Lectures 6-10 Flashcards by Matt Moran

Voltage threshold levels:
VOH, VIH
VOL, VIL
What are they, which one must be bigger/smaller than the other?

VOH: minimum voltage that can be interpreted at output as logic high
VIH: minimum voltage for logic high at input
VOL: max voltage for logic low at output
VIL: max voltage for logic low at input
VOH > VIH
VOL < VIL

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What is noise immunity in relation to voltage thresholds?

Difference between input and output thresholds
Max noise signal that can be added without producing errors
Noise immunity may be different for logic low and high

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Whats propagation delay, race conditions

PD: delay between inputs changing and outputs changing on a digital circuit
RC: when a logic circuit produces erratic output because various inputs have been applied at slightly different times

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Sinking and sourcing current:
Definition
On a circuit diagram, how can you tell if the current is sink or source?

Sink: when current flows INTO an input or output
Source: when current flows OUT of an input or output

Sink: flowing from voltage supply to circuit
Source: flowing from circuit to ground

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I(IL), I(IH) (how do these relate to VIL and VIH?)
IOL, IOH
What are they?

I(IL): current that will be sourced from an input at logic low (at VIL)
I(IH): current that will sink into an input at logic high (at VIH)

IOL: max current that can sink into an output at low
IOH: max current that can be sourced from an output at high

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Output circuits: push-pull/ TTL totem pole

What do they both look like
Pros, cons (same for both)

PP top to bottom: Voltage supply, two p/n type transistors, ground
Output from between transistors
TTL: Same except resistor under voltage supply and transistors are NPN (source and current are drawn at angles)

Pros: switchs fast due to low impedance
-Can sink or source current
Cons: two or more outputs can

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Output circuits: open-collector/drain outputs

What does it look like
Pros, cons

Voltage supply goes through pull-up resistor to ‘bus’ (dotted line)
Multiple transistors with source connected to bus, drain connectedto a ground
Pros: multiple digital devices can pull the line to logic low
Cons: Slow to switch due to pull-up resistor

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Output circuits: tri-state

What does it look like
Pros, cons

A load of push-pull circuits with outputs connected to a bus.
Pros: Fast, same data bus
Cons: Only one circuit can be in the ‘ON’ state

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TTL: Whats supply voltage usually?
Power consumption?
Max/min voltage input thresholds?
Output voltages in comparison to power supply rails?

-Usually fixed at 5V
-High
-Usually 0.8 V and 2.0 V
Usually just a few hundred millivolts difference

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CMOS: Whats supply voltage usually?
Power consumption?
Max/min voltage input thresholds?
Output voltages in comparison to power supply rails?

Flexible, between 1.8V and 5V
A function of clock speed (zero with no input/output changes)
Usually 1/3 and 2/3 of power supply voltage
Essentially equal to power supply (very close)

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Microcontroller digital inputs:
What happens if left unconnected?
What do some microcontrollers have to help with this?
What do you need to remember about this help?

Input will ‘float’ and be sensitive to noise

Pull-up resistors which can be applied with software

Resistors should not be more than 10% of input impedance and not less than the value that would cause output current to reach 90% of the recommended limit

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Connecting outputs and inputs at different voltage levels-example:
5V output to 3.3V input
3.3V output to 5V input
Solutions for each

Potential dividor (speed is reduced tho)
Direct connection if the lower circuit can handle the voltage
Bi-directional level shifter IC

Direct connection if the VOH for the 3.3V device is higher than VIH for the 5V device
Bi-directional level shifter/buffer IC

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Load Switching

Revise

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Why is serial communication more popular than parallel?

Requires multiple input-output connections which is impractical
As distance increases individual bits in parallel will arrive at different times

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Synchronous vs asynchronous:
Whats the difference
What does each need to work

S: each data bit sent is synchronised with the transmission
Requires some form of common timing clock
AS: each data bit sent is NOT synchronised with the transmission
Requires two clocks to be generated which will eventually get out synch
Requires prior agreement for the transfer rate (baud rate)

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Full-duplex vs half-duplex

FD: communication can take place in both directions simultaneously
HD: two directions but not simultaneously

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UART: what duplex type is it
What type of communication is it
What was the original called?
How do you start and stop a transmission?
What attributes do you need to set in software?

