Transcript EDGE - Rochester Institute of Technology

Practical Design Considerations
I/O Circuits
Connecting input and output devices to
electronics.
prepared by Prof. George Slack (EE)
Copyright © 2006 Rochester Institute of Technology
All rights reserved.
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Prerequisite: Schematic Guidelines
Start your schematic(s), if not already.
Sketch a drawing or visualization your project including the following:
• Interface Devices
• ALL inputs and outputs (don’t stop at the board’s edge!)
• power On/Off, reset and safety switches, sample select switch
• removable AC power cord
• connectors
• harnesses (& cables)
• board(s)
– test points
– LEDs (more is better)
– Jtag, USB, RS232, Ethernet (may not need the form factor)
– accessory power connectors
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Steps in Designing I/O Circuits
Step 1. Interface Specifications
Step 2. Specification Sheets & Application Notes
Step 3. Analysis I/O
Step 4a. Synthesis Output Devices
Step 4b. Synthesis Input Devices
Step 5. Analysis Noise immunity
Step 6 Analysis Harness Signal Response
Step 7 Connectors/ Pins
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Step 1: Interface Specifications
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2.
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6.
7.
8.
9.
10.
11.
Bandwidth, frequency response; rf to dc
Data rate
Analog versus digital (you should know by now)
Circuit protection
Power/ Current
Voltage Levels: Vdd/Vcc, multiple?
Devices (Vdd vary from 1.5 VDC to 18 VDC)
Output Control: Sink or source?
Data Controls: enables, select lines, etc.
EMI Noise
Heat
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Step 2. Specification Sheets & Application Notes
Save Hours of design time!
Exploit these Design Notes!
1.
2.
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4.
5.
6.
7.
8.
Many Specification Sheets have recommended applications that
may apply directly to your design! Use them!
Not sure what you need? …. or researching components?
http://cmpmedia.globalspec.com/
Manufacturer’s Home Pages
Suppliers: DigiKey, Mouser and Allied offer good on-line spec
sheets
http://www.findchips.com/
Google (I must admit, I do this first….)
RIT Library services
If complex solution, don’t start from scratch!
•
Purchase device evaluation/ demo kits (USB,
Microcontroller, Motor controllers)
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(continued Examples)
• CMOS (i.e. MicroController Interfaces) devices may need
protection electronics.
• Responsive to signal processing spec’s.
• Example 1. PIC18F2455 Interface considerations
– See attached pdf.
• Example 2. Freescale Semiconductor Inc - application schematic
– Motor Controller MC3PHACVDW
If the spec sheet document is large, send me the link and I
will print for you. Soon may be 400 pages but DO IT!
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Step 3. Analysis
I/O
1.
Impedance matching to your control electronics
Outputs
Inputs
•
•
•
•
Low
Low
Low
Low
Voltage to High Current
Voltage to High Voltage
Current to High Current
to Current to High Voltage
Higher Voltage to Low Voltage
Higher Current to Low Voltage
2. Apply to Thevenin Equivalent Circuits
– Source electronics
– Device being driven
– RLC loads (& potential for unwanted current and voltage
spikes)
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Step 4a. Synthesis
Output Devices
• Logic: Microcontroller to input of electronics drivers.
• Driver: (voltage to voltage, voltage to current.)
• Device:
– Motors
– Linear Actuators
– Solenoids
– Storage Capacitors
– HVPS
– LEDs
– Heaters
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cont.
Types of Output Driver Components
Matching (Voltage, Impedance, Current):
Components
– Transistor: Single stage and Darlington.
– JFET
– MOS FET
– IGBT DC to AC inverters (hybrid cars, mass transit)
Devices:
– Isolators: Optical Drivers
– H bridge: bi-directional control for DC Motors (See later slide)
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Step 4b. Synthesis
Input Devices
Devices
• Op Amp
• Isolator: Optical and Magnetic
• Level Shifter/ Translator
(other) Power
• Regulate
• DC to DC converters
– voltage boasting/ ± polarity
– current boasting
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Step 5. Analysis Noise immunity
• Circuit noise immunity is the ability of a device or component to
operate in the presence of noise disturbance .
• Electro Static Discharge is the sudden discharge (i.e. transients,
surge). To the circuit, this is a rapid high voltage, low current
situation.
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5 Continued
Where does noise get into electronics?
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•
•
•
through ground connections and loops
through power supply connections
through signal inputs
through inadvertent ESD
– (human touch, lightning)
• through Inductive devices (motors)
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5 Continued
How does noise get into electronics?
Energy Coupling
(Conductive, Inductive, Capacitive)
•
EMI - Current surges
(ElectroMagnetic Interference) An electrical disturbance in a system
due to natural phenomena, low-frequency waves from
electromechanical devices or high-frequency waves (RFI) from
chips and other electronic devices. Allowable limits are governed
by the FCC.

RFI – high impedance devices requiring very limited
current.
(Radio Frequency Interference) High-frequency electromagnetic waves that
emanate from electronic devices such as chips.
If the source is sufficiently strong this can enter your circuit.
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Cont 5
ESD and unwanted signals
• Fatal to Electronics:
• Inadvertent user misuse.
• Extreme cases of user abuse and solutions:
– Bridge to block reverse polarity,
– Schottky diode: very fast switching times and low
forward voltage drop. As low as 0.15 volts for low
ma applications.
– Zener diode across the input.
