Investigation on MMCXFR vs SDXFR in STM32_SDMMC_CLKCR_* setting

junwoo0914 · August 9, 2023, 2:23am

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While reading the ‘board.h’ of the Matek H743 Mini board, I stumbled upon this:

PX4/PX4-Autopilot/blob/ae70f9c640c38d4bfbf32cbb37c756be55f08996/boards/matek/h743-mini/nuttx-config/include/board.h#L291-L300


      
          #if defined(CONFIG_STM32H7_SDMMC_XDMA) || defined(CONFIG_STM32H7_SDMMC_IDMA)
          #  define STM32_SDMMC_MMCXFR_CLKDIV   (6 << STM32_SDMMC_CLKCR_CLKDIV_SHIFT)
          #else
          #  define STM32_SDMMC_MMCXFR_CLKDIV   (100 << STM32_SDMMC_CLKCR_CLKDIV_SHIFT)
          #endif
          #if defined(CONFIG_STM32H7_SDMMC_XDMA) || defined(CONFIG_STM32H7_SDMMC_IDMA)
          #  define STM32_SDMMC_SDXFR_CLKDIV    (6 << STM32_SDMMC_CLKCR_CLKDIV_SHIFT)
          #else
          #  define STM32_SDMMC_SDXFR_CLKDIV    (100 << STM32_SDMMC_CLKCR_CLKDIV_SHIFT)
          #endif

After noticing that the two different seemingly similar macro gets sent independently, I wondered what the difference between these two macros are:

STM32_SDMMC_MMCXFR_CLKDIV
STM32_SDMMC_SDXFR_CLKDIV

Findings

NuttX Macros

The CLKDIV macros above are used in the NuttX in the following part:

github.com

micro-ROS/NuttX/blob/d4b717b73bdaaa8039dd4d609974595c0a912928/arch/arm/src/stm32f7/stm32_sdmmc.c#L176-L189


      
          /* Mode dependent settings.  These depend on clock divisor settings that must
           * be defined in the board-specific board.h header file: STM32_SDMMC_INIT_CLKDIV,
           * STM32_SDMMC_MMCXFR_CLKDIV, and STM32_SDMMC_SDXFR_CLKDIV.
           */
          
          #define STM32_CLCKCR_INIT           (STM32_SDMMC_INIT_CLKDIV      | \
                                               STM32_SDMMC_CLKCR_EDGE | \
                                               STM32_SDMMC_CLKCR_WIDBUS_D1)
          #define STM32_SDMMC_CLKCR_MMCXFR    (STM32_SDMMC_MMCXFR_CLKDIV    | \
                                               STM32_SDMMC_CLKCR_EDGE | \
                                               STM32_SDMMC_CLKCR_WIDBUS_D1)
          #define STM32_SDMMC_CLCKR_SDXFR     (STM32_SDMMC_SDXFR_CLKDIV     | \
                                               STM32_SDMMC_CLKCR_EDGE | \
                                               STM32_SDMMC_CLKCR_WIDBUS_D1)

This then gets used in the stm32_clock() function for following cases below.

Enabling MMC normal clocking:

github.com

micro-ROS/NuttX/blob/d4b717b73bdaaa8039dd4d609974595c0a912928/arch/arm/src/stm32f7/stm32_sdmmc.c#L2046-L2050


      
          /* Enable in MMC normal operation clocking */
          
          case CLOCK_MMC_TRANSFER:
            clckr = (STM32_SDMMC_CLKCR_MMCXFR | STM32_SDMMC_CLKCR_CLKEN);
            break;

SD card normal operation clocking:

github.com

micro-ROS/NuttX/blob/d4b717b73bdaaa8039dd4d609974595c0a912928/arch/arm/src/stm32f7/stm32_sdmmc.c#L2063-L2068


      
            /* SD normal operation clocking (narrow 1-bit mode) */
          
            case CLOCK_SD_TRANSFER_1BIT:
              clckr = (STM32_SDMMC_CLCKR_SDXFR | STM32_SDMMC_CLKCR_CLKEN);
              break;
          }

And these cases seem to be defined in this enum for SDIO clock:

github.com

micro-ROS/NuttX/blob/d4b717b73bdaaa8039dd4d609974595c0a912928/include/nuttx/sdio.h#L871-L880


      
          /* Various clocking used by the SDIO driver */
          
          enum sdio_clock_e
          {
            CLOCK_SDIO_DISABLED = 0, /* Clock is disabled */
            CLOCK_IDMODE,            /* Initial ID mode clocking (<400KHz) */
            CLOCK_MMC_TRANSFER,      /* MMC normal operation clocking */
            CLOCK_SD_TRANSFER_1BIT,  /* SD normal operation clocking (narrow 1-bit mode) */
            CLOCK_SD_TRANSFER_4BIT   /* SD normal operation clocking (wide 4-bit mode) */
          };

SDIO, SDMMC

So it seems that SD card and MMC cards have a different specification, and hence they are called that way.

So for now it just seems like we need to define both cases in the board.h file. However, I am still wondering what the XFR stands for, and why we truly need to set MMC and SD cases separately like it is currently done

Investigation on MMCXFR vs SDXFR in STM32_SDMMC_CLKCR_* setting

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Findings

NuttX Macros

SDIO, SDMMC

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