Transcript No Slide Title

Practical Digital Design
Considerations
Part 2
Last Mod: January 2008
©Paul R. Godin
design 2.1
Circuit Interfaces
◊ A variety of electrical issues must be considered
when interfacing between digital logic circuits, or
between digital logic and analog circuits.
◊ This presentation addresses some specialized
interfaces.
design 2.2
Noise Issues
design 2.3
Digital Signal
◊ Signal must be above or below a threshold voltage
to represent a “1” or “0”. This voltage requires a
reference voltage.
◊ Signal timing must be within a threshold for each
bit to be recognized.
Voltage
or “1”
Minimum Threshold
Unknown Zone
Maximum Threshold
No Voltage
or “0”
Timing
Ideal Signal
design 2.4
Digital Signal Problems
◊ Noise
◊ Resistance
◊ Capacitance
design 2.5
Induced Noise
design 2.6
Induction
◊ Inductance
◊ Can be defined as the transference of electrical energy
from one conductor to another.
◊ Faraday’s law (1831)
◊ “The voltage induced in a turn or coil of a conductor is
proportional to the rate of change of magnetic lines of
force that pass through the coil.”
◊ Faraday’s discovery of the relationship between
electricity and magnetism is often cited as the
most important discovery of the modern era.
design 2.7
Magnetism
◊ Principle:
◊ Magnetic flux lines are
established between the 2
poles of a magnet
◊ The quantity, density and
length of these lines increases
with:
◊ the characteristics of the
medium
◊ the strength of the magnet
N
S
design 2.8
Electromagnetism
◊ Principle:
◊ Magnetic flux lines are
established when current
flows in a conductor
◊ The quantity, density and
length of these lines increases
with current.
◊ The direction of the flux lines
is determined by the direction
of the current.
design 2.9
Electromagnetism
◊ Principle:
◊ If conductors are coiled, the flux
lines of the individual turns
combine to create a bar magnet.
◊ If AC current is applied, the
polarity of the magnet will reverse
◊ Note it takes time for the field
to collapse and re-establish
itself in the other direction.
◊ Inductance impedes current
flow (XL).
◊ The effects of inductance
increase with frequency.
N
S
design 2.10
Total Impedance of a Bus
◊ Bus communications contain Resistance,
Capacitance and Inductance.
◊ The effects of resistance increase with
temperature.
◊ The effects of capacitance and inductance
increase with frequency
Electrical equivalent of a bus
design 2.11
Electromagnetic Induction
◊ Principle:
◊ Magnetic flux lines crossing a
conductor will induce current.
◊ There must be relative
motion.
◊ The quantity of lines that
cross the conductor over a
period of time will determine
the induced current.
◊ The direction of the current is
based on the polarity and
direction of the flux lines.
Source Current
Induced Current
design 2.12
Induction: Source of Noise
◊
Noise is produced when a current is induced on a signalbearing conductor through electromagnetic induction.
◊
This noise may cause erratic operation of digital circuits.
◊
The noise may be picked up via connections on the bus or
in the IC.
design 2.13
EMF, EMR, EMI, RFI and EMP
◊
EMF and EMR:
◊
◊
◊
EMI and RFI:
◊
◊
Electro-Magnetic Field and Electro-Magnetic Radiation,
generally describes the strength of a magnetic field and its
ability to induce current.
May also refer to Electro-Motive Force (Voltage)
Electro-Magnetic Interference and Radio Frequency
Interference describe undesired, induced electrical current.
RFI applied to higher frequencies.
EMP:
◊
Electro-Motive Pulse which is a sudden but short-lived increase
in EMI or RFI. Often destructive. EMP sources include
electrical storms and weapons.
design 2.14
Ambient Noise
◊ Ambient Noise:
◊ Background, steady, predictable noise.
◊ Measuring the frequency of the noise may help
determine the source.
◊ 60Hz = lighting, motors, electrical appliances,
electrical cabling, transformers, etc…
◊ 100Hz to 20 MHz = switching power supplies,
electronic devices, telephone systems, video, etc…
◊ > 20MHz = radio transmitters, cellular telephones,
etc…
◊ Often the source is the power supply.
design 2.15
Transient Noise
◊ Transient Noise:
◊ Irregular and unpredictable noise.
◊ Transient noise is often difficult to pinpoint.
◊ Likely sources are switching circuits, such as
◊ Elevators, photocopiers, welders, electrical storms,
static electricity, switching high-current devices, etc…
◊ Crosstalk:
◊ Noise where within a bus (or cable), one signal-bearing
conductor induces a signal on an adjacent conductor.
design 2.16
Reducing the Effect of Noise
◊
Distance:
◊
◊
◊
Shielding:
◊
◊
◊
Magnetic flux lines are less dense with distance from the source
Reduce the length of conductors
Add conductive shielding around a circuit to either protect it from
EMR, or reducing its emissions of EMR.
