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TOPIC 6
INPUT/OUTPUT AND
INTERFACING
6.1 Input/output Operation
Content
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Definition
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Data Transfering Technique
 Direct transfer
 Interrupt
 Direct Memory Access (DMA).
 I/O mechanisms (Handshaking)
6.1.1
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Definition
In computing, input/output(I/O), refers to the
communication between an information processing
system (such as a computer), and the outside
world possibly a human, or another information
processing system.
Inputs are the signals or data received by the
system, and
Output are the signals or data sent from the
system.
Examples of I/O devices:
Input devices - Keyboard, mouse
Output Devices - video screen, printer and etc.
6.1.2
Data Transfering
Technique
Three technique are possible for I/O
operations:
 Direct Transfer
 Interrupt
 Direct Memory Access
6.1.2.1
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Direct Transfer
Also known as Programmed I/O .
With Programmed I/O , data are exchange
between the CPU and I/O module. The CPU
executes a program that gives it direct control of
the I/O operation.
When the CPU executes an instruction relating to
the I/O, it issues a command to the appropriate
I/O module.
The CPU periodically check the status of the I/O
module until it finds the I/O operation is
complete.
… cont’d
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Programmed I/O takes place when an
instruction in the program performs the data
transfer; for example:
MOVE.B Keyboard,D0
- to read a byte of data from the keyboard
and puts it in D0.
Some microprocessors have special instructions
that are used only for I/O; for example:
OUT 50
6.1.2.2
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Interrupt
A computer executes instructions sequentially unless a
jump or a branch is made.
An interrupt is a process or a signal that stops a
microprocessor/microcontroller from what it is doing so
that something else can happen.
In other words an interrupt is defined as a break in the
normal flow of operation of a computer caused by an
interrupt signal.
An interrupt may be a signal from a peripheral (i.e., a
hardware interrupt) or an internally-generated call to
the operating system (i.e., a software interrupt).
Interrupt Sequence
When CPU receives an
interrupt signal, it takes
a specified action.
Interrupt signals can
cause a program to
suspend itself
temporarily to service the
interrupt.
When the interrupt has
been addressed or
processed, the
computer’s attention can
be returned to the
process or program it
was executing before the
interrupt with the exact
same conditions
prevailing.
Sources of Interrupt
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Internal fault (e.g. divide by zero,
overflow)
Software
External hardware
- maskable
- non maskable
RESET
When a hardware device needs the processor's
attention, it simply sends an electrical signal
(hardware interrupt) through a dedicated pin in the
interrupt controller chip
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Hardware interrupts:
every keystroke generates an interrupt signal.
Interrupts can also be generated by other devices,
such as a printer, to indicate that some event has
occurred.
Software interrupts:
Interrupt signals initiated by programs.
Exceptions belong to a special type of software
interrupts. They are generated by the processor itself
whenever some unexpected critical event occurs.
Three types of exceptions: faults, traps and aborts.
Interrupt categories
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Microcomputer interrupts fall into two
basic categories:
- maskable and
- non-maskable.
The CPU of the microcomputer has
interrupt signal line for each category of
interrupt.
Maskable Interrupt
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External hardware interrupts are maskable
interrupts. The interrupt request signal
indicates the presence of one or more of
these interrupts.
Maskable interrupts can be masked out or
locked out for short periods of time by the
software to allow the CPU to perform critical
operations. The programmer is responsible
for ensuring that interrupts are managed in a
timely manner.
Non-maskable Interrupt
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Non-maskable interrupts cannot be masked out.
They are used for conditions that require immediate
attention by the microcomputer.
Microprocessors sometimes have a special interrupt
request input called a non-maskable interrupt request.
Non-maskable interrupts are necessary when the interrupt
is caused by a critical event that must not be missed;
for example: interrupts from the internal hard disks,
modems, fax cards, and sometimes
out-oftolerance condition. If this
feature is
available, a power out-oftolerance
condition will force the
microcomputer to
execute its save
data program.
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The 68000 reserves its
level 7 interrupt (IRQ7) as
a non-maskable interrupt,
because an interrupt on
IRQ7 is always serviced by
the 68000.
