8085 features: • • • • • • • 8085 is an 8-bit microprocessor. It is capable of addressing 64kbytes of memory. It requires a +5volts of power supply. 8085 operates on 3MHz clock.. It has 16 address lines, out of which 8 address lines are multiplexed with data lines. It is manufactured in NMOS technology It is available in 40 pin dual in line (DIP) package. 8085 Pin diagram: 8085 Signal Description:
Address & data lines: Address bus: 8085 has 16-bit address bus AD0-AD7 and A8-A15. A15-A8, it carries the most significant 8-bits of memory/IO address. In this lower address bus is multiplexed with data bus. A8-A15 lines are unidirectional and AD0AD7 lines are bidirectional. Data bus: AD7-AD0, it carries the least significant 8-bit address and data bus. ALE: Address latch enable. It is an output signal used to give information of AD0-AD7 contents. • • • It is a positive going pulse generated when a new operation is started by uP. When pulse goes high it indicates that AD0-AD7 are address. When it is low it indicates that the contents are data. Control & Status Signals: RD: Read control signal is issued to memory or IO device to read data from it. This indicates that the selected memory location or I/O device is to be read and that the data bus is ready for accepting data from the memory or I/O device WR: Write control signal is issued to memory or IO device to write data into it. This indicates that the data on the data bus is to be written into the selected memory location or I/O device. IO/M: Select memory or an IO device.This status signal indicates that the read / write operation relates to whether the memory or I/O device. • • It goes high to indicate an I/O operation. It goes low for memory operations. S1, S0: these are status signals. Depending on the value on these lines, the type of operation being performed by the processor can be determined. Below table shows that information.
Power Supply and clock signals: Vcc: +5v power supply line Vss: electrical ground signal. X1, X2: crystal is connected between these pins. The frequency is internally divided by 2. The systems operates generally at 3MHz. Hence 6 MHz clock signal needs to be connected between X1, X2 lines. CLK (out): It is the clock output signal from processor, which can be used to clock other peripherals in the microprocessor based system. READY: This is used when the processor is reading or writing data to a slow peripheral. Memory and I/O devices will have slower response compared to microprocessors. When this signal goes low processor inserts wait states, until it goes high. Reset Signals: RESET IN: A low signal is applied on this pin; This signal is used to reset the microprocessor. The program counter inside the microprocessor is set to zero. The buses are tri-stated. RESET OUT: when processor is reset, this signal goes high. This pin is connected to reset input of other peripherals. So when processor is reset, other peripherals are also reset. Serial IO lines: SID: serial input data, used to receive serial data. SOD: serial output data, used to send serial data Interrupt Signals: INTR: They are the signals initiated by an external device to request the microprocessor to do a particular task or work. There are five hardware interrupts called, TRAP RST 7.5 RST 6.5 RST 5.5 INTA INTA: On receipt of an interrupt, the microprocessor acknowledges the interrupt by the active low INTA (Interrupt Acknowledge) signal. RST7.5, RST6.5, RST5.5: These are external interrupts. When these interrupt occurs, processor vectors to a specific location. TRAP: It is a non-mask able interrupt. DMA signals: HOLD: This line is used by DMA controller to request microprocessor for system bus. When this line goes high microprocessor completes its current bus cycle and issues system bus to DMA controller. HLDA: HOLD acknowledging signal. Processor acknowledges DMA request using this signal.
Address bus A15-A8, it carries the most significant 8-bits of memory/IO address. Data bus AD7-AD0, it carries the least significant 8-bit address and data bus. Control and status signals These signals are used to identify the nature of operation. There are 3 control signal and 3 status signals. Three control signals are RD, WR & ALE. • RD: This signal indicates that the selected IO or memory device is to be read and is ready for accepting data available on the data bus. • WR: This signal indicates that the data on the data bus is to be written into a selected memory or IO location. • ALE: It is a positive going pulse generated when a new operation is started by the microprocessor. When the pulse goes high, it indicates address. When the pulse goes down it indicates data. Three status signals are IO/M, S0 & S1. IO/M This signal is used to differentiate between IO and Memory operations, i.e. when it is high indicates IO operation and when it is low then it indicates memory operation. S1 & S0 These signals are used to identify the type of current operation. Power supply There are 2 power supply signals: VCC & VSS. VCC indicates +5v power supply and VSS indicates ground signal.