Full-duplex
Asynchronous
RS-232
Logic 1 as start bit
Logic 0 stop bit and pause
Baud rate and frame length

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I^2C protocol: what type of communication is it

What two things does it need for this to work?

Synchronous

Two lines: clock line (Serial Clock, SCL) and data line (Serial Data, SDA)

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I^2C protocol: What duplex type is it?
What type of system is it? What does this mean can happen?
What is there to prevent conflict?
Which device is in control of the flow of data?

Half-duplex
A bus system
More than two devices (ICs) can be connected to the SCL anf SDA lines
An open-collecter bus scheme
By default SCL/SDA lines are high and any device may pull them low
Usually one device acts as master, others are slaves

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I^2C protocol: what unique feature to with the clock does it have?
Whats its “Standard Mode” speed?

If a receiving device is struggling to keep up, it can slow down the transfer by holding the clock line ‘low’
Called clock stretching
100 kbit/s

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I^2C protocol: equation for bus voltage
Whats V(IH) equation?
Equation for maximum pull-up resistor when given minimum rise time
Whats equation for minimum value of pull up resistor?
What minimum rise time for standard mode?

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Vbus = Vsupply*(1 - e^(-t/RC))
t = rise time (time taken for Vbus to reach V(IH)
R = pull-up resistance
C = bus capacitance

V(IH) = 0.7*Vsupply

As V(IH) = Vbus
R = t/1.2C
R > Vsupply/(3*10^-3)
1μs

SPI: differences to I^2C, pros, cons
Four bus lines, names and what they stand for
How many master and slaves are there?

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Does not use open collecters
Uses tri-state pull-pull
Much simpler, faster due to push-pull
Less flexible
SCLK: Serial Clock
MOSI: Master Output, Slave Input
MISO: Master Input, Slave Output
SS: Slave Select
One master, many slaves

CAN protocol: whats it mainly used in?
What type of system does it use (weird)?
Two conductor names, pros of this
How to get logic high and low

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Cars
Uses open-collector differential signalling
Two conductors: CAN high (CANH) and CAN low (CANL)
Shielded twisted-pair wires, gives noise immunity
When both lines are the same, logic high
When CANH is driven 1V higher and CANL driven 1V lower, logic lower

CAN protocol: how does it decide which master gets to control bus?
What does every device on bus have to do?
Synch or asynch?
Rate the speed for different distances

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Prioritises masters according to their unique address
Most important master has most logic lows (zeros)
Listen to each message
Asynch
Fast at short, slow at long

Whats interference? Whats crosstalk? Whats noise?

Unwanted signals from outside the system Unwanted coupling of signals from within the system Random process which may originate internally or externally

Whats a parasitic element in a circuit? | What stray element in a circuit?

Unwanted by inevitable element-i.e. resistor inductance or capacitor leakage Unwanted element which exists between circuit components-i.e. fields between wires