If a level 7 interrupt is
currently being serviced by
the 68000, a further active
transition on IRQ7 (i.e., a
high-to-low edge) results
in the 68000 servicing the
new level 7 interrupt.
The 68000's interrupt structure
6.1.2.3 Direct Memory Access
(DMA)
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Direct Memory Access (DMA) is where a device is allowed
to take over the main computer bus from the CPU and
transfer bytes directly to main memory.
Normally the CPU would make a transfer from a device to
main memory in a two step process:
1. reading a chunk of bytes from the peripheral device
and putting these bytes into CPU
2. writing these bytes from the CPU to main memory
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With DMA it's a one step process of sending the bytes
directly from the device to memory.
moves data between a
peripheral and the CPU's
memory without the direct
intervention of the CPU itself
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DMA provides the fastest possible means of transferring
data between an interface and memory, as it requires no
CPU overhead and leaves the CPU free to do useful work.
While DMA is going on, the CPU can't do too much since
the main bus is being used by the DMA transfer.
Data transfer with a DMA
controller
DMA Operation
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DMA works by grabbing the data and address buses
from the CPU and using them to transfer data directly
between the peripheral and memory.
During normal operation of the computer, bus switch 1 is
closed, and bus switches 2 and 3 are open. The CPU
controls the buses, providing an address on the
address bus and reading data from memory or writing
data to memory via the data bus.
When a peripheral wishes to take part in an I/O
transaction it asserts the TransferRequest input of the
DMA controller (DMAC).
… cont’d
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In turn, the DMA controller asserts DMArequest to
request control of the buses from the CPU; that is the CPU
is taken off-line.
When the CPU returns DMAgrant to the DMAC, a DMA
transfer takes place. Bus switch 1 is opened and switches
2 and 3 closed.
The DMAC provides an address to the address bus
and hence to the memory. At the same time, the DMAC
provides a TransferGrant signal to the peripheral that is
then able to write to, or read from, the memory directly.
When the DMA operation has been completed, the DMAC
hands back control of the bus to the CPU.
Flowchart for a DMA operation
6.1.3
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Handshaking
Many I/O devices cannot accept data at an arbitrary rate.
For example:
- a Pentium based PC is capable of sending several hundred
million characters a second to a printer, but that printer is
(probably) unable to print that many characters each
second.
- Likewise, an input device like a keyboard is unable to
provide several million keystrokes per second (since it
operates at human speeds, not computer speeds).
The CPU needs some mechanism to coordinate data
transfer between the computer system and its peripheral
devices.
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One common way to coordinate data transfer is
to provide some status bits.
Using status bits to indicate that a device is
ready to accept or transmit data is known as
handshaking
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Initially, the computer makes the data available
and then asserts data DAV to indicate that the
data is valid.
The peripheral receiving the data sees that DAV
has been asserted, indicating that new data is
ready.
The peripheral asserts its acknowledgement, DAC
(data accepted) and reads the data. The data
accepted signal is a reply to the computer
informing it that the data has been accepted.
Once the data has been read by the peripheral,
the DAV and DAC signals may be negated and
the data removed. This sequence of events is
known as handshaking.
6.2
Interface
Content:
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Definition
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Concept of interfacing
 Serial and parallel data
transmission techniques
 RS232 standard
 A/D and D/A Converters
 Interface chips (serial & parallel)
6.2.1
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Definition
I/O Interface is required whenever the I/O
device is driven by the processor.
An interface is an
assembly of electronic circuits that make
the computer compatible with the peripheral
units. This compatibility permits the computer
and the peripheral
units to communicate intelligently. The comp
atibility involves logic levels, timing or speed,
and control.
… cont’d
1. When digital data is transmitted between two
units, the binary voltage or current levels must be
compatible. Logic-level conversion is often required
to properly interface different types of logic circuits. For
example, logic-level shifting is often required to
properly interface bipolar and MOS circuits.
2. The speed of the data transmission must also be
compatible. Some type of temporary storage between
the two units may be required as a buffer to match the
high-speed CPU to a low-speed peripheral unit
3. Control is another function of the interface.
There are status lines that tell when the computer
or peripheral unit is ready or busy, and strobe lines that
actually initiate the data transfers. This process is
often referred to as “handshaking.”
