Interface

canolaokahumpkaElectronics - Devices

Nov 2, 2013 (3 years and 9 months ago)

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ELEX165

Microcontroller Interfacing
Techniques

Interfacing


‘Interfacing’ is a term used to describe the
hardware and software needed to connect
devices together efficiently. There are four
basic types of interface:


Digital input


Digital output


Analog input


Analog output

Digital inputs


Simple switches and push buttons can be
connected to the microcontroller parallel
I/O port configured as an input port. Each
switch connects to one input pin and is able
to change the logic state (low/high)


Digital sensors can be similarly connected
to indicate various physical states (e.g. door
open, temperature high etc.)

Digital Outputs


The output from the microcontroller I/O
port is a logic signal (0 or 5Volts normally).
This signal can be used to drive LEDs or
low power digital devices


For higher power applications the logic
signal must be ‘amplified’ to increase the
voltage/current available

Output Interfacing


Power transistor/FET


For low/medium power output


Relay/solenoid


For high power applications (e.g. lamp, motor)


Power driver circuit (e.g. LM293 chip)


For driving DC or stepper motors


Servo


For mechanical position control


Other Digital Interfaces


Serial I/O


Many microcontrollers include serial
input/output hardware that is used to
send/receive data over a serial communications
link


Generally used over short distances (<100m) or
over any distance via an external modem

Other Digital Interfaces


Serial Peripheral Interface (SPI)


A ‘synchronous’ serial interface used to
connect external peripherals to the
microcontroller chip over short distances (<30
cm)


Saves on I/O pin usage and inter
-
chip
connections when data transfer rate between
devices is relatively low

Analog Inputs


Many microcontrollers incorporate an
Analog to Digital Converter (ADC) to
convert analog voltages to a quantized
digital value


An external ADC may also be connected
via the SPI or using the external
data/address bus lines (if available)

Analog Sensors


An ADC allows the microcontroller to
interface to sensors such as:


Temperature sensors


Pressure sensors


Light intensity sensors


Sound sensors (microphone)


Any transducer with a voltage/current output

Input Signal Conditioning


ADCs generally have an input voltage range
of 0
-
10V (uni
-
polar) or +/
-

10V (bipolar)


The transducer output (which could be mV
or a small change in resistance etc.) must be
‘conditioned’ by an input signal
conditioning circuit before it can be used by
the microcontroller ADC

Analog Output


An output signal (analog voltage) can be
produced using a Digital to Analog
Converter (DAC)


The DAC takes a digital word (normally 8
-
16 bits) and converts it to a voltage


By converting a series of digital words in
sequence a signal can be produced


Few microcontrollers include a DAC on
-
chip

Analog Output (cont.)


The output of a DAC is commonly 0
-
10
Volts or +/
-

10 Volts (5V range may also be
used), and is suitable for driving low
-
power
devices


The DAC signal may be passed through an
external amplifier for driving higher power
devices

Output Signal Conditioning


The DAC output signal may need to be
amplified/filtered/converted in order to
drive an external device e.g.


Loudspeaker


Hydraulic servo (position/load control)


RF modulator


Motor drive unit

Exercise


Using the internet or other resources, find
transducers for the following purposes:


Convert temperature to voltage


Convert strain to voltage


Convert position to voltage


Convert strain to voltage


Convert voltage to sound

Exercise (cont.)


For each type of transducer research what
signals are produced/required and how each
one could be interfaced to a microcontroller.


Draw and label a block diagram of the
components required in each case.