munklefish
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Max SD Card Speed

Tue Dec 24, 2013 10:34 pm

Hi,

Whats the fastest read/write speed of and sd-card can the pi theoretically handle on the sdcard slot?

Is this controlled by the usb, and thus limited to 40ish mb speeds?

Any ideas/facts would be appreciated.

Thanks & MERRY CHRISTMAS!!!
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Dilligaf
Posts: 283
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Re: Max SD Card Speed

Tue Dec 24, 2013 11:06 pm

You'll find the sd maxes out at 20 to 25 http://elinux.org/RPi_SD_cards#SD_card_performance

munklefish
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Re: Max SD Card Speed

Tue Dec 24, 2013 11:34 pm

Thanks for the link.

I meant from a technical perspective though. I'm wondering what the theoretical max speed is. From what I know of the pi things like the land are controlled by the USB so are essentially USB devices meaning the USB max bus speed is the limiting factor.

If that's the same for the SD card then speeds could never go beyond the max speed as defined by USB 2.0.
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rpdom
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Re: Max SD Card Speed

Wed Dec 25, 2013 3:41 am

Only the LAN and USB ports are connected to the USB bus. Everything else, including the SD card interface, has its own connections to the GPU chip.

I don't know much about the SDMMC interface and how it is implemented, but I think one of the limiting factors is the clock speed.

munklefish
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Re: Max SD Card Speed

Wed Dec 25, 2013 1:02 pm

OK thanks.

Any idea how the clock speed relates to the sdcard speeds?
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munklefish
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Re: Max SD Card Speed

Thu Dec 26, 2013 5:14 pm

bump
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jdb
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
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Re: Max SD Card Speed

Thu Dec 26, 2013 5:43 pm

The highest supported speed at a 3.3V IO interface is "HS" mode (SDHC spec 1.1). This is a 50MHz clock with SDR data signalling.

We have a 4-bit data connection to SD cards which implies a maximum data bus throughput of 23.8MiB/s. In reality, the inter-block gap times (card-dependent) and protocol turnaround times limit the useable throughput to a bit less than this.

Note that the SD host controller is directly connected to the internal chip bus.
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munklefish
Posts: 75
Joined: Tue Oct 02, 2012 5:35 pm
Location: Neumarkt in der Oberpfalz
Contact: Website

Re: Max SD Card Speed

Thu Dec 26, 2013 8:00 pm

Thanks for the great response.
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