The type of information exchanged between the
I/O unit and the peripheral devices includes data,
addressing, and control signals. In some
computers, peripheral units are addressed as storag
e locations, and all memory reference instructions can
be used in performing I/O operations. No special I/O
instructions are used in these computers
Relationship between a
computer and a peripheral
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The peripheral interface chip looks just like a memory location to the
CPU (i.e. read or write data to it). However, this interface chip must
have necessary logic to interpret the device address generated by the
processor. that allows it to communicate with the peripheral.
Method of transmitting
digital data
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There are two methods of transmitting
digital data :
- parallel and
- serial transmissions.
Microprocessors are by nature parallel
machines. They transmit/receive data in parallel
bits (8,16,32,64). It is required sometimes to
send the data serially.
Parallel data transmission
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all bits of the binary data are transmitted
simultaneously.
For example, to transmit an 8 –
bit binary number in parallel from one unit to
another, eight transmission
lines are required. Each bit requires its own se
parate data path. All bits of a word are transmitted
at the same time.
… cont’d
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This method of transmission can move a significant
amount of data in a given period of time.
Its disadvantage is the large number of
interconnecting cables between the two units. For
large binary words, cabling becomes complex and
expensive. This is particularly true if the distance
between the two units is great. Long multiwire cables
are not only expensive, but also require special
interfacing to minimize noise and distortion
problems.
Examples: connections between a computer and a printer
(Most printers are within 6 meters or 20 feet).
Serial data transmission
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Serial data transmission is the process of
transmitting binary words a bit at a time. Since the
bits time-share the transmission medium, only one
interconnecting lead is required
While serial data transmission is much simpler
and less expensive because of the use of a
single interconnecting line, it is a very slow method
of data transmission.
… cont’d
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Serial data transmission is useful in systems where
high speed is not a requirement. Serial
data transmission techniques are widely used in
transmitting data between a computer and its
peripheral units.
While the computer operates at very high speeds,
most peripheral units are slow because of
their electromechanical nature. Slower serial data
transmission is more compatible with such devices.
Since the speed of serial transmission is more than
adequate in such units, the advantages of low cost and
simplicity of the signal interconnecting line can be
obtained.
Differential between Serial &
Parallel data transmission
Serial
Parallel
sending an entire byte of data in the time it takes to send a
single bit
sending the eight bits in a byte of data simultaneously
low cost
high cost
slows the speed of transmission
highs the speed of transmission
long distance
short distance
smaller data bus
wider data bus
Serial transmission can be either synchronous or
asynchronous . In synchronous transmission, groups of bits
are combined into frames and frames are sent continuously
with or without data to be transmitted. In asynchronous
transmission, groups of bits are sent as independent units
with start/stop flags and no data link synchronization, to
allow for arbitrary size gaps between frames.
The timing for parallel transmission is provided by a constant
clocking signal sent over a separate wire
within the parallel cable; thus parallel transmission is
considered synchronous
Synchronous Communication
(synchronised transmit & receive clocks)
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There are two methods for serial communication:
- Synchronous &
- Asynchronous.
In Synchronous serial communication the
receiver must know when to “read” the next bit
coming from the sender, this can be achieved by
sharing a clock between sender and
receiver.
… cont’d
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In most forms of serial Synchronous communication, if
there is no data available at a given time to transmit, a fill
character will be sent instead so that data is always being
transmitted.
Synchronous communication is usually more efficient
because only data bits are transmitted between
sender and receiver, however it will be more costly
because extra wiring and control circuits are required to
share a clock signal between the sender and receiver.
More complex interface (high data rates supported up
to ~ 10 Gbps)
Used for: Connections between computer and telephony
networks
Asynchronous Communication
(independent transmit & receive clocks)
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Simple interface (limited data rate, typically < 64
kbps)
No clock sent (Tx & Rx have own clocks)
Asynchronous transmission allows data to be
transmitted without the sender having to send a
clock signal to the receiver.
Instead, special bits will be added to each word in
order to synchronize the sending and receiving of the
data.