Leave trace material on a circuit board for ground.
Other
◊
Employ circuit-based methods for reducing the effects of EMR,
such as filters, improved noise margins, other practical design
considerations, specialized trace techniques, etc…
design 2.17
Grounding and Shielding
Electromagnetic Field
Metallic Shield
Metallic Shield
Induced Current
Induced Current
design 2.18
Effective Grounding
design 2.19
Grounds
◊ An earth ground is a point or conductor that has
zero electrical potential.
◊ A common ground is a reference point for signals.
◊ Grounds are important for circuit operation and for
safety. They must be properly configured and used.
◊ Most systems maintain several levels of grounds.
design 2.20
Ground Types
◊ There are 3 basic types of grounds in electrical
systems:
◊ Earth ground: primary role is safety. It is also the
absolute reference point for voltage. The potential of an
earth ground is considered 0 Volts.
◊ Chassis ground: usually used for safety and for noise
shielding. Meant to have a potential of 0 Volts.
◊ Common ground: a point of reference for signals and
voltages on the circuit board. Often a relative reference
point for voltage.
Earth Ground
Chassis Ground
Common Ground
design 2.21
Ground Impedance
◊
The primary consideration for an effective ground system is
maintaining low impedance, especially low resistance.
VCC
5
50
Load
Power
Supply
Describe how ground resistance can cause problems with
electrical or electronic circuits.
design 2.22
Resistance of Conductors
Conductor resistance in circular copper conductors
AWG
Ω per 100ft
(30.5
meters)
12
0.16
14
0.26
16
0.41
18
0.65
20
1.04
22
1.65
24
2.62
26
4.16
28
6.62
30
10.52
design 2.23
Ground Loops
◊ Ground Loops are multiple paths to ground.
◊ Ground loops interfere with the effectiveness of a
ground.
◊ The different grounds may be at different potential,
drawing current through the shield.
◊ The voltage reference may not be at 0 Volts
Metallic Shield
Current
design 2.24
Ground Potential Difference
Ground potential differences cause data
communications problems. Here is an example:
System 1
System 2
Building frame
Building frame
◊
ground resistance
design 2.25
Reducing Ground Problems
◊ Maintain separate grounds for those devices that
may create noise problems.
◊ Maintain low impedance ground connections and
conductors.
◊ Maintain separation from sources of EMI/RFI and
ESD.
◊ Do not create ground loops (connections between
grounds) near or through your circuits.
◊ Ground cable at one end only. Ground the cable to
chassis ground, not signal ground.
◊ Maintain proper power supply and circuit board
techniques.
design 2.26
Circuit Isolation
design 2.27
Circuit Isolation
◊ Loads may affect other circuits.
V
Vcc
Motor
Noise
design 2.28
Isolation
◊ Exposing digital circuits to unwanted electrical
signals (noise) can cause erratic operation or
damage the devices.
◊ Noise may be transferred between circuits.
◊ The source of these undesirable electrical signals
can be:
◊ the input or output devices to which the digital circuit is
attached
◊ the digital circuit itself provides unwanted signals to the
input and output devices
◊ the voltage reference may be to blame.
design 2.29
Electrical Isolation
◊ Electrical isolation is a means of preventing a
direct electrical path between devices.
◊ Isolating the grounds prevents ground line noise
from interfering with digital devices.
◊ Ground line noise can be caused by differences in
grounding, and by switching high current devices.
◊ Electrical isolation may be used for safety
reasons, or if the reference voltages are different.
design 2.30
Isolation
◊ One method for isolation is to use common
grounds instead of earth or chassis ground for
signal reference.
◊ Another is to use independent power supplies.
design 2.31
Optical Isolator
◊ Optical isolator utilizes light as the transmission
medium between 2 digital devices.
◊ The device usually consists of a circuit that
converts digital signals to light pulses, and a
circuit that converts the light received back to a
digital signal.
TX
RX
design 2.32
Optical Isolator
Opto-isolators frequently contain both the
transmitter and receiver in the same unit.
design 2.33
Decoupling
design 2.34
Decoupling
◊ Coupling happens when two devices share the
same power source. This can present a problem,
as noise on the power supply may affect digital
logic function.
◊ Decoupling describes the act of removing the
problems associated with noise on the power
source.
design 2.35
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
design 2.36
Power Supply Noise
◊ Solutions for power supply noise:
◊ Add decoupling or bypass capacitors between VCC/VDD
and ground. Place these capacitors as close to the
offending ICs as possible. Recommended is one 0.01F
for each IC.
◊ Add a 0.1F capacitor for every 5 ICs.
◊ Maintain short power supply buses.
◊ Ensure the power supply buses have low resistance.
◊ Maintain a separate power supply for high-current
devices.
