Tuesday, 6 March 2012

8085 features, Signal description

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. 8085 A-2 operates on maximum clock frequency 5MHz.
  • 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. In this lower address bus is multiplexed with data bus. A8-A15 lines are unidirectional and AD0-AD7 lines are bidirectional.
Data bus: AD7-AD0 is 8-bit bidirectional data bus. It is multiplexed with lower order address bus.
ALE: Address latch enable. It is used to de-multiplex AD0-AD7. It is connected to strobe input of latch which is used to separate address and data bus lines. It is issued in first T-state.
Control & Status Signals:
RD: Read control signal is issued to memory or IO device to read data from it.
WR: Write control signal is issued to memory or IO device to write data into it.
IO/M: It is a signal which is used to distinguish between IO operation and memory operation. It is also used in generating memory and IO, read and write control signals.
 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. When this signal goes low processor inserts wait states, until it goes high.
Reset Signals:
RESET IN: when low signal is applied on this pin, 8085 resets and the microprocessor boots from 0000h location in memory i.e. PC is loaded with 0000h location.
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: interrupt request is general purpose interrupt signal. The interrupting device needs to send the vector address also.
INTA: is interrupt acknowledging signal. This signal indicates that processor has accepted the interrupt.
RST7.5, RST6.5, RST5.5: These are external vectored 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.

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