microprocessor operates on a single +5V power supply connected to VCC at pin number 40. The ground reference is connected to VSS at pin number 20.

Like all other systems, 8085 microprocessor-based systems are also organized around the system buses. The bus is defined as the collection of wires which are used to communicate the address, data, or other information. The 8085 microprocessor has 16-bit address lines which are denoted by (A0-A15). These address lines are divided into higher-order address bus A8-A15 and lower-order address bus A0-A7. High-order address buses (A8-A15) are unidirectional signal lines. These lines are exclusively used to send the high-order address (most significant 8-bit of 16-bit address) to the peripheral or memory. In the 8085 microprocessor, a higher-order address bus is available from pin 21 to pin 28.

The lower order address bus (A0-A7) is multiplexed with an 8-bit data bus (D0-D7). It means that the data and address are sent on the same line but at different instants of times. This configuration is done to reduce the number of pins on the 8085 microprocessor IC. This multiplexed bus is denoted by AD0-AD7. The signal lines AD0-AD7 are bidirectional.

In executing an instruction, during the earlier part of execution, the multiplexed bus is used as the lower-order address bus. Whereas during the later part, it is used as the data bus. Demarkation of the execution time for the address bus as well as for the data bus is obtained by an appropriate control signal. In practical applications, separate data and address bus are required so these have to be demultiplexed. Demultiplexed address and data bus can be obtained by using a latch or buffer with an appropriate control signal. Pin 12 to Pin 19 are sued for multiplexed address/data bus in 8085 microprocessor.

Control signal lines are commonly known as control buses although they are individual signals. The control signals are as follows:

1. Read (RD): The read control signals are used to control the reading operation of the microprocessor. This is an active low signal, meaning that after the RD signal goes low, the external device places the data on the data bus and the microprocessor reads this data. It is important to note that if the data is not placed on the data bus and RD is low, the microprocessor will read whatever is available which may be garbage.
2. Write (WR): The write control signal is similar to the read control signal (RD). it is also an active low signal. the microprocessor places the data on the data bus and makes WR signal low. this is the responsibility of the external device that when the (WR) signal goes low, it should read the data from the data bus.
3. Address Latch Enable (ALE): Address latch enable (ALE) is one special type of control signal. it is used to latch the address generated during the earlier part of the execution of any instruction. it is a positive-going pulse generated every time the 8085 microprocessor begins an execution. it indicates that the bits on multiplexed address/data bus (AD0 – AD7) are address bits.

1. IO/M signal: Input-output/(Memory) (IO/M) signal shows that the microprocessor is communicating (reading or writing) with the I/O port or memory. Pin 34, in 8085 microprocessor IC, works for IO/RD signal. it is active in both states. when it is high it indicates an I/O operation. when it is low, indicates a memory operation. this signal is combined with reading (RD) and write (WR) control signals to generate input/output read, input/output write, and memory write control signals.
2. S1 and S0: S1 and S0 are two status signals. these status signals are used to indicate the internal operation of the microprocessor. these are rarely used in small systems. the complete operation of the microprocessor can be studied by the combination of three status signals S1, S0, and IO/M. it becomes valid at the beginning of the execution of the instruction and remains stable throughout the complete execution. all the operations and their associated status signals are listed in the given below table.

IO/M	S1	S0	Status
0	0	1	Memory write
0	1	0	Memory read
1	0	1	I/O write
1	1	0	I/O read
0	1	1	Opcode fetch
1	1	1	Interrupt Acknowledge
Z	0	0	Halt
Z	X	X	Hold
Z	X	X	Reset

These signals are initiated by the user or peripheral. these are asynchronous in nature. 8085 microprocessor has four different categories of this type of signal.

1. RESET (RESET IN, RESET OUT)
2. Ready (READY)
3. Interrupt (TRAP, RST 7.5, RST 6.5, RST 5.5, INTR, INTA)
4. Direct memory access (HOLD, HLDA)

1. RESET IN: this is an input signal, when the signal on this pin goes low (0). the program counter is set to zero (0000 H). control unit transfers the contents of the program counter to the address latch, providing the address of the first instruction to be executed. however, other registers (A, F, B, C, D, E, H, and L) are not cleared by resetting, although they may be altered.
2. RESET OUT: This signal is actually generated by the microprocessor in response to the signal RESET IN when RESET IN is logic 0, and RESET OUT is logic 1. this signal indicates that the microprocessor is being reset. this signal is used to reset external devices in a microprocessor-based system in a synchronous manner.

This is an active-high signal. it is used by microprocessors to sense whether a peripheral is ready or not for data transfer. if this signal is high, it means that the peripheral is ready for communication. if it is on low, it means the peripheral is not ready and the microprocessor will wait until it goes high. READY signal plays an important role to synchronize the slower peripheral with the faster microprocessor.

The microprocessor 8085 has five interrupt signals. these interrupt signals can be used by the peripherals to break the sequencing of the main program.

1. TRAP: TRAP is a non-maskable interrupt. it has the highest priority. it is a level or edge-sensitive signal. TRAP interrupt is usually used during critical situations such as power failure.
2. RST 7.5, RST 6.5, RST 5.5: These interrupts are maskable interrupts. these are also known as restart interrupts. these are vectored interrupts and transfer the program control to the specific memory locations. priority order among the three interrupts is RST 7.5, RST 6.5, and RST 5.5.
3. INTR: This interrupt is also a maskable interrupt. this is used as a general-purpose interrupt. it has the lowest priority among all the interrupts. INTR is a level-sensitive interrupt pin. it is the only non-vectored interrupt. Non-vectored means that the microprocessor does not know from where it has to execute when interrupted through this pin.
4. INTA: Interrupt acknowledge (INTA) is an active low signal made active by the processor. it is used to acknowledge the peripheral that the microprocessor has recognized interrupt through the INTR pin.

Also, read

1. Demultiplexing of address/data bus
2. Memory organization in 8085 Microprocessor
3. Operations of Microprocessor
4. 8085 microprocessor architecture
5. Microprocessor as CPU

Direct memory access (DMA) is used to transfer data from memory to peripheral or from peripheral to memory without the intervention of the microprocessor.

1. HOLD: HOLD is an active high signal. The High (1) signal on this pin indicates that the other device (e.g. DMA controller) is requesting DMA operations. after receiving the HOLD signal, the microprocessor releases the address and data bus to be used by another device like the DMA controller.
2. HLDA: This signal acknowledges the HOLD request. it is an active high signal. this signal is made inactive by the microprocessor after the I/O device has completed the DMA operation and makes the HOLD signal inactive.

The 8085 microprocessor is the only 8-bit microprocessor that has two separate pins for serial communication.

1. SID (Serial Input Data): SID is a pin through which serial data are brought into the microprocessor RIM instruction is executed to read the serial data from the peripheral.
2. SOD (Serial Output Data): The SOD pin is used by the microprocessor to send the data serially to the external world. serial data is sent out of the microprocessor by executing SIM instructions.

1. X1 and X2: A crystal oxillator is used to provide the clock frequency to the microprocessor. the clock frequency is used to synchronize the operation of the 8085 microprocessor. the crystal is connected across X1 and X2 at pin numbers 1 and 2 of the 8085 microprocessor. the internal operation of the microprocessor will be half the crystal frequency. therefore, to operate a system at 3 MHZ the crystal should have a frequency of 6 MHZ.
2. CLK OUT: This is an output pin. this signal is generated by the microprocessor and can be used as the system clock for other devices.