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//Microprocessor Unit 8085//

 

1. INTRODUCTION:

MPU is defined as a device or a group of devices that can communicate with peripherals provide timing signals, direct data flow and perform computing tasks as specified by the instructions in memory. This unit will have the necessary address bus, the data bus, the control signals, and would have require only a power of supply and a crystal to be functional.

The 8085A is an 8-bit general purpose microprocessor capable of addressing 64KB of memory. The device has 40 pins, requires +5V single power supply. All the signals of 8085 can be classified into 6 groups.

They are :

         Address Bus

         Data Bus

         Control and Status Signals

         Power supply and frequency Signals

         Serial I/O Ports

 

The lower order address bus 8085 is multiplexed with the data bus. The bus need to be demultiplexed. Appropriate control signal need to be generated to interface memory and I/O with the 8085.

2. ADDRESS BUS:

The 8085 has 8 signal lines, A15-A8 which are unidirectional and used as the higher order address bus.

 

3. MULTIPLEXED ADDRESS/DATA BUS:

The signal lines AD7-AD- are bidirectional, they serve dual purpose. They are used as the low order address bus as well as the data bus. In executing an instruction, during the earlier part of the cycle, these lines are used as the low order address bus. During the later part of the cycle, these lines are used as the data bus. However, the low order address bus can be separated from these signals by using a latch.

 

4. CONTROL AND STATUS SIGNAL:

This group of signals includes two control signals (RD and WR), three status signal (IO/M, S1, S0) to indicate the beginning of the operation.

 

5. POWER SUPPLY AND CLOCK FREQUENCY:

         Vcc: +5V power supply.

         Vss: Ground reference.

         X1, X2: A crystal is connected at these two pins. The frequency is internally divided by two; therefore, to operate a system at 3MHz, the crystal should have a frequency of 6MHz.

         CLK (Out): Clock output this signal can be used as the system clock for other devices.