CES 520 - WEEK 6 September 26, 2006

CMOS issues

• Current consumption
• Normally power supply current is directly proportional to clock rate. DC current ~= 0.
• Floating inputs can cause large DC current, expecially for higher-voltage (5V) parts.
• CMOS latchup
• Most modern CMOS parts have less of a problem - requires 100's of mA to cause latch up
• Beware mixed-voltage interfaces, e.g. 5V CMOS driving 3.3V or 1.8V CMOS

Driving Hardware

• Typical CMOS output current specified at about 4 or 5 mA
• Bipolar transistor must be driven into saturation for lowest "on" collector voltage
• Don't use the published non-saturated beta (current gain) value
• Darlington transistor needs much less drive current
• Note that the output voltage is higher - V(BE) of output transistor + V(CE)sat of input transistor
• Bipolar drive transistors may need a pull-down resistor on the base input to prevent unwanted switching
• ULN2003 has 7 darlington transistors with built-in input pulldown resistors
• LEDs
• Forward current determines brightness. Typically 10-20 mA for small indicator LEDs.
• Forward voltage (Vf) typically about 1.5-3V.
• Relays
• Used for switching high-current or AC loads
• The relays for the lab take 5V / 500 Ohms = 10 mA
• Beware inductive kickback.
• Clamp diode increases relay turn-off time.
• Place Zener diode in series with clamp diode.
• Opto isolators / solid-state relays
• Give near-perfect isolation between driver and load
• No common ground connection required
• Reduces noise and provides safety isolation
• Solid-state relays may be able to drive AC loads
• Compared to electromechanical relays
• Require only a few mA drive current
• Much more reliable (with careful design)
• Much faster
• Buttons and keypads
• Contact bounce causes switches and relays to have noise upon closing and opening
• Can cause the software to report multiple key presses
• Software solution: Read the port several times until a stable reading is achieved
• Hardware solution: Add a capacitor in parallel and use a Schmidt-trigger input to the latch
• Keypad matrix
• Processor continuously scans a "high" across the rows, one row at a time
• Looks for a high on the column outputs to determine which switch was pressed
• Liquid-Crystal Displays (LCD)
• Most have driver/encoding electronics integrated into the module
• Backlight requires high-current power supply
• Digital potentiometers
• Uses tapped resistor and analog switches to simulate a potentiometer
• Unlike a DAC, all three terminals are floating
• LCD contrast adjustment is a typical application
• Non-volatile memory and very low standby current = almost like a real pot
• More reliable and stable than a pot
• Allows calibration under software control
• Tend to have high/non-linear wiper resistance. Better to use as a potentiometer vs variable resistor
• Not good for high power.

Interfacing to the external world

• Crossing clock boundaries
• Metastability
• May occur if a signal violates setup or hold times of a flip-flop
• Increases propagation delay of flip flop, possibly enough to cause an error
• Solution is to add one or two flip-flops to synchronize the input signal
• 
• MTBF = Mean Time Between Failure for single synchronizer
• MTBF2 = Mean Time Between Failure for two synchronizers
• tr = Resolve Time = clock period - path delay (including FF clk-to-data-out and tsu of next device)
• Gamma = Flip-flop metastability window = tsu + thold, tsu = Setup time, thold = Hold time
• F_D = Data Frequency, F_C = Clock Frequency, T_P = Flip-flop Propagation delay
• Typical MTBF can vary from seconds to years. (delta t = clock period)
• FIFO buffer
• Used to smooth out intermittent data flows
• Needed even in fully-synchronous systems if data source can ever temporarily out-pace the receiver
• ESD (Electro-Static Discharge)
• It's a big problem in manufacturing.
• A good web site for more information on ESD
• Some devices are more susceptible than others
• Discrete MOSFET and JFET devices
• Microwave devices such as small-signal Schottky diodes
• Small-geometry digital ICs
• Precision analog ICs
• Precision thin-film resistors
• Human body model (HBM)
• Simulates a human touching the device
• 100 pF / 1.5 KOhm
• Machine Model
• Simulates an object touching the device
• 200 pF / 0.5 uH
• A potential problem for any circuitry that communicates to the outside world
• Connectors of all kinds
• Recessed pins and grounded shell help
• AC or DC power inputs
• Any circuitry in a user-accessible space
• Panel switches and controls
• ESD can jump through any non-insulated orifice
• Protection devices
• Series resistor/shunt diodes
• Must be clamped to a voltage outside the normal signal range
• Switching diode gives fast response but fairly limited protection
• Power Schottky diodes are fast but expensive and have considerable leakage current
• Zener diodes. "Transorb" is trade name for Zener-based ESD protection device.
• Selectable clamp voltage.
• Fast switching but typically has large capacitance.
• Should be protected with a fuse if overvoltage condition is not transient
• Metal-oxide varistor (MOV) - non-linear resistor similar to a Zener diode
• Can typically dissipate more power than a Zener
• Gas discharge tubes
• Use internal spark gap - good for high-power ESD events
• Becomes almost a short circuit once it fires

Regulatory issues:

• Safety
• UL - Underwriters' Labs
• UL is a private company. No government regulations require UL certification.
• "UL Listed" applies to equipment
• "UL Recognized" applies to components (e.g. line cords)
• CSA - Canadian Standards Association
• Not Canada only. Often specified in USA and other other countries as well.
• CE - Covers the European Economic Area
• Certifies that the equipment meets electromagnetic compatibility as well as safety regulations
• EMI
• United States: Regulations specify radiation limits only. Part 15 and 18 of FCC regulations.
• Class A is for commercial and scientific equipment
• Class B is for home or consumer equipment. Much more stringent requirements.
• Europe: Regulations specify both radiation and susceptibility. IEC/EN standards.
• Based on IEC and CISPR recommendations
• Most other nations follow CISPR recommendations in developing their standards
• Design techniques to reduce radiation and susceptibility
• Filter cables and wires that go between PC board assemblies
• Bypass wires right where they exit a shield enclosure
• Use low-voltage differential signalling (LVDS) between assemblies if possible
• Common-mode chokes reduce RF interference without degrading the signal
• Ground loops are a source of low-frequency interference
• Use differential signalling
• Ground the two chassis or PC boards together. Use single-point ground.
• Use opto-isolators or transformers to break the ground loop
• Transformers don't work at DC or low frequencies
• Coaxial cable works well at RF frequencies
• Keep clock and other high-speed traces short
• Use multi-layer PC board with power/ground planes on inner layers
• Put small (50-100 ohm) resistors on output of high-speed drivers - reduces ringing
• Rabbit 3000 Low-EMI features
• Separate power pins for core and I/O
• Includes an external I/O bus separate from the memory bus
• Clock spreader option smears out the clock spectrum and its harmonics
• Some internal clocks can be gated off when not needed.
• An internal clock doubler allows the external crystal oscillator to operate at 1/2 the frequency.

Assignments:

• Read Embedded Systems Design, Chapter 5, pp 116-143.
• Lab 4: Add relays and LED hardware and software. (Due in 2 weeks)