The 8085 PIN Function

The 8085 is Intel’s 8-bit microprocessor and is 40-pin DIP (Dual In line Package) integrated chip. The pin diagram of the 8085 is shown in figure. The 8085 has 16-bit address bus which can address maximum 64 K bytes (216 =64 K bytes) of memory.

The signals of 8085 are divided into following six groups.
The 8085 PIN Function
PIN Function

Address bus: The 8085 has 16 address lines. The upper eight address lines A15-A8 are unidirectional and used as higher order address bus
Multiplexed Address/Data bus: the eight lower bits of address bus are multiplexed with data bus. These eight lines AD7-AD0 either contain 8-bit data or lower byte of 16-bit address. During the earlier part of instruction execution, it contains lower order address.

Control and status signals: Two signal RD and WR are used to control the read/write operations. When read control signal RD is low, the microprocessor is reading either from the memory device or input port. When write control signal WR is high the microprocessor writes into the storage device or sending data to the output port.
The 8085 PIN Function
The 8085 PIN Function

A special signal ALE (Address Latch Enable) is employed to demultiplex the multiplexed address/data bus. When ALE is high multiplexed address/data bus contains lower byte of the address and when it's low multiplexed bus contains 8-bit data.

To distinguish between the memory and peripheral , the 8085 uses IO/M signal. it's high when microprocessor communicates with peripheral and it remains low when it communicates with storage device .The RD and WR signals are combined with IO/M to generate control signals IOR (I/O read), IOW (I/O write), MEMR (memory read) and MEMW (memory write) which are used to activate I/O device or memory device for read and write operations respectively. For example, when IO/M is low, RD is low and WR is high, the operation is memory read and MEMR signal is generated which activates memory device for read operation.

The status signals S1 and S0 are used to identify the operation currently being performed by the 8085 microprocessor. These signals in combination with the IO/M signal show current status of the microprocessor as shown in table. The status of 8085 indicates the machine cycle, the 8085 is performing.
The 8085 PIN Function
The 8085 Machine Cycles

Serial I/O signals: the 8085 has two signals namely SID (Serial Input Data) and SOD (Serial Output Data) to supply serial communication with serial device like character printer , tty device etc.
Power supply and clock: The 8085 requires only +5 V power supply. The Vcc is connected to +5 V power supply and Vss is connected to ground. The 8085 has its own on-chip clock generator. The crystal is to be connected between x1 and x2 pins. The 8085 operates at half of the clock frequency as:
Operating frequency – 1/2 *crystal frequency                                                                                   
The CLK (OUT) is an output signal which provides system clock to other devices in the system.
Interrupt and other signals: whenever interrupt occurs on one of the interrupt pin, microprocessor suspends all the operations, saves the current status and jumps to special service routine known as Interrupt Service Routine (ISR). There are five interrupt pins TRAP, RST 7.5, RST 6.5, RST 5.5 and INTR.
The TRAP is non-maskable interrupt and can’t be stopped either be means of hardware or software. It is generally used for emergency situation like power outage to save lots of the important values.
The RST 7.5, RST 6.5 and RST 5.5 are referred to as restart interrupts. The INTR is general purpose interrupt. All the four are maskable and may be masked by software instruction like DI (Disable Interrupt) and unmasked using instruction EI (Enable Interrupt).
The 8085 acknowledges the interrupt through the pin INTA (Interrupt Acknowledge output pin.
The 8085 has two signals HOLD and HLDA to transfer the control of buses to other device like DMA controller. A HOLD input indicates that other device wants to use the address and data bus. After receiving HOLD input, the 8085 completes the data transfer on buses, puts RD, WR and IO/M in tri-state and indicate to peripheral devices through HLDA output that it will leave the control of buses in next clock cycle.

The READY input signals is used to synchronize the operations with the slower peripheral devices. It is used by peripheral devices to inform the microprocessor that it is ready to send or receive data. When it's low, the microprocessor goes into wait state then waits for a response from device. When devices is prepared to transfer the info , it sends READY signal high to microprocessor.
When RESET IN input pin goes low, the microprocessor resets itself.The microprocessor can send the reset signal to other peripheral devices by RESET OUT pin to point that's being reset.
The functions of every pin of 8085 are briefly given below:
A15 –A8: they represent upper half of the address bus of 8085 microprocessor and carries upper byte of 16-bit address.
AD7-AD0: They represent multiplexed address/data bus. When ALE is high, they're used as lower half the address bus and carries lower byte of 16-bit address. When ALE is low, they're used as 8-bit data bus to transfer data to and from microprocessor.
ALE: It stands for Address Latch Enable. It is used to demuliplex multiplexed address/data bus (AD7-AD0). When it is high, AD7-AD0 carries lower byte of and address and when it is low, AD7-AD0 carries 8-bit data. ALE goes high in start of each machine cycle and stays high during first T-state. At the end of T1, it goes low.
IO/M: it stands for Input-Output/Memory. It differentiates between I/O and Memory operation. When it's high, 8085 performs I/O operation (either read or write) and when it's low, 8085 performs Memory operation (either read or write).
RD,WR: the control signals RD and WR are wont to perform the read and write operations respectively. When RD is low, 8085 is reading from memory or data input device . When WR is high, 8085 is writing to memory location or output device. These signals along side IO/M are wont to generate control signals MEMR, MEMW, IOR and IOW.

S1, S0: They are status signals and IO/M, S1 and S0 together represents different machine cycles 8085 can perform. For example, IO/M=0, S1=0 and S0=1 represents memory write cycle.
SID, SOD: SID stands for Serial input file and SOD represent Serial Output Data. SID is employed to read the bits serially, while SOD is employed to send data serially bit-by-bit. they're used for serial communication between 8085 and serial device.
READY: it is used to synchronize the operation with slower peripherals. When it's high, the device is prepared to perform the read or write operation.
RESET IN, RESET OUT: when RESET IN is low, microprocessor resets itself and initializes Program Counter to 0000h. RESET OUT is employed to tell all other peripherals during a system that microprocessor is being reset.
TRAP: it is non-maskable interrupt and used only in emergency like power outage .Once it occurs, it can not be stopped by software or hard ware means.
RST 7.5, RST 6.5, RST 5.5: They are referred to as restart interrupts and are maskable. they're controlled by EI/DI instruction also as RIM and SIM instructions.
INTR: it is general purpose interrupt and also maskable. it's enabled and disabled by EI and DI instructions.
INTA: it is interrupt acknowledge pin and used by microprocessor to acknowledge the acceptance of an interrupt of an interrupt request by device.
X1, X2: 8085 contains in-built clock generator and X1 and X2 are used to connect crystal for this internal clock generator, using it, 8085 generates system clock. All the operations performed by 8085 are synchronized to the present clock.
CLK OUT: It is wont to provide the system clock to other devices during a system.
Vcc, Vss: They are power signals. Vcc is connected to +5 Volt, while Vss is connected to ground.

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