• 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
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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
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In class activity
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Voltage Current Voltage Current.5 V 4V1V 5V1.5V 6V2V 7V2.5V 8V3V 9V3.5V 10V
Ohms Law
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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
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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.
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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.
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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)
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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
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Power Calculation for transistors switching on/off
• What will cause power to go up?
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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
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