Where does a chip's power go?
Chips use power in two main ways: each time switches flip (switching power), and by leaking current even when idle (leakage).
1. Two ways to use power
A chip holds billions of tiny on/off switches, and it uses power in two main ways. Switches use power each time they flip, and current leaks through them even at rest.
2. What sets switching power
Switching power grows with how many switches flip, how often they flip (the clock speed), and the voltage. Voltage counts twice: twice the voltage gives four times the power.
3. Slower and lower
Half the clock speed halves the switching power, but a job takes twice as long, so that alone saves no energy. The lower voltage that a slower clock allows is what saves energy.
4. Leakage flows while power is on
Leakage flows whenever a part has power, even when it does nothing. It grows as the chip gets hotter or its voltage rises.
5. What stops each kind
Stopping the clock stops the switching power. Only cutting the power supply stops almost all the leak too.
Words to know
- Switching power
- Power used each time switches flip on or off. Engineers also call it dynamic power.
- Leakage
- Current that leaks through switches that are off, whenever the part has power.
- Current
- The flow of electricity, like water flowing through a pipe.
- Voltage
- The electrical push that moves current through the chip and makes its switches flip.
- Clock speed
- How many times a second the chip's clock ticks.
Where you meet it at work
Testers first ask which kind of power a feature cuts, and when: while a part works or while it sits idle. Then they measure power at that moment with the feature on and off.
Check yourself
Why does stopping the clock not stop all power use?Think first, then tap to see the answer.
Leakage keeps flowing while the part has power. Only cutting the power stops almost all of it.