fundamental
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Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 9:26 pm

Hello, a late in the day design query or two here....!

I wondered why an sd card slot was provided on the pi at all, since thumb drives cost peanuts these days and can be booted from. Wouldn't it make more sense and save costs for both manufacturer and consumer to have usb only? If sd card were really needed then an sdcard-to-usb adapter could be made use of.

As a secondarym related question: would it have been technically possible to have designed it so that the usb could be booted if no sd card was present. It seems a little cumbersome to be forced to make use of an sdcard, (particularly when there is no clear benefit of having one at all).

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Mortimer
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 9:29 pm

Because the Raspberry Pi can only boot from an SD-card. You can run one from a USB memory stick, but you still need an SD-card to kick things off.
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redhawk
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 9:44 pm

The PI cannot boot from USB because there isn't any code in the boot process to utilise the USB ports only code for loading from an SD Card slot. - https://github.com/dwelch67/raspberrypi

Richard S.

fundamental
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 10:12 pm

Why from a design point of view? 'There isn't any code'... or 'just because' doesnt address the question.
Why design it that way?

lavers
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 10:36 pm

fundamental wrote:Why from a design point of view? 'There isn't any code'... or 'just because' doesnt address the question.
Why design it that way?
The System on a Chip (SoC) that powers the Raspberry Pi is, to my understanding, designed for mobile phones and the like. Therefore it's probably only been designed at the hardware level to boot from an SD card and nothing else.

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redhawk
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 10:43 pm

This is a purely speculative guess but I would image the code required to support SD card booting was a lot simpler than the code for supporting USB flash drive bootable media.

Why it's that way?? I don't know maybe too hard or not enough memory space for code to exist inside the PI.

Richard S.

pygmy_giant
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 11:06 pm

Could have made it a micro SD though - wouldnt stick out so much.

Looks like the board is blowing a raspberry - at all of us.

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Dweeber
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 11:14 pm

pygmy_giant wrote:Could have made it a micro SD though - wouldnt stick out so much.

Looks like the board is blowing a raspberry - at all of us.
Cost. Fully sized SDcards are cheap all over.
Availability. Same with Full Sized SDcards... Easy to obtain just about anywhere.
Size. Imagine in a school environment. Very small easy to lose.

You could use a SDCard adapter (a short one at that) and use a MicroSDHC card (I do), but I would imagine in a class type environment it might be a real problem.
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pygmy_giant
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 11:19 pm

Not convinced.

1) In a class environment I would want the SD card locked away inside the case where naughty kids cant tamper with it or accidentally wrench it.

2) Micro SD's are no longer more expensive than chunky ones.

so those are actually 2 reasons for micro SD

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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 11:21 pm

Actually, In a class environment, i would want an SD card. That way each student could have their own card with their projects saved on it, saving writes to the SD card which has a limited number of writes, having many students using the same card could cause you to go through more cards than it is worth. They can then plug the card into any PI and use their own computer essentially making the larger harder to lose SD card superior.
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pygmy_giant
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 11:27 pm

Dont think the sd card sockets would last long with that sort of punishment.

Better off giving the students flash drives to plug in a hub.

dknute
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 11:42 pm

If you doubt SD socket can take it then why are you advocating micro SD?

pygmy_giant
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 11:46 pm

Because it can be bolted shut inside the case so that nobody can remove it.

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redhawk
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Re: Naive question: Why have an sd card slot at all?

Thu Nov 29, 2012 11:51 pm

I wouldn't mind soldering a Micro-SD holder instead of the SD card holder but I can't find any sold via eBay.

Could anyone recommend a site that sells them (preferably the spring loaded type)??

Richard S.


cnt
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Re: Naive question: Why have an sd card slot at all?

Fri Nov 30, 2012 7:06 am

redhawk wrote:I wouldn't mind soldering a Micro-SD holder instead of the SD card holder but I can't find any sold via eBay.

Could anyone recommend a site that sells them (preferably the spring loaded type)??

Richard S.
adafruit has an awesome "short" adapter so the card doesn't stick out. Their idea is super simple and works great.
https://www.adafruit.com/products/966

adlambert

Re: Naive question: Why have an sd card slot at all?

Fri Nov 30, 2012 8:57 am

cnt wrote:
redhawk wrote:I wouldn't mind soldering a Micro-SD holder instead of the SD card holder but I can't find any sold via eBay.

Could anyone recommend a site that sells them (preferably the spring loaded type)??

