The 8085 PIN Function
The 8085 is Intel’s 8-bit microprocessor
and is 40-pin DIP (Dual In line Package) integrated chip. The pin
diagram of the 8085 is shown in figure. The 8085 has 16-bit address bus which
can address maximum 64 K bytes (216 =64 K bytes) of memory.
 |
| PIN Function |
Address bus: The 8085 has
16 address lines. The upper eight address lines A15-A8
are unidirectional and used as higher order address bus
.
Multiplexed Address/Data bus:
the eight lower bits of address bus are multiplexed with data bus. These eight
lines AD7-AD0 either contain 8-bit data or lower byte of
16-bit address. During the earlier part of instruction execution, it contains
lower order address.
Control and status signals:
Two signal RD and WR are used to control the read/write operations. When read
control signal RD is low, the microprocessor is reading either from the memory
device or input port. When write control signal WR is high the microprocessor writes into
the storage device or sending data
to the output port.
 |
| The 8085 PIN Function |
A special signal ALE
(Address Latch Enable) is employed to
demultiplex the multiplexed address/data bus. When ALE is high multiplexed
address/data bus contains lower byte of the address and when it's low multiplexed bus contains 8-bit data.
To distinguish
between the memory and peripheral ,
the 8085 uses IO/M signal. it's high
when microprocessor communicates with peripheral and
it remains low when it communicates with storage
device .The RD and WR signals are combined with IO/M to
generate control signals IOR (I/O read), IOW (I/O write), MEMR (memory read)
and MEMW (memory write) which are used to activate I/O device or memory device
for read and write operations respectively. For example, when IO/M is low, RD
is low and WR is high, the operation is memory read and MEMR signal is
generated which activates memory device for read operation.
The status
signals S1 and S0 are used to identify the operation currently being performed
by the 8085 microprocessor. These signals in combination with the IO/M signal
show current status of the microprocessor as shown in table. The status of 8085
indicates the machine cycle, the 8085 is performing.
 |
| The 8085 Machine Cycles |
Serial I/O signals: the 8085 has two
signals namely SID (Serial Input Data) and SOD (Serial Output Data) to supply serial communication with serial
device like character printer , tty
device etc.
Power supply and clock:
The 8085 requires only +5 V power supply. The Vcc is connected to +5 V power
supply and Vss is connected to ground. The 8085 has its own on-chip clock
generator. The crystal is to be
connected between x1 and x2 pins. The 8085 operates at half of the clock
frequency as:
Operating
frequency – 1/2 *crystal frequency
The CLK (OUT) is
an output signal which provides system clock to other devices in the system.
Interrupt and
other signals: whenever interrupt occurs on one of the interrupt pin,
microprocessor suspends all the operations, saves the current status and jumps
to special service routine known as Interrupt Service Routine (ISR). There are
five interrupt pins TRAP, RST 7.5, RST 6.5, RST 5.5 and INTR.
The TRAP is
non-maskable interrupt and can’t be stopped either be means of hardware or
software. It is
generally used for emergency situation like power
outage to save lots of the
important values.
The RST 7.5, RST 6.5
and RST 5.5 are referred to as restart
interrupts. The INTR is general purpose interrupt. All the four are
maskable and may be masked by
software instruction like DI (Disable Interrupt) and unmasked using instruction
EI (Enable Interrupt).
The 8085
acknowledges the interrupt through the pin INTA (Interrupt Acknowledge output
pin.
The
8085 has two signals HOLD and HLDA to transfer the control of buses to other
device like DMA controller. A HOLD input indicates that other device wants to
use the address and data bus. After
receiving HOLD input, the 8085 completes the data transfer on buses, puts RD,
WR and IO/M in tri-state and indicate to peripheral devices through HLDA output
that it will leave the control of buses in next clock cycle.
The READY input
signals is used to synchronize the operations with the slower peripheral
devices. It is used by peripheral devices to inform the microprocessor that it
is ready to send or receive data. When it's low,
the microprocessor goes into wait state then waits
for a response from device. When devices is
prepared to transfer the info ,
it sends READY signal high to microprocessor.
When RESET IN input
pin goes low, the microprocessor resets itself.The microprocessor can send the
reset signal to other peripheral devices by RESET OUT pin to point that's being
reset.
The functions of every pin of 8085 are briefly given below:
The functions of every pin of 8085 are briefly given below:
A15
–A8: they represent upper half of the address bus of 8085
microprocessor and carries upper byte of 16-bit address.
AD7-AD0:
They represent multiplexed address/data bus. When ALE is high, they're used as lower half the address bus and carries lower byte of
16-bit address. When ALE is low, they're used
as 8-bit data bus to transfer data to and from microprocessor.
ALE: It stands for Address
Latch Enable. It is used to demuliplex multiplexed address/data bus (AD7-AD0).
When it is high, AD7-AD0 carries lower byte of and
address and when it is low, AD7-AD0 carries 8-bit data. ALE goes high in
start of each machine cycle and
stays high during first T-state. At the end of T1, it goes low.
IO/M: it stands for
Input-Output/Memory. It differentiates between I/O and Memory operation. When it's high, 8085 performs I/O operation (either
read or write) and when it's low,
8085 performs Memory operation (either read or write).
RD,WR: the control signals RD and WR are wont to perform the read and write operations
respectively. When RD is low, 8085 is reading from memory or data input device . When WR is high, 8085 is
writing to memory location or output device. These signals along side IO/M are wont
to generate control signals MEMR, MEMW, IOR and IOW.
S1, S0:
They are status signals and IO/M, S1 and S0 together
represents different machine cycles 8085 can perform. For example, IO/M=0, S1=0
and S0=1 represents memory write cycle.
SID, SOD: SID stands for
Serial input file and SOD represent Serial Output Data. SID is employed to read the bits serially, while
SOD is employed to send data
serially bit-by-bit. they're used
for serial communication between 8085 and serial device.
READY: it is used to
synchronize the operation with slower peripherals. When it's high,
the device is prepared to perform
the read or write operation.
RESET IN, RESET OUT: when RESET IN is low,
microprocessor resets itself and initializes Program Counter to 0000h. RESET
OUT is employed to tell all other peripherals during a system that microprocessor is being
reset.
TRAP: it is non-maskable
interrupt and used only in emergency like power
outage .Once it occurs, it can not be stopped by
software or hard ware means.
RST 7.5, RST 6.5, RST 5.5:
They
are referred to as restart
interrupts and are maskable. they're controlled
by EI/DI instruction also as RIM
and SIM instructions.
INTR: it is general
purpose interrupt and also maskable. it's enabled
and disabled by EI and DI instructions.
INTA: it is interrupt
acknowledge pin and used by microprocessor to acknowledge the acceptance of an
interrupt of an interrupt request by device.
X1, X2: 8085 contains
in-built clock generator and X1 and X2 are used to connect crystal for this
internal clock generator, using it, 8085 generates system clock. All the operations
performed by 8085 are synchronized to the
present clock.
CLK OUT:
It
is wont to provide the system clock
to other devices during a system.
Vcc, Vss:
They are power signals. Vcc is connected to +5 Volt, while Vss
is connected to ground.
Post a Comment
Please do not enter any spam link in the comment box.