Mutual capacitance: What is it? Capacitence equation for two paralell wires per metre length

``` When mutual capacitance occurs between two points C = πε/ln(d/a) ε = permitivity d = distance between wires a = radius of wires ```

Mutual inductance: What is it? Effects of this for high loop and low loop impedance circuits

When two current-carrying conductors are in proximity, there will be a mutual inductance between them High loop: series coupled voltage source Low loop: current flow which produces voltage drops

Capacitive coupling vs inductive coupling: what kind of signals make each likely to happen How to minimise each

CC: high frequency voltage signals Place a grounded screen/shield around circuit IC: low frequency current signals Max distance between likely sources of interference and circuit

Balanced lines: What are they Requirements for it to work

System of two conductors which have the same impedance with respect to a common ground Both conductors must have same exposure to any source of interference Must use differential signals

Balanced lines: why do they work?

They work because any interference will affect each signal the same, meaning the difference between each will remain the same

How do they deal with interference in: Professional audio systems Modern network cables (whats the name of these)

Balanced lines with three conductors, two being the lines and one being a grounded shield Multiple pairs of conductors twisted together. Each pair is twisted by different amounts to minimise crosstalk CAT5

Ground loops: how are they formed? | What does it do to a circuit?

Connecting signals from two different pieces of equipment together and creating a duplicate ground connection Adds voltage to ground-equivalent circuit is a voltage source placed in series with ground

Ground loops: solutions (bad to good)

Break the loop Disconnect mains protective ground Use isolated inputs/outpupts i.e. use a transformer to transfer the signal

Shared ground paths: how are they formed | Solutions

When ground paths are connected to several current-drawing loads, causing IR voltage drops Separate ground paths for sensitive circuits

Things to consider when choosing a power supply for a circuit-3 things

How much current/voltage does the circuit draw? How much you'll need to protect your circuit from over voltage/current Mains or battery?

Whats current compliance in a voltage source?

The amount of load current a reulated voltage source can maintain supply voltage up to

Converting mains AC to DC: transformer-based power supplies, 3 components How to tell output power Regulated or unregulated?

Consists of a step-down transformer, full bridge rectifier and a smoothing capacitor Use ratio of transformer i.e. 20:1 will convert 240AC to 12DC Unregulated

Converting mains AC to DC: switching power supply 3 components Pros, cons vs transformers

Rectifier, smoothing capacitor, switching using PWM Pros: more compact, efficient, easy to adapt to different power-line voltages Cons: more complicated, quality of DC output is worse

Things to consider when choosing batteries: three things

Use single-discharge (primary) or rechargeable (secondary) Primary batteries for suitable for circuits with low average power consumption Also check battery source resistance

Linear voltage regulators: What do they do? Why do they get hot? Whats headroom/drop-out voltage?

They reduce input voltage (+/-10V to +/-5V) It is constantly dissipating energy Minimum difference between input and output voltage

Linear voltage regulators: power dissipation equation Whats an alternative to LVR's (think power electronics) How to get a voltage reference without a LVR

Preg = (Vin - Vout)*I(load) Its ya boi buck and boost converters (DC-DC) Use a zener diode (when Vin > V(zener))

Thermal management: equation for max junction temperature 4 parameters How to rearrange to get thermal resistance for a heatsink if needed?

``` Tj = P(R(θJC) + R(θCA)) + Ta Tj = max junction temp P = power dissipation (remember equation from earlier) R(θJC) = thermal resistance, junction to case R(θCA) = thermal resistance, case to ambient Ta = ambient temperature ``` Rearrange for R(θCA), this is the value you need

What is: failure mode Fail-safe design 4 things to be concerned about with electronic circuits

A possible fault condition that may arise A design approach where if any failure modes occur, consequences are not dangerous Overheating Component rupture Unsafe output voltage Unsafe controlled functions

Power supply failure: what to worry about if power goes Over-voltage happens Over-current happens

Need back-up safety if it fails (e.g. doors) Circuits that produce safety warnings should automatically warn if power fails Use 'crow-bar' circuit using a thyristor to short circuit supply Use fuses (i.e. PPTC fuse)

Connection failure: why can this happen | What to do about it

Oxidising, bad solder, thermal cycling Design it so the system is aware if this happens Can use weak pull-down resistors to avoid floating inputs

Processor/logic failure: What is it What to do about it

Microcontrollers can fail, software can have bugs Use timers to reset if processor stops responding Write code so that if anything fails, system reverts to safe state

Component failure: What components are risky? What to do about it

Capacitors as they store a lot of energy Resistors and inductors can fail from faults elsewhere in the system Don't short circuit them Use over-current protection or bigger resistors

Load faults: what are they | What to do about it

Loads which become short-circuited | Use capacitors and resistors rated for the max possible voltage and power in a circuit

Human interaction: common problems What to do about them

Connecting power wrong (battery wrong way round, power connected wrong way round) Use a series diode or bridge rectifier (although this causes voltage drop) Devices where you don't want them left on Make it so you constantly require input (i.e. drills)

Lectures 6-10 Flashcards

(50 cards)