Richard S.
adafruit has an awesome "short" adapter so the card doesn't stick out. Their idea is super simple and works great.
https://www.adafruit.com/products/966
ModMyPi also do one which I use and I am buying more for all my pis:

https://www.modmypi.com/shop/sd-cards-a ... rd-adaptor

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tedhale
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Re: Naive question: Why have an sd card slot at all?

Fri Nov 30, 2012 9:48 am

cnt wrote:
redhawk wrote:I wouldn't mind soldering a Micro-SD holder instead of the SD card holder but I can't find any sold via eBay.

Could anyone recommend a site that sells them (preferably the spring loaded type)??

Richard S.
adafruit has an awesome "short" adapter so the card doesn't stick out. Their idea is super simple and works great.
https://www.adafruit.com/products/966
I got one to try out, since it seemed like a good idea. The uSD adapter is soldered on top of an SD card connector (as apposed to being built inside of it.)
This makes the adapter too fat to fit in any of my cases. The slot is only big enough for a normal SD card to fit through.

Just a warning, since it didn't "work great" for me. This also won't work for a lot of other people.
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Re: Naive question: Why have an sd card slot at all?

Fri Nov 30, 2012 11:15 am

The SoC requires an SD card to boot from.

uSD sockets are much more fragile that normal SD sockets, the cards are smaller and easier to use, and at the time of design, were relatively uncommon. Also you can get adapters to SD from uSD, but not the other way round, for obvious reasons! My personal preference is for SD, I find the uSD's just too easy to drop and lose.
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pygmy_giant
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Re: Naive question: Why have an sd card slot at all?

Fri Nov 30, 2012 11:15 pm

I agree, but I worry about these easy to remove cards in a classroom setting. Any teacher would need to have a strong discipline regime in place to make sure that they didn't end up in little johnny's MP3 player or swiped out of the Pi being used by the freckly girl with braces who everyone picks on as a joke.

I think the ability to tuck the card inside a case out of temptation's way could reduce distraction in lessons and teacher's stress levels, which could make the Pi more appealing to the educational market.

Perhaps that is more an argument for an educational case/adapter combo than changing the board.

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Gert van Loo
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Re: Naive question: Why have an sd card slot at all?

Fri Nov 30, 2012 11:30 pm

Somebody told me micro SD cards are not allowed in schools or need special precautions because of their size.
Probably because they fall in the category of 'everything small enough that it can be swallowed and obstruct the airway'.

By the way: The BCM2835 CAN boot from USB but NOT trough the LAN chip. Only from a USB directly connected (e.g. model A).
The chips on the Raspberry-Pi are just not programmed to do that.

Joe Schmoe
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Re: Naive question: Why have an sd card slot at all?

Sat Dec 01, 2012 12:47 am

Gert van Loo wrote:By the way: The BCM2835 CAN boot from USB but NOT trough the LAN chip. Only from a USB directly connected (e.g. model A).
The chips on the Raspberry-Pi are just not programmed to do that.
That's very interesting. You are saying that the Model A could boot from USB - but only if the USB drive is directly connected to the (one and only) USB port on the board.

So, no keyboard or mouse...
And some folks need to stop being fanboys and see the forest behind the trees.

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redhawk
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Re: Naive question: Why have an sd card slot at all?

Sun Dec 02, 2012 11:57 am

As I understand he means the BCM2835 has USB bootable capability but it's not coded into the PI to do so.
Even if USB booting was possible there's no way to connect a keyboard, mouse and LAN port since this would require a hub and as already stated you cannot perform USB booting via a hub.

Unless the PI has 2 USB controllers (which will never happen) USB booting is simply not possible.

Richard S.

Twinkletoes
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Re: Naive question: Why have an sd card slot at all?

Sun Dec 02, 2012 12:09 pm

Regarding classrooms, if little Johnny deletes the OS from his card, and Susan comes in to the next class, she's using her own SD card, so Johnny hasn't caused her any problems. Cue discussion about the lack of IT support in schools to rebuild PC hard drives between lessons...

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Re: Naive question: Why have an sd card slot at all?

Sun Dec 02, 2012 5:12 pm

I think the "SoC can only boot from an SD card" is slightly wrong.

The SoC can only boot from flash memory connected to its bus that is built for that purpose. In a phone (or a set-top box), that would almost certainly be on-board flash. In the case of the Pi, there is no on-board flash...it's all on the removable card.

There is at least one similar tiny, single board computer (though not identical with respect to SoC) that has enough flash on it's PCB to boot from. For the Pi's intended use, this is less convenient.

(We need a generic term for "Pi like" computers... Pico-computers, perhaps?)

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