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Z
Z_yamada
Visitor II
March 11, 2026
Question

About eMMC memory connection for STM32MP157AAB3

  • March 11, 2026
  • 1 reply
  • 94 views

Is it okay to use the following connection for eMMC memory on the STM32MP157AAB3?

Also, is it possible to set the eMMC memory to boot mode with this connection?

PB14:SDMMC2_D0

PB15:SDMMC2_D1

PB3:SDMMC2_D2

PB4:SDMMC2_D3

PB8:SDMMC2_D4

PB9:SDMMC2_D5

PC6:SDMMC2_D6

PC7:SDMMC2_D7

PC1:SDMMC2_CK

PG6:SDMMC2_CMD

1 reply

Z
ZAKARIA_C1
Technical Moderator
March 23, 2026

Hello @Z_yamada,

The official documentation specifies the following default pin assignments for connecting eMMC to SDMMC2 on the STM32MP157AAB3:

Signal STM32 Pin
SDMMC2_D0 PB14
SDMMC2_D1 PB15
SDMMC2_D2 PB3
SDMMC2_D3 PB4
SDMMC2_D4 PA8
SDMMC2_D5 PA9
SDMMC2_D6 PE5
SDMMC2_D7 PD3
SDMMC2_CK PE3
SDMMC2_CMD PG6

Your proposed connection uses PB8, PB9, PC6, PC7, and PC1 for D4-D7 and CK, which does not match the documented configuration. Therefore, it is not recommended to use your proposed pinout for eMMC on STM32MP157AAB3.

2. Boot Mode Support with eMMC

For eMMC boot mode, the STM32MP157AAB3 boot ROM expects the eMMC to be connected to SDMMC2 using the default pin assignments listed above.
The minimum set of signals required by the boot ROM during eMMC boot are these default pins.
Using a different pinout (alternate mapping) is not supported by default for boot unless you have specifically programmed the OTP (One-Time Programmable) memory to change the AFmux (alternate function multiplexing) configuration.

  • If the eMMC memory interface OTP field is not set (value is 0), the ROM code uses the default pin mapping.
  • If you have programmed the OTP fields for alternate mapping, the ROM code will use those settings. Otherwise, only the default mapping is supported for boot.

3. Alternate Pin Mapping for Boot

Alternate pin mapping for SDMMC2 (e.g., using PB8, PB9, PC6, PC7, PC1) is only possible if you have configured the appropriate OTP fields to inform the boot ROM of the new mapping. If you have not programmed these OTP fields, the boot ROM will not recognize the alternate mapping, and eMMC boot will fail.

Summary

  • Your proposed pinout is not recommended for eMMC connection on STM32MP157AAB3.
  • The boot ROM supports only the default pin mapping for eMMC boot unless you have programmed the OTP for alternate mapping.
  • For reliable boot and operation, use the default pin assignments as documented.


The proposed configuration is valid it matches the reference schematic used on AN5031 figure 37. eMMC™ connection example. Here is the full analysis.

SDMMC2 Pinout Validation

Each signal must use the correct Alternate Function (AF) in the device tree (TF-A, U-Boot, Linux):

Signal Pin AF Recommended Mode
SDMMC2_D0 PB14 AF9 Pull-Up, High Speed
SDMMC2_D1 PB15 AF9 Pull-Up, High Speed
SDMMC2_D2 PB3 AF9 Pull-Up, High Speed
SDMMC2_D3 PB4 AF9 Pull-Up, High Speed
SDMMC2_D4 PB8 AF10 Pull-Up, High Speed
SDMMC2_D5 PB9 AF11 :warning: Pull-Up, High Speed
SDMMC2_D6 PC6 AF10 Pull-Up, High Speed
SDMMC2_D7 PC7 AF10 Pull-Up, High Speed
SDMMC2_CK PC1 AF12 No Pull, High Speed
SDMMC2_CMD PG6 AF10 Pull-Up, High Speed

:warning: Watch out: PB9 uses AF11 (not AF10) for SDMMC2_D5 — this is a very common mistake in custom device trees.

eMMC Boot: Default ROM Configuration Issue

The STM32MP157 Boot ROM uses these default pins for eMMC boot over SDMMC2:

Signal Default ROM boot pin
SDMMC2_CK PE3 (AF9) ← differs from your design!
SDMMC2_CMD PG6 (AF10) :white_heavy_check_mark:
SDMMC2_D0 PB14 (AF9) :white_heavy_check_mark:

Your design uses PC1 for SDMMC2_CK instead of PE3. You must therefore program the OTP words to declare a non-default AFmux configuration.

Required OTP Configuration

OTP WORD3 — eMMC Interface Selection

OTP_WORD3[2:1] = 0b10 (value = 2) → SDMMC2 with non-default AFmux defined in OTP words:

OTP WORD3 : bit[2]=1, bit[1]=0 → contribution = 0x00000004

OTP WORDS 5–6 — Custom AFmux (minimum 3 pins for boot)

The Boot ROM only uses D0, CMD, and CK during the initial boot phase. Each OTP WORD holds 2 pin configurations (16 bits each) using the following format:

Bits[31:16] = pin1 : [31:28]=port | [27:24]=pin# | [23:20]=AF | [19:16]=mode Bits[15:0] = pin0 : [15:12]=port | [11:8]=pin# | [7:4]=AF | [3:0]=mode

Port codes: 1=A, 2=B, 3=C, 4=D, 5=E, 6=F, 7=G
Mode codes: 2=AF/NoPull/HighSpeed, 5=AF/PullUp/HighSpeed

OTP WORD5 → PC1 (SDMMC2_CK) + PG6 (SDMMC2_CMD):

pin0 = PC1, AF12, No Pull, High Speed → port=3, pin=1, af=0xC, mode=2 → 0x31C2 pin1 = PG6, AF10, Pull-Up, High Speed → port=7, pin=6, af=0xA, mode=5 → 0x76A5 OTP WORD5 = 0x76A531C2

OTP WORD6 → PB14 (SDMMC2_D0):

pin0 = PB14, AF9, Pull-Up, High Speed → port=2, pin=14 (0xE), af=0x9, mode=5 → 0x2E95 pin1 = unused → 0x0000 OTP WORD6 = 0x00002E95

BOOT Pins Hardware Level

To enable eMMC boot on STM32MP157, set the hardware boot pins as follows:

BOOT[2:0] = b010 → BOOT2=0, BOOT1=1, BOOT0=0



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