• Notes: Prior to class, instructor will need to obtain a set of power supplies with current measurement from tech support as well as a set of 50 ohm ¼ W resistors and a set of 50 ohm larger resistors as well.

• You will also need a modified SunROM board with interchangeable crystals

CS2710 Computer Organization 1

Lecture Objectives:

Ohms Law and Power

1) Explain the relationship between current, voltage, and resistance (Ohms Law)

2) Calculate the power dissipated in a resistor based on voltage and current

3) Explain the relationship between clock rate and dissipated power.

In Class Activity

• Your instructor will pass out a power supply and a resistor– Wire the following circuit (Your instructor will aid

you)– Starting at 0 V, measure the current as you increase

the voltage between 0 and 10 V.– Plot the results

CS2710 Computer Organization 3

In class activity

CS2710 Computer Organization 4

Voltage Current Voltage Current.5 V 4V1V 5V1.5V 6V2V 7V2.5V 8V3V 9V3.5V 10V

Ohms Law

CS2710 Computer Organization 5

V I R

V=> Voltage, which is electrical “pressure”I=> Current, which is rate of electron flowR=> Resistance (of a material to flow of electrons through it)wa

Power

CS2710 Computer Organization 6

2 /

W V I

W V R

W=> Power (Watts), electrical work done per unit time (Joules/s)V=> VoltageI=> CurrentR=> Resistance

What is power?

• Power is the energy used by an electronic device to do work– For computers, it’s byproduct is heat.

CS2710 Computer Organization 7

Demo part 2

• Using a modified SunROM board, measure the power used to run the board at 1Mhz, 4 Mhz, and 16 MhZ.

• Sample code for the program is available.

CS2710 Computer Organization 8

CS2710 Computer Organization 9

0 2000000 4000000 6000000 8000000 1000000012000000 14000000 16000000180000000

5

10

15

20

25

30

35

40

45

50

f(x) = 1.37064407211569E-06 x + 21.4060648531567R² = 0.992074254384158

ATMEGA 32 Current versus Crystal Frequency

CurrentLinear (Current)

Crystal Frequency (Hz)

Micr

opro

cess

or C

urre

nt (m

A)

CS2710 Computer Organization 10

2 2.5 3 3.5 4 4.5 5 5.50

50

100

150

200

250

f(x) = 3.42804303590267 x^2.58744148306454R² = 0.998420723836924

Power versus Microprocessor Voltage

PowerPower (Power)

Microprocessor Voltage

Micr

opro

cess

or P

ower

(mW

)

Power and computers

CS2710 Computer Organization 11

Power Calculation for transistors switching on/off

• What will cause power to go up?

CS2710 Computer Organization 12

2Power CapacitiveLoad Voltage ClockFrequency

Why do you care?

• Overclocking– Setting your CPU and memory to run at speeds

higher than their official speed grade. – Intel Core i7 860 • 2.80GHz out of the box.• Overclocked if pushed to a clock speed higher than

2.80GHz

CS2710 Computer Organization 13