Pin Configuration of 8085 (cont.)

6. Interrupts and Externally initiated operations:

  • 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,

                                                       
  • On receipt of an interrupt, the microprocessor acknowledges the interrupt by the active low INTA (Interrupt Acknowledge) signal.
  • Reset In (input, active low)
    • 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 (Output)
    • It indicates CPU is being reset.
    • Used to reset all the connected devices when the microprocessor is reset.

READY (input)
  • Memory and I/O devices will have slower response compared to microprocessors.
  • Before completing the present job such a slow peripheral may not be able to handle further data or control signal from CPU.
  • The processor sets the READY signal after completing the present job to access the data.
  • The microprocessor enters into WAIT state while the READY pin is disabled.

7. Single Bit Serial I/O ports:
  • SID (input)            -  Serial input data line
  • SOD (output)        -  Serial output data line
  • These signals are used for serial communication.

Pin Configuration of 8085


8085 is a 40 pin IC, DIP package.The microprocessor can be divided into three segments for the sake clarity, arithmetic/logic unit (ALU), register array, and control unit.

  • The signals from the pins can be grouped as follows

  1. Power supply and clock signals
  2. Address bus
  3. Data bus
  4. Control and status signals
  5. Interrupts and externally initiated signals
  6. Serial I/O ports

1. Power supply and Clock frequency signals:

  • Vcc        + 5 volt power supply
  • Vss        Ground
  • X1, X2 :    Crystal or R/C network or LC network connections to set the frequency of internal clock generator.
  • The frequency is internally divided by two. Since the basic operating timing frequency is 3 MHz, a 6 MHz crystal is connected externally.
  • CLK (output)-Clock Output is used as the system clock for peripheral and devices interfaced with the microprocessor.
2. Address Bus:

  • A8 - A15   (output; 3-state)
  • It carries the most significant 8 bits of the memory address or the 8 bits of the I/O address;

3. Multiplexed Address / Data Bus:

  • AD0 - AD7 (input/output; 3-state)
  • These multiplexed set of lines used to carry the lower order 8 bit address as well as data bus.
  • During the opcode fetch operation, in the first clock cycle, the lines deliver the lower order address A0 - A7.
  • In the subsequent IO / memory, read / write clock cycle the lines are used as data bus.
  • The CPU may read or write out data through these lines.

4. Control and Status signals:

  • ALE  (output) - Address Latch Enable.
  • This signal helps to capture the lower order address presented on the multiplexed address / data bus.
  • RD (output 3-state, active low) - Read memory or IO device.
  • 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 (output 3-state, active low) - Write memory or IO device.
  • This indicates that the data on the data bus is to be written into the selected memory location or I/O device.
  • IO/M (output) - 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.

5. Status Signals:

  • It is used to know the type of current operation of the microprocessor.

8085 Microprocessor



Introduction to Microprocessor



                It is a program controlled semiconductor device (IC}, which fetches,decode and executes instructions. It is a computer processor on a microchip and is a multipurpose, programmable device that uses digital data as input and provides results as an output once it processes the input according to instructions stored in its memory. Microprocessors use sequential digital logic as they have internal memory and operate on numbers and symbols represented in the binary numeral system. They are designed to perform arithmetic and logic operations that make use of data on the chip.
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