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CO527 Anonymous Questions and Answers Keyword Index |
This page provides a keyword index to questions and answers. Clicking on a keyword will take you to a page containing all questions and answers for that keyword, grouped by year.
To submit a question, use the anonymous questions page. You may find the keyword index and/or top-level index useful for locating past questions and answers.
Keyword reference for electronics
2014 |
Submission reference: IN3377
Will the content covered today (first lecture) be on the exam, as you mentioned it is background content, or is it to help build understanding of things to come?
The material covered today is not directly examinable (rather, won't be directly examined). I.e. you won't encounter any questions such as:
Though these might be good questions! Its inclusion is to allow you to build a better understanding of what comes later, particularly to have an insight into what happens at the physical level and what is feasible/infeasible in terms of building a processor architecture. If you took some knowledge or understanding away from today's lecture, that's great :-), but if not, it's not critical — that said, things learned later on may simply remain as facts (e.g. that increasing the frequency of a processor increases power consumption) rather than having an understanding of why (which is because of, amongst other things, the brief periods of short-circuit that occur when switching totem-pole MOSFET structures, and more switching per unit time means more time spent in short-circuit, thus more current draw and more power consumed).
Keywords: electronics
2013 |
Submission reference: IN2897
What is high impedance?
High impedance can be thought of as "high resistance" if it helps. In simple terms, it effectively means that whatever is being talked about is disconnected — i.e. if it were connected directly to Vss or Vdd then little or no current would flow (because of the high resistance). In digital electronics, and elsewhere, high-impedance inputs can be used to "sense" the voltage on a wire (0 or 1) without interfering with it (much at least). Oscilloscope probes are typically high-impedance things (millions of Ohms) allowing one to prod around in a physical circuit to see what the signals are without interfering with them.
Resistance isn't quite impedance though — Wikipedia can tell you more.
Keywords: electronics
Submission reference: IN2896
Will electronic things such as diodes come up in the exam?
Not directly, no. However, having an understanding of how a processor works, in terms of functional units, clocks and other signals, needs at least some knowledge about the electrical signals involved (mostly whether they are logically 0 or 1, physically Vss or Vdd).
Keywords: exam , electronics
Submission reference: IN2894
This is a follow up of Question 39 (2013). Which way will the current flow if there is a fork?
It depends.. In the case of the inverter, current will flow either in or out of the "out" connection, depending on whether the inverter's input is 0 or 1. Usually this would be connected to the gate of another FET, in which case the current involved is very small (because the gate terminals are mostly insulated in a FET — the transistor works on field effect rather than current amplification like a BJT). If that wasn't what you meant, can you draw a picture? (ASCII art will do!).
Keywords: electronics
Submission reference: IN2892
What is a transistor with a small circle next to it? As in the third transistor from the top in output latch slide of the second lecture.
It's just another transistor, but of the other type. I.e., here there are 3x n-MOSFETs and 1x p-MOSFET. See the description in lecture 2 for the difference.
Keywords: electronics
Submission reference: IN2891
If input is high/1/Vdd and there are two transistors will the lower one always be the one which is switched on? Is this because high is dragged to low because Vdd always flows to Vss and vice versa?
Finally, are things like gates built on top of a diode which allows the current of the gates to only flow one way?
With regard to your first question, it's not entirely clear what you're talking about here! It sounds like you are asking about something like the 'not' gate (inverter) from lecture 2. In this sort of setup, and as explained on that slide, one of the transistors is always on and one one is always off, assuming the input is in a steady-state (itself 1 or 0). Current flows as a result of potential difference, so if both the transistors were ever on simultaneously, there would be a short circuit from Vdd to Vss through those transistors — held for any appreciable amount of time, this will undoubtedly destroy the circuit. When something like an inverter switches, there is usually a brief period (nano-seconds) of short-circuit — either direct or from capacitance inherent in the circuit: thus, switching costs power.
Regarding diodes and transistors, see lecture 1. Logic gates (e.g. and/or/not) are built out of transistors (see lecture 2), and may contain diodes as extra bits if needed, but not always. Both BJT and FET transistors act as diodes in some sense (see any introductory electronics texts/Wikipedia/etc.) but as far as building digital circuits goes, transistors (used to build gates) and diodes are quite different.
Keywords: electronics
Referrers: Question 41 (2013)
Submission reference: IN2766
From the lab, how does the computer decide which LED to turn on? I was unsure of how the computer says that it wants to turn on red but not turn on yellow and green as they are all connected to the same circuit.
Labeling the AVR as a "computer" sounds a little odd! (micro-controller would be a more appropriate noun here). But to answer your question, there are essentially three circuits (electronically) when the 3 LEDs are connected — each starts with +5v (Vcc) passes through a resistor (360-ohm) then one of the LEDs, and finishes at one of the AVR's I/O pins. The way things are wired up in the practical labs, these are the AVR's "PORTB" pins 5, 4 and 3. On the Arduino these are the sockets numbered 13, 12 and 11.
In software, a program can use something like this to turn the LEDs into a particular state:
cbi PORTB, 5 ; PB5 low -> red LED on
sbi PORTB, 4 ; PB4 high -> yellow LED off
sbi PORTB, 3 ; PB3 high -> green LED off
The sbi and cbi instructions set and clear individual bits in I/O registers (the port B port-register in this case, which when configured for output, drives those particular pins high or low).
Keywords: avr , electronics
Submission reference: IN2637
Why the names "Vdd" and "Vss"?
Tradition mostly; "Vdd" is for drain and "Vss" for source in FET terminology.
Keywords: electronics
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Maintained by Fred Barnes, last modified Wed May 25 15:07:20 2016 |
