USB Powered Devices vs Main Powered (CPU Impact)

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This thread could go in many of the subforums so I basically just threw a dart to choose.

I have many devices that can be, and are, powered by USB.
These devices often also have AC powered options to allow for standalone usage.

If I choose to add mains power to these devices whilst also being connected via USB will there be any CPU reduction (however negligible)?
"I was wondering if you'd like to try Magic Mushrooms"
"Oooh I dont know. Sounds a bit scary"
"It's not scary. You just lose a sense of who you are and all that sh!t"

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no.


Battery management should be handled by hardware.

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jackoo wrote:no.


Battery management should be handled by hardware.
Interesting. So adding AC adaptors to these devices as I had planned is a complete waste of money. Thx.
"I was wondering if you'd like to try Magic Mushrooms"
"Oooh I dont know. Sounds a bit scary"
"It's not scary. You just lose a sense of who you are and all that sh!t"

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Maybe.

Many CPUs use a burst mode where they need to access more power.

If you have a laptop or a computer with a power supply not adequate for the task, you could deprive your CPU of power it needs if you don't avail yourself of the AC powered option.

Also, the hotter your machine is the slower it will run. Everything you can do to offload work it has to do and keep the CPU cooler, including sending power to USB devices, will speed the machine up.
Last edited by billcarroll on Thu Apr 17, 2014 3:41 pm, edited 1 time in total.
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billcarroll wrote:Maybe.

If you have a laptop or a computer with a power supply not adequate for the task, you could deprive your CPU of power it needs if you don't avail yourself of the AC powered option.
This is even more interesting.

I do have a laptop and it seems to handle it ok.
I am trying to relieve it of any unneccessary stress.
"I was wondering if you'd like to try Magic Mushrooms"
"Oooh I dont know. Sounds a bit scary"
"It's not scary. You just lose a sense of who you are and all that sh!t"

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Mushy Mushy wrote:
billcarroll wrote:Maybe.

If you have a laptop or a computer with a power supply not adequate for the task, you could deprive your CPU of power it needs if you don't avail yourself of the AC powered option.
This is even more interesting.

I do have a laptop and it seems to handle it ok.
I am trying to relieve it of any unneccessary stress.
Take the MacBook Pro for example. It has to access the battery to get enough power to handle CPU bursts. The less power available, the slower the machine will run. The hotter it gets, the slower it will run.
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billcarroll wrote:Take the MacBook Pro for example. It has to access the battery to get enough power to handle CPU bursts. The less power available, the slower the machine will run. The hotter it gets, the slower it will run.
This does make sense and is basically what I expected.

I will continue with Plan A then, adding AC adaptors.
I think I only need two more. One for the Midex8 and another for the MKS25.

Thanks for your help.
"I was wondering if you'd like to try Magic Mushrooms"
"Oooh I dont know. Sounds a bit scary"
"It's not scary. You just lose a sense of who you are and all that sh!t"

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billcarroll wrote:The less power available, the slower the machine will run.
I don't think so.
It either runs or it doesn't (shutting down completely in case of unacceptable undervoltage). Why would it run slower? Do you have a reference for that?

Unless you're running on batteries and you have software that underclocks you CPU to save power. Is this what you meant? But then, that software would always be on, regardless of what you have connected to your USB.

Also, your laptop shouldn't get hotter when you have more USB devices attached. The power is dissipated into the USB devices themselves, not your laptop. You're just discharging your battery faster (yes, wearing it out).

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jackoo wrote:
billcarroll wrote:The less power available, the slower the machine will run.
I don't think so.
It either runs or it doesn't (shutting down completely in case of unacceptable undervoltage). Why would it run slower? Do you have a reference for that?

Unless you're running on batteries and you have software that underclocks you CPU to save power. Is this what you meant? But then, that software would always be on, regardless of what you have connected to your USB.

Also, your laptop shouldn't get hotter when you have more USB devices attached. The power is dissipated into the USB devices themselves, not your laptop. You're just discharging your battery faster (yes, wearing it out).
Hi Jackoo, I never run it battery only. It's always AC powered.
You believe quite strongly then it would make zero difference.

confused.com now
"I was wondering if you'd like to try Magic Mushrooms"
"Oooh I dont know. Sounds a bit scary"
"It's not scary. You just lose a sense of who you are and all that sh!t"

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Mushy Mushy wrote:
jackoo wrote:
billcarroll wrote:The less power available, the slower the machine will run.
I don't think so.
It either runs or it doesn't (shutting down completely in case of unacceptable undervoltage). Why would it run slower? Do you have a reference for that?

Unless you're running on batteries and you have software that underclocks you CPU to save power. Is this what you meant? But then, that software would always be on, regardless of what you have connected to your USB.

Also, your laptop shouldn't get hotter when you have more USB devices attached. The power is dissipated into the USB devices themselves, not your laptop. You're just discharging your battery faster (yes, wearing it out).
Hi Jackoo, I never run it battery only. It's always AC powered.
You believe quite strongly then it would make zero difference.

confused.com now
I'm not very knowledgeable on the subject, but I would agree with jackoo...

also, how much power can you draw through the usb bus?

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@ pdxindy: re power through the bus.

Surely if less power is required through the bus, this capacity can be reappropriated to data movement. Thus making it faster.

Although if it's already operating at capacity, then it just becomes stress relief.
"I was wondering if you'd like to try Magic Mushrooms"
"Oooh I dont know. Sounds a bit scary"
"It's not scary. You just lose a sense of who you are and all that sh!t"

Post

pdxindy wrote:how much power can you draw through the usb bus?
The voltage specification for USB is 5V +- 5%. For USB 2.0 maximum DC current is 500mA, and for USB 3.0 this has been increased to 900mA.

Therefore, power capability per USB port is:
USB 2.0: 2.5W
USB 3.0: 4.5W
Mushy Mushy wrote: Surely if less power is required through the bus, this capacity can be reappropriated to data movement. Thus making it faster.

Just moving data across devices requires very little power. If less power is required from the bus, it will simply not be consumed, it won't matter to your data transfer rate.

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Mushy Mushy wrote:If I choose to add mains power to these devices whilst also being connected via USB will there be any CPU reduction (however negligible)?
No. What in the world would current at the USB ports have to do with CPU cycles? :?

ew
A spectral heretic...

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Mushy Mushy wrote:@ pdxindy: re power through the bus.

Surely if less power is required through the bus, this capacity can be reappropriated to data movement. Thus making it faster.

Although if it's already operating at capacity, then it just becomes stress relief.
The power adapters on modern a macbook pro cannot deliver enough power to handle boost mode of the CPU, for example a 2.3GHz quad-core Intel Core i7 processor might have "Turbo Boost" up to 3.5GHz, so the laptop must access the battery in addition to the power provided by the adapter. Take the battery out of your macbook pro and run it off power adapter only and you will see the machine run at what seems to be about 1/2 speed.

Using your laptop to power a bunch of external power devices will slow things down when your laptop's power adapter can't even provide enough power for the badass CPUs we slap in them these days. All of this will also murder your battery, and you will end up cutting battery lifespan dramatically.

Not only is a lack of power an issue, but heat is also another major player. The harder your laptop has to work to power external devices the hotter it will get and the slower it will run.

If you can power your USB devices with an adapter while working you will be better off.

I speak from direct experience and testing.
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