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Max SD Card Speed

Posted: Tue Dec 24, 2013 10:34 pm
by munklefish
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!!!

Re: Max SD Card Speed

Posted: Tue Dec 24, 2013 11:06 pm
by Dilligaf
You'll find the sd maxes out at 20 to 25 http://elinux.org/RPi_SD_cards#SD_card_performance

Re: Max SD Card Speed

Posted: Tue Dec 24, 2013 11:34 pm
by munklefish
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.

Re: Max SD Card Speed

Posted: Wed Dec 25, 2013 3:41 am
by rpdom
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.

Re: Max SD Card Speed

Posted: Wed Dec 25, 2013 1:02 pm
by munklefish
OK thanks.

Any idea how the clock speed relates to the sdcard speeds?

Re: Max SD Card Speed

Posted: Thu Dec 26, 2013 5:14 pm
by munklefish
bump

Re: Max SD Card Speed

Posted: Thu Dec 26, 2013 5:43 pm
by jdb
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.

Re: Max SD Card Speed

Posted: Thu Dec 26, 2013 8:00 pm
by munklefish
Thanks for the great response.