Used for connecting: Printer, Terminal, Modem,
home connections to the Internet
Asynchronous serial
communication characterictics
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Serial communication requires the following four
parameters:
The baud rate of the transmission
The number of data bits encoding a character
The sense of the optional parity bit
The number of stop bits
Baud rate is a measure of how fast data are moving
between instruments that use serial communication.
The baud rate is identical to the maximum number of bits
of information, including control bits, that are transmitted
per second.
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When a word is given to the UART for Asynchronous
transmissions, a bit called the "Start Bit" is added to the
beginning of each word that is to be transmitted. The
Start Bit is used to alert the receiver that a word of
data is about to be sent, and to force the clock in the
receiver into synchronization with the clock in the
transmitter.
After the Start Bit, the individual bits of the word of
data are sent, each bit in the word is transmitted for
exactly the same amount of time as all of the other bits
… cont’d
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When the entire data word has been sent, the
transmitter may add a Parity Bit that the
transmitter generates. The Parity Bit may be used
by the receiver to perform simple error checking.
Then at least one Stop Bit is sent by the
transmitter.
If the Stop Bit does not appear when it is supposed
to, the UART considers the entire word to be
garbled and will report a Framing Error.
Example of serial data transmission
Each transmitted character is packaged in a character
frame that consists of a single start bit followed by the
data bits, the optional parity bit, and the stop bit or bits.
***
The standard serial communications hardware in the PC
does not support Synchronous operations.
RS 232 Standard
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There are many different recommended standards of
serial port communication, including the following most
common type:
RS-232,
RS-449, RS-422, RS-423
RS-232 (Recommended Standard 232) is a standard for
serial binary single-ended data and control signals
connecting between a DTE (Data Terminal Equipment)
and a DCE (Data Circuit-terminating Equipment).
It is commonly used in computer serial ports.
RS-232-compatible port was a standard feature for serial
communications, such as modem connections, on many
computers.
… cont’d
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The RS-232 standard includes electrical signal
characteristics (voltage levels), interface mechanical
characteristics (connectors), functional description of
interchange circuits (the function of each electrical
signal), and some recipes for common kinds of terminalto-modem connections.
The most frequently encountered revision of this
standard is called RS-232C.
In personal computer peripherals it has largely been
supplanted by other interface standards, such as USB.
RS 232C connector
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Devices that use serial cables for their communication
are split into two categories. These are DCE and DTE.
DCE are devices such as a modem, TA adapter, plotter,
and so on, while DTE is a computer or terminal.
RS-232 serial ports come in two sizes, the D-Type 25-pin
connector and the D-Type 9-pin connector.
RS 232C connector
– DE9
The connector on the PC has male pins, therefore the cable needs to
terminate in a DE9/F (Female pin) connector.
RS 232C connector– DB25
The DB-25 connector is the standard RS-232
connector, with enough pins to cover all the
signals specified in the standard. Table shows
only the core set of pins that are used for most
RS-232 interfaces.
RS- 232 Interface
Function of RS 232C connector
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The RS-232C standard defines a set of rules
to provide for the orderly exchange of data in
a serial bit-stream.
It also defines signals to allow the connected
devices to tell each other when they are
ready to send or receive data.
These signals are called flow-control or
'handshake' signals.
Analog and Digital Converter
Analog to Digital Converter
(ADC)
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An analog-to-digital converter is a device that
converts a continuous quantity to a discrete digital
number.
The basic principle of operation is to use the comparator
principle to determine whether or not to turn on a
particular bit of the binary number output.
ADC is an electronic device that converts an input analog
voltage (or current) to a digital number proportional to
the magnitude of the voltage or current.
Some non-electronic or only partially electronic devices,
such as rotary encoders, can also be considered ADCs.
Digital to Analog Converter
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A digital-to-analog converter (DAC or D-to-A) is
a device that converts a digital (usually binary) code
to an analog signal (current, voltage, or electric
charge).
A DAC converts an abstract finite-precision number
(usually a fixed-point binary number) into a concrete
physical quantity (e.g., a voltage or a pressure).