◊ Use a properly designed power supply with input and
output capacitors.
design 2.37
Other sources for Noise
design 2.38
Transmission Line Noise
◊
If the transmission line are not properly impedance-matched
to the system, some signal reflection will occur.
◊
On some cables, if a terminator is not present, the signal will
reflect from the end of the conductor, resulting in ringing.
design 2.39
Switching Loads
design 2.40
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
Load
design 2.41
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 or High I Circuit
N
Low V, Low I Circuit
S
design 2.42
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. These 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.
design 2.43
Relays
◊ Special consideration must be taken into account
when using relays:
◊ EMR coils produce a high voltage at the moment
they are de-energized. Diodes often prevent these
voltages from appearing on the circuit.
◊ Typically, a transistor or other semiconductor switch is
used to isolate the digital circuit from the relay. Most
digital circuits cannot be directly connected to the coil of
a relay without a driver.
◊ Relays are much slower than semiconductor components
and are not particularly well suited to a high degree of
switching. Relays often bounce.
◊ Mechanical relays do wear and may be susceptible to
dust, humidity and temperature effects. Resistance may
be present at the contact points.
design 2.44
Signal Conditioning
design 2.45
Signal Requirements
◊ Digital devices rely on digital signals that:
◊
◊
◊
◊
◊
have proper logic voltage levels
are able to source or sink the appropriate current
have the correct timing
have fast rising and falling edges
are free from noise
design 2.46
Signal Conditioning
◊ Signal conditioning is a means of ensuring that all
digital signal problems may be corrected.
◊ Digital signals may need to be re-constructed,
filtered or amplified in order to satisfy the
requirements of the digital logic devices.
design 2.47
Signal Conditioning and Passive Devices
◊ A passive device is one that requires no external
power source.
◊ Passive devices include RC circuits. These circuits
may:
◊
◊
◊
◊
filter noise
add a delay
create an edge
create a set or reset state on power-up
design 2.48
Filters
◊ Which of these circuits will filter high frequencies,
and which will filter low frequencies?
Use EWB to determine the operation of these circuits.
design 2.49
Filters
◊ What is the operational response of this circuit?
How can these filters be used to condition signals?
design 2.50
Delay
◊ Every electronic device and digital IC adds delay
to the signal.
◊ In some instances, it may be necessary to add
delay to a circuit to ensure signals arrive with the
proper timing.
◊ An example of a circuit delay requirement is the
oscilloscope.
design 2.51
Example: Oscilloscope Delay
◊
The oscilloscope processes the incoming signal with the
vertical section. This circuit provides an output to the
horizontal section and to the CRT.
◊
The horizontal section has extra circuitry and therefore adds
delay to the signal. A matching delay circuit is required for the
vertical section so that both the horizontal and the vertical
components of the signal are rejoined at the CRT with the
same timing.
design 2.52
Delay circuit
◊ A digital delay circuit may make use of an
external RC network, as in the figure below.
◊ Short-delay circuits utilize an array of selectable,
cascaded gates.
Time Delay Circuit
design 2.53
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:
Describe how this circuit operates.
design 2.54
Signal Conditioning (active)
◊ Active devices can also accomplish signal
conditioning by regenerating a digital signal.
Regenerating VO
Regenerating VO and edges
design 2.55
Unused I/O
design 2.56
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
design 2.57
TTL
◊ Open inputs to most TTL devices float high,
however this is not a certainty.
◊ logic gates may see a low input if Vcc abruptly
changes its voltage level, even just fractions of a
volt, as may happen when other devices on the
circuit are switching.
design 2.58
TTL
◊ Tie the inputs so that the device will utilize the
least amount of power.
◊ IIH usually requires less current than IIL  tie inputs
high.
◊ ICCH is usually less than ICCL  tie the inputs so the
output is a logic level high.
design 2.59
CMOS
◊ CMOS devices are sensitive to voltage levels.
Unlike TTL, floating CMOS inputs are neither
high nor low.
◊ Inputs to CMOS require very little current to
provide a logic level. The leads can pick up
static charges from the environment, or have
voltages induced from adjacent conductors
and circuits. These charges can then
adversely affect the output of a device.
design 2.60
CMOS
◊ Feedback (output being fed back to the input)
can cause the device to begin oscillating, causing
the IC to overheat and the power requirements to
increase.
◊ CMOS devices are static sensitive. A static
charge on one of the open inputs can create
breakdown of the entire device.
◊ Tie any unused inputs to VDD or VSS.
design 2.61
Electro-Static Discharge (ESD)
design 2.62
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)
design 2.63
Review Questions
◊ Explain inductance.
◊ What is the most important rule in regards to
grounding?
◊ What problems does noise cause with electrical
signals?
◊ How can we help isolate the source of noise?
◊ More review questions in class!
design 2.64
END
©Paul R. Godin
prgodin @ gmail.com
design 2.65