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Pin diagram of 8085 and description of various signals

    This is one of the favourite topic for the examiners.  As you can see almost every year comes a question based on the pin diagram of microprocessor 8085 and its various pins description. If you see previous years sample papers of Microprocessor and interfacing, you will be able to find it how important this topic/concept is. So i do recommend to prepare this question for the coming December semester  2018. Students can prepare this topic through the notes they may have made during the semester if any case one can find all the useful and important concepts, topics, notes here on my website. For more updates on the topics, notes or latest Sample papers of B.Tech keep on watching my website. So here's your topic below happy reading/self studying.

Pin diagram of 8050 and description of various signals

     It is an 8-bit general purpose microprocessor capable of addressing 64kb of memory.  The device has 40 pins and works on +5v single power supply.  It is a TTLIC. Lets see the pin diagram of 8085 with direction of signals.
Pin diagram of 8085 Microprocessor

The various signals can be classified into six groups :-
  1. Address bus
  2. Data bus
  3. Control and status signals
  4. Power supply and frequency
  5. Externally initiated signals
  6. Serial I/O ports.

  • A8 - A15 : Address bus :-  These are unidirectional O/P tristate signals, used as higher order 8 bit of 16 bit address.  These signals are unidirectional meaning that the address is given by 8085 to select a memory or an I/O device. Address line A8 - A15 are used to send higher order 8 bit address from microprocessor to memory.  These lines are tristated by 8085 in response of following signals : (1) Reset (2) Hold (3) Halt.
  • AD0 - AD7 : Multiplexed address/data bus : These are bidirectional i.e. input/output. Tristate signals having two set of signals they are address and data. The lower 8 bits of 16 bit address (i.e. A0 - A7) is multiplexed or time shared with data bus ( D0 - D7). From same 8 bit line two types of signals are transmitted.
     All the operations of the microprocessor are performed sequentially with reference to the clock.  Microprocessor perform an operation in a specific period, that is known as operation cycles.
    In an operations cycle during earlier part it is used as lower address and in later part it is used as data bus. But for peripheral devices we want separate address and data signals so these signals are demultiplexed by using latch and ALE signal.  These lines are tristated by 8085 for condition same as A8 - A15.

  Various signals are as follows :
  1. Power supply signals
  2. clock signals
  3. DMA request signals
  4. reset signals
  5. interrupt signals
  6. status and control signals
  7. serial I/O signals
(1). Power Supply signals :

(i) Vcc and Vss
  • Vcc is to be connected to +5v power supply.
  • Vss - Ground reference.

(2). Clock Signals :-

(i) X1, X2 :
  • These are clock input signals, connected to crystal. LC or RC network.  The crystal, LC or RC is connected between these two pins.
  • The X1 and X2 pins drive the internal clock generator circuit.  Hence, externally only one crystal is enough.
  • The frequency is divided by 2 and used as operating frequency.  Generally the 6.014 MHz crystal is connected to X1 and X2, this is divided by 2.  So the operating frequency of 8085 is 3.07 MHz.

(ii) CLK OUT :
  • This is an output signal, used as a system clock.
  • The internal operating frequency is available on CLK OUT pin.
  • This pin can be used by the peripherals as a system clock input for their operation. Hence, there will be synchronisation between the different peripherals and the microprocessor.
(3) Reset Signals

  • This is an active low, input reset signal.  When RESETIN = 0, it clears program counter i.e 0000 and makes address, data and control lines tristated.  After reset the status of internal register and flags are unpredictable.  The instruction register is reset. Halt flip-flop is reset. The program counter is reset.  All maskable interrupts are disabled. Also, other peripherals along with 8085 are reset.
  • The CPU is held in the reset condition as long as RESETIN is applied.
  • After reset the microprocessor starts executing instructions from 0000 H on wards.
(ii) Reset Out :
  • This is an active high, output signal used to indicate that the microprocessor is reset.
  • This signal is used as system reset, to reset other devices connected in system.

(4) Interrupt Signals :

(i) TRAP
  • This is an active high level and edge triggered, non maskable, vectored highest priority interrupt.
  • When TRAP line is active microprocessor performs internal restart automatically at vector address 0024 H.
(ii) Reset interrupts (RST 7.5, RST 6.5, RST 5.5) :
  • These are active high level, triggered, vectored, maskable interrupt.  They cause an internal restart to be automatically inserted.
  • The priorities of these are RST 7.5, RST 6.5 or RST 5.5.
  • When RST 7.5, RST 6.5 or RST 5.5 is active microprocessor performs internal restart automatically at vector addresses 003C H, 0034 H, 002C H respectively.
(iii) INTR :
  • INTR is an active high, level triggered general purpose, non-vectored interrupt.
  • It has the lowest priority.
  • Whenever a device requires a service it has to request service on this pin by making it's logic "I".
  • The interrupting device has to state where the interrupt service is placed in the memory.
(iv) INTA :
  • It is an output signal.
  • INTA is used to indicate that the microprocessor has received an INTR interrupt.
(5) Status and Control Signals :

(i) Address latch enable (ALE) :
  • This is an output signal, used to give information of AD0 - AD7 contents.
  • It is a positive going pulse generated during the first clock cycle of a machine cycle.
  • When pulse is high it indicates that the contents of AD0 - AD7 (i.e. demultiplex) to A0 - A7 are address.  When it is low it indicates that the contents are data.
  • The ALE signal is used to separate AD0 - AD7(i.e demultiplex) to A0 - A7 and D0 - D7. To do this separation an external latch is connected to AD0 - AD7 lines and this latch is controlled by ALE signal.
(ii) Input output/memory (IO/M) :
  • This is an output status signal, used to give information of operation to be performed with memory or I/O device.
  • If IO/M = 0, the microprocessor is performing a memory related operation.
  • If IO/M = 1, the microprocessor is performing an I/O device related operation.
(iii) Read (RD)
  • This is an active low signal.
  • It is an output control signal that is used to read data from the selected memory location or an I/O location via data bus.
  • A low on this pin indicated that an operation performed is a read operation.
(iv) Write (WR) :
  • It is an output control signal used to write data to selected memory location or an I/O location via data bus.
  • This is an active low signal.
  • A low on this pin indicated that an operation performed is write operation.
(v) 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. If peripherals are fast enough it is tied to Vcc. If it is left open, 8085 enters in the wait state.

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