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Digital Interfacing
Digital Interfacing
EE 264 EE 264
Dr. Samuel Russ Dr. Samuel Russ
Why do microprocessors
Why do microprocessors
need interface circuits?need interface circuits?
Drive strengthDrive strength
Single microprocessor pin can generally only Single microprocessor pin can generally only
drive a small number of outputs drive a small number of outputs
Example:
Not strong enough to drive a single
Example:
Not strong enough to drive a single
LED! LED!
Voltage differencesVoltage differences
Different digital devices run at different Different digital devices run at different
voltage levels voltage levels
Examples:
5V, 3.3V, 2.5V,
Examples:
5V, 3.3V, 2.5V,
±±
12V, etc.12V, etc.
Safety issuesSafety issues
Lightning Lightning
Other extremely high voltages Other extremely high voltages
Drive
Drive
Strength: Drivers
Strength: Drivers
Special buffers that have high outputSpecial buffers that have high output currentcurrent
Example:
Clock driver
Example:
Clock driver
Has built
Has built
in
in
circuits to synchronize the output tocircuits to synchronize the output to the input (0 delay)the input (0 delay)
LED:
10’s of
LED:
10’s of
mA
mA
Relay:
100’s of
Relay:
100’s of
mA
mA
Voltage
Voltage
Level Change
Level Change
OneOne
way:
Open
way:
Open
drain drivers drain drivers
These also increase drive strength These also increase drive strength
Only drive to ground (0V) which is a nearly Only drive to ground (0V) which is a nearly
“universal” voltage level “universal” voltage level
TwoTwo
way:
Pass transistor circuits
way:
Pass transistor circuits
Lets either side drive to ground (bidirectional) Lets either side drive to ground (bidirectional)
RSRS
232 driver circuit232 driver circuit
Special case:
Drives ±12V outputs off of a
Special case:
Drives ±12V outputs off of a
single +5V supply single +5V supply
Isolation / Safety Circuits Isolation / Safety Circuits
Isolation:
Provide voltage isolation
Isolation:
Provide voltage isolation
between input and output between input and output
Protection:
Two forms
Protection:
Two forms
Disconnects power when current draw is Disconnects power when current draw is
too large too large
Shorts out and dissipates when voltage Shorts out and dissipates when voltage
rises above a certain level rises above a certain level
Specific Examples of Driver Specific Examples of Driver
Circuits Circuits
Highlights choices available toHighlights choices available todigital digital
system designerssystem designers
Consult datasheetsConsult datasheets
and feel free toand feel free to
find your own solutions! find your own solutions!
Common Example:Common Example:
Electromechanical Relay Electromechanical Relay
Digital output drives anDigital output drives an electromagnetelectromagnet
The magnet moves a switch backThe magnet moves a switch back and forth between contactsand forth between contacts
Electromagnetor “Coil”
Digital Output
Switch (SPDT)
From 120 VAC
To Light Bulb
Example: Light Switch
Clamping Diode
Relays, Relays,
ctd ctd
. .
The switch is a simple shortThe switch is a simple short
circuitcircuit
Can be AC, DC, radio Can be AC, DC, radio
frequency, etc. frequency, etc.
Switch is completely isolated from theSwitch is completely isolated from the digital driverdigital driver
no DC connectionno DC connection
The coil produces an inductive “kick” whenThe coil produces an inductive “kick” when it is openit is open
circuited circuited
Attach a diode from Attach a diode from
to + to absorb the to + to absorb the
energy energy
Coil voltages:
+5V, +12V
Coil voltages:
+5V, +12V
Coil currents:
10’s to 100’s of
Coil currents:
10’s to 100’s of
mAmA
Voltage Isolation: Voltage Isolation:
Opto Opto
Isolators Isolators
OptoOpto
isolators are combinations ofisolators are combinations of
LED’s (or LED’s (or
IRED’sIRED’s
) and photocells) and photocells
No DC connection from input toNo DC connection from input tooutput output
only photons!only photons!
Typical isolation is 3,500 voltsTypical isolation is 3,500 volts
Output can be DC or ACOutput can be DC or AC
DC type:
Darlington
DC type:
Darlington
pair outputpair output
AC type: AC type:
TriacTriac
output output
Other Safety DevicesOther Safety Devices
One type is the simplest:
The Fuse!
One type is the simplest:
The Fuse!
Numerous variations, such as slow Numerous variations, such as slow
blow blow
Provides a current limit on the system Provides a current limit on the system
Several devices turn into short circuitsSeveral devices turn into short circuits
Telephony: Telephony:
SidactorsSidactors
Power strips (etc.):
Metal
Power strips (etc.):
Metal
OxideOxide
VaristorsVaristors
( (
MOV’s MOV’s
) )
I don’t likeI don’t like
MOV’sMOV’s
They can burst into
They can burst into
flame!!flame!!
Adding current limits makes this more likely! Adding current limits makes this more likely!
Needless to say, READ THE DATASHEETNeedless to say, READ THE DATASHEET when designing for safety!when designing for safety!
