eMMC module

techny4287
Posts: 1
Joined: Thu Mar 12, 2020 6:41 pm

Re: eMMC module

Postby techny4287 » Thu Mar 12, 2020 7:00 pm

I know this topic has been dead for almost a year now, but im trying to figure out if the same sd -> emmc 5.x is faster than a uch class I/III sd card. After reviewing the 4.0 esp-idf the speed limitation of the smmc library is both the number of lines and frequency from the clock divider. So, my question is... How can a emmc chip write/read faster if the limitiations of the read and write speed libary do exceed 8 lines and 80k frequency? A Samsung evo sd card maximum clock speed is 100MHz.

marchingband
Posts: 30
Joined: Wed Mar 13, 2019 9:20 pm

Re: eMMC module

Postby marchingband » Wed Apr 08, 2020 8:05 am

my understanding is that there is a latency overhead with both read and write operations associated with the 'S' for secure in SD ... My speed tests support this.

ESP_igrr
Posts: 1656
Joined: Tue Dec 01, 2015 8:37 am

Re: eMMC module

Postby ESP_igrr » Wed Apr 08, 2020 11:24 am

Hi techny4287,

the current limitations for SD and eMMC from the driver perspective are as follows (docs):

Default Speed (20 MHz), 4-line/1-line (with SD cards), and 8-line (with 3.3 V eMMC)
High Speed (40 MHz), 4-line/1-line (with SD cards), and 8-line (with 3.3 V eMMC)
High Speed DDR (40 MHz), 4-line (with 3.3 V eMMC)

(Note that we use 20MHz/40MHz for default/high speed modes rather than 25/50 suggested by the standard, since the ESP32 doesn't have a way to generate precisely 25 and 50MHz clocks for SD peripheral, source frequency being 160MHz).

So the highest theoretical throughput with and SD card is 40MHz * 4 bit = 20 MBps. Due to some software overhead, you will typically get up to around 16-17MBps read throughput with large buffers (64kB) and fast SD cards.

With eMMC, the highest theoretical throughput is 40 MHz * 8 lines (or 40 MHz * 4 lines * 2 (DDR)) = 40MBps. Practical speeds (again, with large 64kB buffers) are around 23-24MBps at the default optimization level.

franzhoepfinger
Posts: 3
Joined: Wed Mar 04, 2020 7:14 pm

Re: eMMC module

Postby franzhoepfinger » Sun Apr 19, 2020 7:56 pm

did you solve this ?

emmc only works in Emmc Mode.

SDcard works in SPI mode and emmc mode.

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