– Circuit breaker – GFI
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5. Synthesis
CMOS gate
Reduce Transients
Protect from?
• Misuse during debug and testing.
• ESD and reverse polarity
– Solution: Diode circuits to protect against
• reduce transients
– Component Specification Sheets
– Solution: Low pass filter
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5. Isolating Noise
• Analog and Digital Optocoupler /Optoisolators Somewhat
expensive ($1/ channel) but good isolation.
– an electronic device that uses optics to transfer a signal while
keeping the receiving and transmitting circuits electrically isolated .
– Analog Devices:
• http://www.analog.com (Analog Devices)
• http://www.analog.com/en/subCat/0,2879,767%255F827%255
F0%255F%255F0%255F,00.html
• http://www.analog.com/en/prod/0,2877,ADuM2401,00.html
– OPTO:
• http://www.optoinc.com/
• http://www.optoinc.com/optocouplers1.html
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5 Example of Driver Circuit
•
• Driver Circuit – Opto
22 ODC5
– See diagram below for
application.
– Output Device – 2.5a,
50 vdc Inductive Load
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5. Polarity Protection
(+)
+
Schottky diode 1N5822 or
1N5817
–
(–)
1N4001
Input Port
+
Input Port
–
see 7.4.1 page 238, MOSFET solution
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5. Over-voltage Protection
Fuse
(+)
+
(–)
input port
–
Zener diode
(also MOV)
1N5339 (5.6V for a 5.0V input)
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5. Over-voltage Protection for
Digital Inputs
Vdd
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5. Minimize Bandwith
One solution: Lowpass Filter
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5. CMOS and Noise
characteristics Print out the specification sheets.
• 1 uA input? Not sure? Get the spec sheet!...
• Draw the equivalent circuit for each internal
pin. That is, CMOS versus TTL input
impedance, output is pull-up or pull-down
circuit, current limiting resistor value.
• CMOS inputs have very high input impedance
which is good for low power consumption for
well protected electronics but susceptible to
misuse when connecting to the outside world.
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5. Decoupling Capacitors
• Know your performance response needs. Try
to eliminate frequencies outside your
performance needs (i.e. filtering, smooth out
spikes in DC power of IC’s).
• Common practice is to isolate IC’s with
capacitors. See manufacturer’s Spec Sheet
for capacitor values.
• Filters current transients when transistors
switch with a digital logic gate.
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Step 6 Analysis
Harness Signal Response
Design Considerations: data rate, distance, noise,
parasitic capacitance, reflections.
1. Frequency Response/ Rise Time needs
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DC to 100 khz Open Wire
DC to 40 MB/s Ribbon Cable (less than 3’)
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DC to 300 mhz Twisted Pair - unshielded, ribbon
DC to 100/1000 MB/s 10/ 100/ 1000 BaseT
ethernet Cat 5 minimum spec. RJ45 connector.
Coax, VHF 3000 megahz (scope probe, cable TV)
attenuation, reflectance, Cable TV, Rf
DC to 4 gigahz Fiber Optics
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SCSI, SPI-3 applications
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Step 7 Synthesis
Harness -Twisted Pair
When purchasing harness cable:
• Unshielded Twisted Pair (UTP)
• Shielded Twisted Pair (STP)
Study the manufacturer’s Specification to match to
your needs.
Practical Design Considerations
http://www.cirris.com/testing/twisted_pair/twist.html
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Cont 7 Harness -Twisted Pair
What is does: Cancels out crosstalk from neighboring wires and
electromagnetic interference from external sources
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7 Practical Harness Considerations
1.
Current, Voltage Needs
• Gauge of wire, insulation
2. Use and Abuse
• Connect/ disconnect needs
• Routing, protecting
• Mounting, vibration and stability
• Gold versus Tin; environment, corrosion, current rating
• Instrumentation quality for long life
3. Color Coding for ease in Debug and future use.
As an example;
1. Red Vcc/Vdd
2. Black Grd/ Vss
3. Org for signal
4. Fabricating: Harness board and nails
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8 Connectors/ Pins
Add all needed I/O connectors to your schematic(s).
1. Connect/ disconnect needs – screw terminators, push type
2. Major Manufactures: Amp, Molex
3. Location
– between assemblies
– interface to other projects (collaborate with other teams)
– instrumentation
4. Get crimp compatibility to pin manufacturers (style and gauge).
5. Pins: crimp versus solder
6. Current rating rule of thumb is 10x. i.e. 100 ma purchase 1 amp
pins
7. Types; ribbon, D shell, PCB mount
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Misc: (H Bridge Considerations)
• Motors or whenever you need to direct current.
• Introduction to H bridge operation:
– http://www.dprg.org/tutorials/1998-04a/
• Designing an H-Bridge and PWM Circuits and Code
– http://www.learn-c.com/experiment7.htm
• Debugging: Initial reset may close a short circuit or stress on
the H bridge. Symptom: Motor shaft may pulse or flinch at
power on. May cause immediate failure.
• Design Needs: Inductive loads and protection diode.
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Summary:
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Start your schematic(s), if not already.
Sketch a drawing or visualization your project including the following:
Define I/O Design Needs
Specification Sheets & Application Notes
Analysis I/O
Design Output Devices
Design Input Devices
Consider Noise immunity
Harness; Signal Response
Connectors/ Pins
Next Steps:
•
Define your core electronics and software development
EDGE™