40 PIN Diagram.
This microprocessor has 40 PINs. this diagram shows the logic or functional pin out of 8085 micro-processor.All the 40 signals are classified into 6 groups they are:-
- Address Bus
- Multiplexed address data bus
- Control and status signal
- power supply and frequency signal
- Interrupt and peripheral initiated signal.
- Serial I/O signal
[I] Address Bus (A8-A15):
8085 has 8 address lines A8-A15.these address bus are used to address the memory location.
[II] Multiplex Address/Data Bus (AD)-AD7):
This signal lines are bi-directional which are used for two purpose,that is it can be used as lower order address bus A0 to A7 and 8 bit data bus D0 to D7. When instruction is executed,during early part of execution cycle,these line are used as address bus and during later part of cycle;this lines are used as data bus. Thus address/Data bus operates in timeshare mode.This technique is called as multipl-exing.Multiplexing means many into one operation.
[III] Control and status signal:
This group includes two control signals:
(1) Read (RD).
(2) Write (WR).
And three status signals
(1) S1
(2) S0
(3) IO/M
ALE (Address Latch Enable/PIN 30):-This positive pulse generated every time the 8085 starts the operation.This signal is used to capture the lower address presented on multiplexed address and data bus.
RD (PIN 32):-This is active low input generally used for reading operation.
WR (PIN 31):-This is active low input used for writing operation.
IO/M (PIN 34):-This pin is used to select the memory or input-output through which we want to communicate the data. When this signal is low it indicates memory operation and when this signal is high it indicates I/O operation.
S0 and S1 (PIN 33,29):-It is used for the giving status signal in microprocessor.So that the outside world can know what is the current operation being performed by 8085.
[IV] Power supply and frequency signal:
The 8085 requires only +5V and ground ( VSS /substrate voltage)
VCC (PIN 40):-VCC is to be connected to +5V power supply.
VSS (PIN 20):-Ground reference
X1 and X2(PIN1,2):- This pin is used for providing the clock frequency to the microprocessor.The input frequency of he crystal is internally divided by 2. CLK OUT is used to synchronize the rest of the system to the 8085.
The basic operating timing frequency of the microprocessor is 3 MHz so 6 MHz frequency is applied.
[V] Interrupt and peripheral initiated signal:
They are the signals initiated by an external device to request the microprocessor to do a particular task or work.
8085 has five hardware interrupt signals that can be used to interrupt program execution.these signals are
(1)TRAP
(2)RST 7.5
(3)RST 6.5
(4)RST 5.5
(5)INTR/INTA
And three externally initiated signals those are:
[V] Interrupt and peripheral initiated signal:
They are the signals initiated by an external device to request the microprocessor to do a particular task or work.
8085 has five hardware interrupt signals that can be used to interrupt program execution.these signals are
(1)TRAP
(2)RST 7.5
(3)RST 6.5
(4)RST 5.5
(5)INTR/INTA
And three externally initiated signals those are:
(1)READY
(2)RESET(reset in and reset out)
(3)DMA request signals (HOLD/HLDA)
RST 7.5, 6.5, 5.5 (PIN 7,8,9):
These are restart interrupts or vectored interrupt. these are maskable interrupt and has lower priority than TRAP, among these RST 7.5 has the highest priority and RST 5.5 having the lowest.
INTR (PIN 10):
Interrupt request - this signal is used as a general purpose interrupt.it has lowest priority and is a maskable interrupt.
INTA (PIN 11):
This is interrupt acknowledgement signal sent by the processor when INTR is recognized. This is active low signal.
READY (PIN 35):
This is used by the microprocessor to check if a peripheral is ready for the data transfer.
RESET Signals:
8085 has two signal for the use of serial data communication.
(1)SID (serial input data)
(2)SOD (serial output data)
SID (PIN 5): This is serial input data line.
SOD (PIN 4): This is serial output data line.
(2)RESET(reset in and reset out)
(3)DMA request signals (HOLD/HLDA)
TRAP (PIN 6):
Trap is highest priority, nonmaskable interrupt, when trap is received microprocessor starts the program execution from location (4.5*8) =0024H. this is why it is also called as vectored interrupt.RST 7.5, 6.5, 5.5 (PIN 7,8,9):
These are restart interrupts or vectored interrupt. these are maskable interrupt and has lower priority than TRAP, among these RST 7.5 has the highest priority and RST 5.5 having the lowest.
INTR (PIN 10):
Interrupt request - this signal is used as a general purpose interrupt.it has lowest priority and is a maskable interrupt.
INTA (PIN 11):
This is interrupt acknowledgement signal sent by the processor when INTR is recognized. This is active low signal.
READY (PIN 35):
This is used by the microprocessor to check if a peripheral is ready for the data transfer.
RESET Signals:
- Reset in (PIN 36): This is active low input. This pin is used to reset the microprocessor. An active low signal applied to this pin reset the program counter inside the microprocessor. The buses are tri-stated.
- Reset out (PIN 3): If we want to reset the external devices connected to the microprocessor then a signal applied to this pin resets the external devices.
DMA request signals:
- HOLD (PIN 39): HOLD is an active high input signal used by the other controller to request microprocessor about use of address, data and control signal. On receiving the hold request the CPU release the use of the bus as soon as the completion of the current Bus transfer.
- HLDA (PIN 38): HLDA means Hold acknowledge. This indicates that the cpu has received the hold request and it will release the bus in next clock cycle.
[VI] Serial I/O signal :
8085 has two signal for the use of serial data communication.
(1)SID (serial input data)
(2)SOD (serial output data)
SID (PIN 5): This is serial input data line.
SOD (PIN 4): This is serial output data line.
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