Transcript Electrical Interface of Logic Devices

Electrical Interface of Logic
Devices
Technician Series
Created Feb 2015
©[email protected]
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Basic Electrical Interfaces
• Digital electronic devices need to connect and
communicate with the analog world. They need to
switch high loads, deal with noise effectively and
communicate properly.
• This presentation addresses some basic devices
and circuits that help the digital circuit achieve its
objectives.
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Tri-States
• A Tri-State buffer (or a 3-state buffer) utilizes a second
input to electrically disconnect the output.
• The third state is called “high impedance”, or simply “Z”.
• The control input is often labeled “OE”, or Output Enable.
A
OE
Y
A
OE
Y
0
1
0
1
1
1
X
0
Z
Status
Enabled (A to Y)
Disabled (high Z)
IEEE/ANSI Symbol for Tri-State
3
Example of Tri-State use
Only one of these
Output Enables will be
enabled at a time
Tri-state device
OE2
Tri-state device
OE1
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Buffers and Drivers: Storing Buffers
The 74374 contains 8 DFlip-Flops with buffered
clock and OE inputs, and
a tri-state buffered
output.
OE
D0
D
Clock
D1
Q
Q0
Q
Q1
D
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Interfacing Independent Circuits
• Two digital circuits that utilize different power supplies
need to be interfaced.
• Should there be a common ground between the power supplies?
• What are some considerations?
Power
Supply 1
Power
Supply 2
Digital Circuit 1
Digital Circuit 2
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Circuit Isolation
• Loads may affect other circuits.
• Ground noise is a common problem.
V
Vcc
Motor
Noise
7
Optical Isolator
Opto-isolators usually contain both the transmitter and
receiver in the same unit.
Digital-Side
Opto-isolators
Analog-Side
8
Power Supply Noise
• Power supply noise is often created by the logic switching
of devices where current requirements may change
abruptly.
• This noise appears on both the VCC/VDD and the ground
conductor buses.
• The noise may cause some devices to fault on their logic.
Typical Power Supply Noise
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Decoupling Capacitors
Small capacitors are used to filter out noise present on the
Vcc of a digital circuit. Small caps respond fast and are
very effective.
image: prgodin
The rule is 0.01μF for each IC, and 0.1μF for every 5 to 10.
Decoupling Caps in a circuit: blue=0.01μF, yellow are higher values
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Transistors
• Transistors are frequently used to switch devices with
higher current or voltage requirements.
• In digital electronics, transistors are used as switches.
For the example below, a logic high will turn it on; a low
will turn it off.
V
Bias
V
Pull-up
Load
from logic gate
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Relays
• Relays are used to isolate one electrical circuit from
another and also allow circuit ground isolation.
• Typically, relay are used to switch relatively higher
voltages and currents with relatively smaller signals.
High V, High I Circuit
N
Low V, Low I Circuit
S
12
Relays
• Relays used in digital electronics vary considerably
in physical construction, electrical specifications and
application.
• Types include:
• Solid State Relays (SSR’s) that do not rely on mechanical
motion but utilize semiconductor components to make
electrical connection. They are more limited in variety but
are becoming increasingly popular.
• Electro-Magnetic Relays (EMR’s) that utilize an energized
coil to create motion and physically pull contacts together.
Very common device that has many applications, especially
for very high voltages and currents. Has several
disadvantages, including mechanical failure, high back
voltage, transient voltage spikes, etc.
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Power-up state
• When power is initially applied to an IC, sometimes
the output isn’t in a predictable or desired state. A
set or reset may need to be applied immediately on
power-up. The RC circuit below accomplishes a
reset:
Can you describe how this circuit operates?
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Handling Unused I/O’s
• Unused inputs and outputs must be handled correctly.
Problems with incorrectly handling input or outputs may
result in:
• Faulty logic
• Damage to the IC
• Higher power supply requirements
• Generally unused inputs are tied to either ground or Vcc
• Unused outputs should be left disconnected in most
cases.
15
ESD
• Electro-Static Discharge may cause damage to digital
components, especially to CMOS-type devices.
• Proper handling techniques are discussed by the
manufacturers. These include:
•
•
•
•
•
Grounded work area (ground mats)
Grounded personnel (ground straps)
Proper handling techniques (circuit cards)
Proper packaging (static-free foam, bags, etc)
Proper procedures (power-up phases, unused inputs)
16
Oscilloscope and Grounds
• The oscilloscope probe is connected to earth ground. The
digital circuit has a common ground.
• Can the ground of the scope be connected to the common ground?
Are there any considerations?
Digital Circuit
Oscilloscope
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The End
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