8085 Interfacing with I/O Devices or Memory

8085:
INTERFACING WITH
I/O DEVICE OR
MEMORY
PRESENTED BY-
APUL RANJAN SAIKIA (CSE-08/14)
PARISMITA BHARALI (CSE-37/14)
PARTHA PRATIM BARMAN (CSE-38/14)
SAUMAY PAUL (CSE-50/14)
SEMINAR PRESENTATION ON
MICROPROCESSOR AND MICROCONTROLLERS

INTERFACING MEMORY CHIPS
WITH 8085
Interfacing is a technique through which two separate components
of a computer system exchange information or interact with each
other.
Interfacing a microprocessor is to connect the microprocessor to
various peripherals to perform operations to obtain output.
Two types of interfacing can be done with the 8085 microprocessor
- Memory interfacing
-I/O interfacing

INTERFACING MEMORY CHIPS
WITH 8085
8085 has 16 address lines
Hence, 216 = 64KB of memory locations can be interfaced with it.
The memory address ranges from 0000H to FFFFH.
Control Signals for memory
When is IO/ M high, both memory control signals are deactivated irrespective
of the status of RD and WR signals.

INTERFACE AN IC 2764 WITH 8085 USING NAND GATE ADDRESS DECODER SUCH
THAT THE ADDRESS RANGE ALLOCATED TO THE CHIP IS 0000H – 1FFFH.
Specification of IC 2764:
8 KB (8 x 2 ^ 10 byte) EPROM chip
13 address lines (2 ^ 13 bytes = 8 KB)
Interfacing:
13 address lines of IC are connected to the corresponding address lines of 8085.
 Remaining address lines of 8085 are connected to address decoder formed using logic gates, the
output of which is connected to the CE pin of IC.
 Address range allocated to the chip is shown in Table.
 Chip is enabled whenever the 8085 places an address allocated to EPROM chip in the address bus.
This is shown in Fig..

INTERFACING IC 2764 WITH THE 8085
TABLE ADDRESS ALLOCATED TO IC 2764

INTERFACE A 6264 IC (8K X 8 RAM) WITH THE 8085 USING NAND GATE DECODER
SUCH THAT THE STARTING ADDRESS ASSIGNED TO THE CHIP IS 4000H.
8K x 8 RAM
8 KB = 2 ^ 13 bytes
13 address lines
The ending address of the chip is 5FFFH (since 4000H + 1FFFH = 5FFFH).
When the address 4000H to 5FFFH are written in binary form, the values in the lines A15, A14, A13 are
0, 1 and 0 respectively.
The NAND gate is designed such that when the lines A15 and A13 carry 0 and A14 carries 1, the output
of the NAND gate is 0.
The NAND gate output is in turn connected to the pin of the RAM chip.
A NAND output of 0 selects the RAM chip for read or write operation, since CE2 is already 1 because of
its connection to +5V. Fig. shows the interfacing of IC 6264 with the 8085.

INTERFACING 6264 IC WITH THE 8085

INTERFACE TWO 6116 ICS WITH THE 8085 USING 74LS138 DECODER SUCH THAT THE
STARTING ADDRESSES ASSIGNED TO THEM ARE 8000H AND 9000H, RESPECTIVELY.
2 K x 8 RAM
2 KB = 2 ^ 11 bytes
11 address lines
A0 – A10 lines of 8085 are connected to 11 address lines of the RAM chips.
 Three address lines of 8085 having specific value for a particular RAM are connected to the
three select inputs (C, B and A) of 74LS138 decoder.
 Table shows that A13=A12=A11=0 for the address assigned to RAM 1 and A13=0, A12=1 and
A11=0 for the address assigned to RAM 2.
Remaining lines of 8085 which are constant for the address range assigned to the two RAM are
connected to the enable inputs of decoder.

 When 8085 places any address between 8000H and 87FFH in the address bus, the select inputs
C, B and A of the decoder are all 0. The Y0 output of the decoder is also 0, selecting RAM 1.
 When 8085 places any address between 9000H and 97FFH in the address bus, the select inputs
C, B and A of the decoder are 0, 1 and 0. The Y2 output of the decoder is also 0, selecting RAM 2.

INTERFACING TWO 6116 RAM CHIPS USING 74LS138 DECODER

PERIPHERAL MAPPED I/O
INTERFACING
In this method, the I/O devices are treated differently from
memory chips.
The control signals I/O read ( IOR ) and I/O write ( IOW), are used
to activate input and output devices.
IN instruction is used to access input device and OUT instruction is
used to access output device.
Each I/O device is identified by a unique 8-bit address assigned to
it; a maximum of 256 (28
) input devices and 256 output devices can
be interfaced with 8085.
Generation of IOR and IOW signals

STATUS OF IOR AND IOW SIGNALS IN 8085
PERIPHERAL MAPPED I/O
INTERFACING
INTERFACING AN 8-BIT DIP SWITCH WITH THE 8085 SUCH THAT THE ADDRESS ASSIGNED TO THE DIP SWITCH IS F0H.
 F0H in the lines A0 – A7; copy of it in
A8 – A15
 IOR signal is activated
 Data in the data bus is read and store in
the accumulator
 A0 – A7 lines are connected to NAND gate;
O/P of NAND gate is 0.
 O/P of NAND gate is ORed with the IOR signal.
 O/P of OR gate is connected to 1G and 2G of
the 74LS244.
 74LS244 enabled; data in data bus of 8085.
 8085 read & store data in the accumulator.

MEMORY MAPPED I/O
INTERFACING
8085 uses its 16 bit address bus to identify a memory location.
Memory address space : 0000H to FFFFH
8085 needs to identify I/O devices also .
I/O device can be interfaced using addresses from memory space .
8085 treats such an I/O device as a memory location.
This is called Memory mapped I/O.

MEMORY MAPPED I/O
INTERFACING
Memory mapped i/o instruction:
 i/o device are identified by 16 bit addresses .
8085 communicates with an i/o device as if it were one of the memory
location memory related instruction are use
Ex: LDA,STA
LDA 8000H= load A with data read from input device with 16 bit
address 8000H
STA 8001H= store contents of A to output device with 16 bit address
8001H

INTERFACING 8-BIT DIP
SWITCH WITH 8085
 address lines are connected to AND
gates.
 output of these gates along with MEMR
 signal are connected to a NAND gate,
 when the address F0F0H is placed in the
 address bus and MEMR = 0 its output
becomes 0; enables the buffer 74LS244.
 data from the DIP switch is placed in
the 8085 data bus.
 8085 reads the data from the data bus
and stores it in the accumulator.

8085 Interfacing with I/O Devices or Memory

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