eMMC access on STM32F7

[nickstoughton]

I'm trying to prototype a system where I have a NucleoF746ZG board attached to an eMMC (which has a FAT32 filesystem on it).

I started with the samples/subsys/fs/fat_fs sample, and setup my board overlay:

     &sdmmc1 {
         status = "okay";
         pinctrl-0 = <&sdmmc1_d0_pc8 &sdmmc1_d1_pc9
                 &sdmmc1_d2_pc10 &sdmmc1_d3_pc11
                 &sdmmc1_d4_pb8 &sdmmc1_d5_pb9
                 &sdmmc1_d6_pc6 &sdmmc1_d7_pc7
                 &sdmmc1_ck_pc12 &sdmmc1_cmd_pd2>;
     };

(I tried 1, 4 and 8 bit here, they all behave the same, which is not surprising since the failure occurs in setup when we are in 1-bit mode anyway)

My prj.conf has

CONFIG_DISK_ACCESS=y
CONFIG_LOG=y
CONFIG_FILE_SYSTEM=y
CONFIG_FAT_FILESYSTEM_ELM=y
CONFIG_PRINTK=y
CONFIG_MAIN_STACK_SIZE=2048
CONFIG_DISK_DRIVER_SDMMC=y
CONFIG_SDMMC_STM32=y
CONFIG_SDMMC_LOG_LEVEL_DBG=y
CONFIG_SHELL=y

(I added a shell for debug support).

Digging a bit more deeply at the generated files, the resulting full zephyr.dts has:

sdmmc1: sdmmc@40012c00 {
    compatible = "st,stm32-sdmmc";
    reg = < 0x40012c00 0x400 >;
    clocks = < &rcc 0x3 0x800 >;
    status = "okay";
    label = "SDMMC_1";
    pinctrl-0 = < &sdmmc1_d0_pc8 &sdmmc1_d1_pc9 &sdmmc1_d2_pc10 &sdmmc1_d3_pc11 &sdmmc1_d4_pb8 &sdmmc1_d5_pb9 &sdmmc1_d6_pc6 &sdmmc1_d7_pc7 &sdmmc1_ck_pc12 &sdmmc1_cmd_pd2 >;
	};

Out of the box, the sdmmc_stm32.c zephyr disk driver uses the underlying stm32f7xx_hal_sd.c driver, and the SD_PowerOn() function fails at the CMD8 stage., and the driver fails to find the operating voltage. All commands appear to fail with a timeout.

While investigating this, I realized that there was a second driver in the HAL, stm32f7xx_hal_mmc.c. The interface to this driver appears to be very similar to the SD driver, so I attempted to modify the Zephyr disk driver to use this driver instead.

Now we get a lot further! BUT, it still fails at a CMD8 to get Extended CSD (Card Support Details) with a timeout.

Anyone got any ideas?

--
Nick

[nickstoughton]

I found the answer ... and it is somewhat complex.

First, there is a difference (especially during device initialization) between the MMC (JEDEC) protocol and the SD protocol. The eMMC chips I am using are JEDEC compatible. So CMD3, which is used to assign the card Relative device Address, was setting the RCA to 0 (which is a reserved special value). Then we ask the command for its extended Card Specific Data (with CMD8) without selecting the card, so it refuses to respond. I am preparing patches for these issues.