DACs are often used to convert finite-precision time
series data to a continually varying physical signal.
USART 8251
( Universal Synchronous and Asynchronous
Receiver and Transmitter)
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The 8251 is a USART (Universal Synchronous
Asynchronous Receiver Transmitter) for serial data
communication.
As a peripheral device of a microcomputer system, the
8251 receives parallel data from the CPU and transmits
serial data after conversion. This device also receives
serial data from the outside and transmits parallel data to
the CPU after conversion.
It allows connecting a microcomputer system to a variety
of external devices, e.g. mouse or trackball, serial
keyboards and terminals, printers and plotters with RS232 interface, microcontroller development systems, flashprogrammers, etc.
Block Diagram of 8251A
PPI 8255
(Programmable Peripheral Interface)
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The 8255A programmable peripheral interface (PPI)
implements a general-purpose I/O interface to connect
peripheral equipment to a microcomputer system bus.
The functional configuration of each port is programmed by
the systems software.
Features:
- Three 8-bit Peripheral Ports - Ports A, B, and C
- Three programming modes for Peripheral Ports: Mode 0
(Basic Input/Output), Mode 1 (Strobed Input/Output),
and Mode 2 (Bidirectional)
- Total of 24 programmable I/O lines
- 8-bit bidirectional system data bus with standard
microprocessor interface controls
Block Diagram PPI 82C55A
Operating Modes
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The configuration is required before the chip can be used. The
configuration tells the 8255 whether ports are input or output
and even some strange arrangements called bi-directional and
strobed.
The 8255 allows for three operating modes:
 Mode 0: Ports A and B operate as either inputs or outputs
and Port C is divided into two 4-bit groups either of which
can be operated as inputs or outputs
 Mode 1: Same as Mode 0 but Port C is used for
handshaking and control
 Mode 2: Port A is bidirectional (both input and output) and
Port C is used for handshaking. Port B is not used.
Control Word
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The control word contains information such as “mode”, “bit set”,
“bit reset”, etc., that initializes the functional configuration of
the 82C55A.
Each of the Control blocks (Group A and Group B) accepts
“commands” from the Read/Write Control logic, receives
“control words” from the internal data bus and issues the proper
commands to its associated ports.
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Control Group A - Port A and Port C upper (C7 - C4)
Control Group B - Port B and Port C lower (C3 - C0)
The control word register can be both written and read as
shown in the “Basic Operation” table.
When the control word is read, bit D7 will always be a logic “1”,
as this implies control word mode information.
Basic
Operation
table
Mode 0 (Basic Input/Output)
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This functional configuration provides simple input
and output operations for each of the three ports.
No handshaking is required, data is simply written to
or read from a specific port.
Mode 0 Basic Functional Definitions:
Two 8-bit ports and two 4-bit ports
Any Port can be input or output
Outputs are latched
Input are not latched
16 different Input/Output configurations possible
Mode 0 Configurations
Mode 1 (Strobed Input/Output)
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This functional configuration provides a means for
transferring I/O data to or from a specified port in
conjunction with strobes or “hand shaking” signals.
In mode 1, port A and port B use the lines on port C to
generate or accept these “hand shaking” signals.
Mode 1 Basic Function Definitions:
Two Groups (Group A and Group B)
Each group contains one 8-bit port and one 4-bit
control/data port
The 8-bit data port can be either input or output.
Both inputs and outputs are latched.
The 4-bit port is used for control and status of the
8-bit port.
Mode 1 Configurations
Mode 2 (Strobed Bi-Directional Bus I/O)
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The functional configuration provides a means for
communicating with a peripheral device or structure on a single
8-bit bus for both transmitting and receiving data (bi-directional
bus I/O).
“Hand shaking” signals are provided to maintain proper bus
flow discipline similar to Mode 1.
Interrupt generation and enable/disable functions are also
available.
Mode 2 Basic Functional Definitions:
Used in Group A only
One 8-bit, bi-directional bus Port (Port A) and a 5-bit
control Port (Port C)
Both inputs and outputs are latched
The 5-bit control port (Port C) is used for control and
status for the 8-bit, bi-directional bus port (Port A)
Mode 2 Configurations