mhamam
Posts: 2
Joined: Tue Apr 30, 2019 4:08 pm

SD card reader question

Tue Apr 30, 2019 4:11 pm

Hi,
Does anybody know how the SD card is connected the the CPU? I know in PCs it is on a USB bus, in macs it's on PCIe, but I can find any information on the Raspberry.
Thanks!

PhilE
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 2374
Joined: Mon Sep 29, 2014 1:07 pm
Location: Cambridge

Re: SD card reader question

Tue Apr 30, 2019 5:53 pm

The family of SoCs we use includes two MMC interfaces directly addressable by the processor (i.e. memory mapped). One is used for the SD card or eMMC, while the other, SDIO-capable interface is used to connect the WiFi chip (if there is one).

mhamam
Posts: 2
Joined: Tue Apr 30, 2019 4:08 pm

Re: SD card reader question

Fri May 03, 2019 10:17 am

Thank you. How does it compared in terms of speed and power consumption to a PCIe interface? I'm trying to asses whether for my application using a raspberry would be better (faster and quieter in terms of electrical noise due to PS) than a NUC with the sd card reader on the pcie.

PhilE
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 2374
Joined: Mon Sep 29, 2014 1:07 pm
Location: Cambridge

Re: SD card reader question

Fri May 03, 2019 10:47 am

The SD interfaces are limited to single data rate - 4 bits at 50MHz = 25MB/s theoretical maximum. In practise you lose throughput due to response latencies, but you can overclock the interfaces a bit - many cards will run happily at 80+MHz, but it is technically out of spec.

User avatar
rpdom
Posts: 15194
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: SD card reader question

Fri May 03, 2019 11:56 am

The best way to assess the suitability of the Pi is to buy one and try it. It's not exactly expensive, even when you include the cost of the card and official PSU. Monitors, keyboard and mice (if required) are standard, so pull them off a PC and use those.

W. H. Heydt
Posts: 10884
Joined: Fri Mar 09, 2012 7:36 pm
Location: Vallejo, CA (US)

Re: SD card reader question

Fri May 03, 2019 3:07 pm

rpdom wrote:
Fri May 03, 2019 11:56 am
The best way to assess the suitability of the Pi is to buy one and try it. It's not exactly expensive, even when you include the cost of the card and official PSU. Monitors, keyboard and mice (if required) are standard, so pull them off a PC and use those.
Since this thread is involved with the CM3, one should include the developer carrier board (CMIO3+). Fortunately, even those are a lot cheaper than they used to be and for testing in a commercial environment, the cost is probably trivial. (And, in that context, one needs to remember that the interface is SD card OR eMMC, not both, so the OP should get himself a CM3L/CM3L+ specifically.)

User avatar
rpdom
Posts: 15194
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: SD card reader question

Fri May 03, 2019 3:23 pm

W. H. Heydt wrote:
Fri May 03, 2019 3:07 pm
rpdom wrote:
Fri May 03, 2019 11:56 am
The best way to assess the suitability of the Pi is to buy one and try it. It's not exactly expensive, even when you include the cost of the card and official PSU. Monitors, keyboard and mice (if required) are standard, so pull them off a PC and use those.
Since this thread is involved with the CM3, one should include the developer carrier board (CMIO3+). Fortunately, even those are a lot cheaper than they used to be and for testing in a commercial environment, the cost is probably trivial. (And, in that context, one needs to remember that the interface is SD card OR eMMC, not both, so the OP should get himself a CM3L/CM3L+ specifically.)
I missed the fact that it was about the CM3 when I replied. Actually, on re-reading, I can't see any mention of the CM other than the forum it was posted in.

However the SDIO interface will still be available on the CM3/CMIO, which ever model of CM is chosen.

Return to “Compute Module”