Does 12MHz save power?

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jthole
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Joined: Mon Nov 27, 2017 7:46 pm

Does 12MHz save power?

Post by jthole » Sat Dec 02, 2017 3:16 pm

Does switching the DM15L to 12MHz actually save power? I know that the CPU draws less current, but calculations also take much longer. Do those factors equal each other out? Or is running 12MHz still more energy efficient? Thanks!

Michael
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Re: Does 12MHz save power?

Post by Michael » Sat Dec 02, 2017 4:51 pm

Every operation or calculation uses the same amount of energy regardless of the CPU speed.
But when running at higher speed it draws more current. The more current it draws the bigger is the voltage drop of the battery.
Therefor it is better running at lower speed if the battery gets low.
Once the battery gets really low, the LCD doesn't redraw instantly, but the CPU is still running.
So, in the end it doesn't really matter.

Boub65
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Location: Rabat, Morocco

Re: Does 12MHz save power?

Post by Boub65 » Sat Dec 02, 2017 5:09 pm

Michael wrote:
Sat Dec 02, 2017 4:51 pm
Every operation or calculation uses the same amount of energy regardless of the CPU speed.
Ok Michael for CPU. But what about LCD?
If we run slower (4x) LCD will be "ON" 4x longer for the same calculation? So it will draw 4x time more than when we run faster for the same calculation. No?
2xDM15L, DM41L
DM42 SN#00855

Michael
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Re: Does 12MHz save power?

Post by Michael » Sat Dec 02, 2017 5:25 pm

That's correct, but the LCD draws only about 120uA while the CPU draws 12mA@48MHz. That is two orders of magnitude.
And of course the CPU goes to sleep instantly after every operation, even after each button stroke. In sleep mode the CPU draws about 3uA.

guiddruid
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Re: Does 12MHz save power?

Post by guiddruid » Sat Dec 02, 2017 5:51 pm

Do you drop the voltage to the CPU when you drop the operating frequency?

Michael
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Re: Does 12MHz save power?

Post by Michael » Sun Dec 03, 2017 7:56 pm

guiddruid wrote:
Sat Dec 02, 2017 5:51 pm
Do you drop the voltage to the CPU when you drop the operating frequency?
Please rephrase ...

guiddruid
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Re: Does 12MHz save power?

Post by guiddruid » Sun Dec 03, 2017 9:20 pm

Sorry. I meant - do you make use of "dynamic voltage scaling" to reduce the CPU supply voltage at lower operating frequencies.
Looking at the STM32L476xx datasheet, there's some mention of it, but it's not clear if it's an optional thing.

Michael
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Re: Does 12MHz save power?

Post by Michael » Mon Dec 04, 2017 11:22 am

guiddruid wrote:
Sun Dec 03, 2017 9:20 pm
Sorry. I meant - do you make use of "dynamic voltage scaling" to reduce the CPU supply voltage at lower operating frequencies.
Looking at the STM32L476xx datasheet, there's some mention of it, but it's not clear if it's an optional thing.
The DM15L has a LPC1115 CPU without any "dynamic voltage scaling".
The DM42 has a STM32L476xx and the datasheet does mention that.
But calling two power consumption ranges "dynamic voltage scaling" seems kind of exaggeration ...

jthole
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Joined: Mon Nov 27, 2017 7:46 pm

Re: Does 12MHz save power?

Post by jthole » Mon Dec 04, 2017 8:40 pm

Thanks for the explanation, and additional info!

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