Pin diagram of 8085

Pin diagram of 8085

AU : May - 09, 12, 14, 17, 18, Dec.-12, 14

Fig. 1.3.1 (a) and (b) show 8085 pin configuration and functional pin diagram of 8085 respectively. The signals of 8085 can be classified into seven groups according to their functions.

a) Power supply and frequency signals.

b) Data bus and address bus

c) Control bus

d) Interrupt signals

e) Serial I/O signals

f) DMA signals

g) Reset signals

 

1. Power Supply and Frequency Signals

i) V_CC : It requires a single +5 V power supply.

ii) V_SS : Ground reference.

iii) X₁ and X₂ : A tuned circuit like LC, RC or crystal is connected at these two pins. The internal clock generator divides oscillator frequency by 2, therefore, to operate a system at 3 MHz, the crystal of tuned circuit must have a frequency of 6 MHz.

iv) CLK OUT : This signal is used as a system clock for other devices. Its frequency is half the oscillator frequency.

 

2. Data Bus and Address Bus

A) AD₀ to AD₇ : The 8-bit data bus (D₀ - D₇) is multiplexed with the lower half (A₀ - A₇) of the 16-bit address bus. During first part of the machine cycle (T1), lower 8 bits of memory address or I/O address appear on the bus. During remaining part of the machine cycle (T₂ and T₃) these lines are used as a bi-directional data bus.

B) A₈ to A₁₅ : The upper half of the 16-bit address appears on the address lines A₈ to A₁₅. These lines are exclusively used for the most significant 8 bits of the 16-bit address lines.

 

3. Control and Status Signals

A) ALE (Address Latch Enable) : We know that AD₀ to AD₇ lines are multiplexed and the lower half of address (A₀ - A₇) is available only during T₁ of the machine cycle. This lower half of address is also necessary during T₂ and T₃ of machine cycle to access specific location in memory or I/O port. This means that the lower half of an address must be latched in T₁ of the machine cycle, so that it is available throughout the machine cycle. The latching of lower half of an address bus is done by using external latch and ALE signal from 8085.

A low on  indicates that the data must be read from the selected memory location or I/O port via data bus. A low on p indicates that the data must be written into the selected memory location or 1/O port via data bus.

 indicates whether I/O operation or memory operation is being carried out. S₁ and S₀ indicate the type of machine cycle in progress.

D) READY : It is used by the microprocessor to sense whether a peripheral is ready or not for data transfer. If not, the processor waits. It is thus used to synchronize slower peripherals to the microprocessor.

 

4.  Interrupt Signals

The 8085 has five hardware interrupt signals : RST 5.5, RST 6.5, RST 7.5, TRAP and INTR. The microprocessor recognizes interrupt requests on these lines at the end of the current instruction execution.

The  (Interrupt Acknowledge) signal is used to indicate that the processor has acknowledged an INTR interrupt.

 

5.  Serial I/O signals

A) SID (Serial I/P Data) : This input signal is used to accept serial data bit by bit from the external device.

B) SOD (Serial O/P Data) : This is an output signal which enables the transmission of serial data bit by bit to the external device.

 

6. DMA Signal

A) HOLD : This signal indicates that another master is requesting for the use of address bus, data bus and control bus.

B) HLDA : This active high signal is used to acknowledge HOLD request.

 

7. Reset Signals

A) : A low on this pin

1) Sets the program counter to zero (0000H) and clears the INTE flag.

2) Resets the interrupt enable and HLDA flip-flops.

3) Tri-states the data bus, address bus and control bus. (Note : Only during RESET is active).

4) Affects the contents of processor's internal registers randomly.

B) RESET OUT : This active high signal indicates that processor is being reset. This signal is synchronized to the processor clock and it can be used to reset other devices connected in the system.

Review Questions

1. Write about the pin configuration of 8085 processor and explain them in detail.

AU : June-09, May-17, Marks 16

2. Draw the signal configuration of 8085 and explain the purpose of each signals.

AU : May-14, Marks 16

3. Draw the pin diagram of 8085 microprocessor.

AU : June-12, Marks 4

4. Explain different control signals used by 8085.

AU : Dec.-14, Marks 8

5. What is the use of ALE signal ?

6. What is the use of CLKOUT and RESET OUT signals of 8085 processor ?

7. Describe the function of following pins in 8085.

8. Explain the signals used in DMA operation in 8085.

9. Draw the pin diagram of 8085 microprocessor.

AU : May-12, Marks 4

10. Draw the pin diagram of 8085 microprocessor and list the classification of signals.

AU : Dec.-12, Marks 8

11. Draw and explain the building blocks and its signal of 8085 processor.

AU : May-18, Marks 13