8085 Pin Description

Properties:

· It is a 8-bit microprocessor

· Manufactured with N-MOS technology

· 40 pin IC package

· It has 16-bit address bus and thus has 216 = 64 KB addressing capability.

· Operate with 3 MHz single-phase clock

· +5 V single power supply

The logic pin layout and signal groups of the 8085nmicroprocessor are shown in Fig. 6. All the signals are classified into six groups:

· Address bus

· Data bus

· Control & status signals

· Power supply and frequency signals

· Externally initiated signals

· Serial I/O signals

Address and Data Buses:

· A8 – A15 (output, 3-state): Most significant eight bits of memory addresses and the eight bits of the I/O addresses. These lines enter into tri-state high impedance state during HOLD and HALT modes.

· AD0 – AD7 (input/output, 3-state): Lower significant bits of memory addresses and the eight bits of the I/O addresses during first clock cycle. Behaves as data bus during third and fourth clock cycle. These lines enter into tri-state high impedance state during HOLD and HALT modes.

Control & Status Signals:

· ALE: Address latch enable

· RD : Read control signal.

· WR : Write control signal.

· IO/M , S1 and S0 : Status signals.

Power Supply & Clock Frequency:

· Vcc: +5 V power supply

· Vss: Ground reference

· X1, X2: A crystal having frequency of 6 MHz is connected at these two pins

· CLK: Clock output

Externally Initiated and Interrupt Signals:

· RESET IN : When the signal on this pin is low, the PC is set to 0, the buses are tristated and the processor is reset.

· RESET OUT: This signal indicates that the processor is being reset. The signal can be used to reset other devices.

· READY: When this signal is low, the processor waits for an integral number of clock cycles until it goes high.

· HOLD: This signal indicates that a peripheral like DMA (direct memory access) controller is requesting the use of address and data bus.

· HLDA: This signal acknowledges the HOLD request.

· INTR: Interrupt request is a general-purpose interrupt.

· INTA : This is used to acknowledge an interrupt.

· RST 7.5, RST 6.5, RST 5,5 – restart interrupt: These are vectored interrupts and have highest priority than INTR interrupt.

· TRAP: This is a non-maskable interrupt and has the highest priority.

Serial I/O Signals:

· SID: Serial input signal. Bit on this line is loaded to D7 bit of register A using RIM instruction.

· SOD: Serial output signal. Output SOD is set or reset by using SIM instruction.