Introduction to logic design By Sajjan G. Shiva

(c)abhijith marathakam

COMPUTER SYSYTEM ORGANISATION AND DESIGN.

abhijith.marathakam@gmail.com

First law of thermodynamics!

A closed system is one where no material is transferred
across the system boundaries. Only heat and work are transferred to the
system as shown in the animation above.

As work is done on a real gas, the temperature and pressure
increase and some heat will be transferred out of the system.

As a real gas expands, it does work on the surroundings
and the temperature and pressure decrease. Heat is transferred to the system.

We balance the changes in the internal energy of the gas
with the amount of heat transferred to/from the gas and work done by/on
the gas. This energy balance is called the first law for a closed system.
It is written

differential form dU = dQ - dW

integrated form (1)

The sign conventions for heat transfer and work are

Heat transferred to the system is POSITIVE

Work done on the system is NEGATIVE

If the processes by which heat is transferred or work done
on/by the system are reversible,
then we can calculate numerical values of for the Q and W in equation (1)
and work out the internal energy change.

If the processes by which heat is transferred or work done
on/by the system are irreversible then, values at the start and end of the
irreversible process are required to calculate the change in internal energy.

LCD And PLasma (electronx assgnmnt)

Topic of the week (22 oct 2008)

Capacitors and charging

The voltage on a capacitor depends on the amount of charge you store on its plates. The current flowing onto the positive capacitor plate (equal to that flowing off the negative plate) is by definition the rate at which charge is being stored. So the charge Q on the capacitor equals the integral of the current with respect to time. From the definition of the capacitance,

v_C = q/C, so

Now remembering that the integral is the area under the curve (shaded blue), we can see in the next animation why the current and voltage are out of phase.

Once again we have a sinusoidal current i = I_m . sin (ωt), so integration gives

(The constant of integration has been set to zero so that the average charge on the capacitor is 0).

Now we define the capacitive reactance X_C as the ratio of the magnitude of the voltage to magnitude of the current in a capacitor. From the equation above, we see that X_C = 1/ωC. Now we can rewrite the equation above to make it look like Ohm's law. The voltage is proportional to the current, and the peak voltage and current are related by

V_m = X_C.I_m.

Note the two important differences. First, there is a difference in phase: the integral of the sinusoidal current is a negative cos function: it reaches its maximum (the capacitor has maximum charge) when the current has just finished flowing forwards and is about to start flowing backwards. Run the animation again to make this clear. Looking at the relative phase, the voltage across the capacitor is 90°, or one quarter cycle, behind the current. We can see also see how the φ = 90° phase difference affects the phasor diagrams at right. Again, the vertical component of a phasor arrow represents the instantaneous value of its quanitity. The phasors are rotating counter clockwise (the positive direction) so the phasor representing V_C is 90° behind the current (90° clockwise from it).

Recall that reactance is the name for the ratio of voltage to current when they differ in phase by 90°. (If they are in phase, the ratio is called resistance.) Another difference between reactance and resistance is that the reactance is frequency dependent. From the algebra above, we see that the capacitive reactance X_C decreases with frequency . This is shown in the next animation: when the frequency is halved but the current amplitude kept constant, the capacitor has twice as long to charge up, so it generates twice the potential difference. The blue shading shows q, the integral under the current curve (light for positive, dark for negative). The second and fourth curves show V_C = q/C . See how the lower frequency leads to a larger charge (bigger shaded area before changing sign) and therefore a larger V_C.

Thus for a capacitor, the ratio of voltage to current decreases with frequency. We shall see later how this can be used for filtering different frequencies.

(SOURCE:http://www.physclips.unsw.edu.au/~jw/AC.html)

Topic of the WEEK

Direct Mapping

This is the simplest among the three techniques. Its simplicity
stems from the fact that it places an incoming main memory block into a specific
fixed cache block location. The placement is done based on a fixed relation between
the incoming block number, i, the cache block number, j, and the number of cache
blocks, N:
j = i mod N

According to the direct-mapping technique the MMU interprets the address issued
by the processor by dividing the address into three fields as shown in Figure below:






The lengths, in bits, of each of the fields in Figure are:
1. Word field = log2 B, where B is the size of the block in words.
2. Block field = log2 N, where N is the size of the cache in blocks.
3. Tag field = log2 (M/N), where M is the size of the main memory in blocks.
4. The number of bits in the main memory address = log2 (B x M)
It should be noted that the total number of bits as computed by the first three
equations should add up to the length of the main memory address. This can be
used as a check for the correctness of your computation.

(extracted from book:FUNDAMENTALS OF COMPUTER ORGANIZATION AND ARCHITECTURE by Mostafa Abd-El-Barr & Hesham El-Rewini )

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