Repurposing Disabled Peripheral Pins as GPIOs on Verdin AM62 – Boot Issue

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Hello Uday,

Great question! GPIO numbering in Linux, especially on platforms like the Verdin AM62, can be confusing at first. Let’s clarify how to map the SD card pins (after you have repurposed them to GPIO via device tree) to the Linux GPIO numbers you actually use with tools like gpioinfo or libgpiod.

1. Understanding GPIO Chip and Line Numbering

  • GPIO Chip:
    In Linux, GPIOs are organized into “chips” (controllers), each exposed as /dev/gpiochip0, /dev/gpiochip1, etc.
    On Verdin AM62, the main GPIOs are usually under /dev/gpiochip0, but you can check on your running system with:

    gpiodetect

  • GPIO Line:
    Each chip controls a range of lines (pins), and lines are 0-based indexed. For example, &main_gpio0 78 refers to line 78 of the first GPIO controller.

  • Combined Legacy Numbering:
    Some older guides refer to a “global number” (chip * N + offset). Modern tools use (chip, offset).

2. Mapping SD1 Pins to GPIO in Device Tree and Linux

Your repurposed SD1 pins (from the AM62x datasheet) are, for example:

SODIMM Pin SoC Ball GPIO Function Linux DT Number
SODIMM 74 MMC1_CMD GPIO0_78 78
SODIMM 78 MMC1_CLK GPIO0_77 77
SODIMM 80 MMC1_DAT0 GPIO0_76 76
SODIMM 82 MMC1_DAT1 GPIO0_75 75
SODIMM 70 MMC1_DAT2 GPIO0_74 74
SODIMM 72 MMC1_DAT3 GPIO0_73 73

In the device tree, you reference them as:

gpios = <&main_gpio0 78 GPIO_ACTIVE_HIGH>;

This means: chip = &main_gpio0, line = 78.

3. Seeing the Mapping in Linux

  • After boot, run:
    gpiodetect
gpioinfo
  • You will see something like:
    gpiochip0 - 192 lines:
  ...
  line  73:      unused   "SD1_DAT3"
  line  74:      unused   "SD1_DAT2"
  line  75:      unused   "SD1_DAT1"
  line  76:      unused   "SD1_DAT0"
  line  77:      unused   "SD1_CLK"
  line  78:      unused   "SD1_CMD"
  ...

The numbers match what you used in your Device Tree overlay.
You then control, for example, MMC1_CMD as GPIO 78 of gpiochip0 (i.e., /dev/gpiochip0, line 78).

4. Summary Table (for easy reference)

Pin Name SODIMM AM62 Ball Linux Reference (DT/gpioinfo)
MMC1_CMD 74 A21 78
MMC1_CLK 78 B22 77
MMC1_DAT0 80 A22 76
MMC1_DAT1 82 B21 75
MMC1_DAT2 70 C21 74
MMC1_DAT3 72 D22 73

5. References and Next Steps

In summary: Use the GPIO line number from the device tree (as above), it matches what you’ll see in gpioinfo under gpiochip0. There is no “mapping math” needed for AM62—the Linux line is the same as the SoC GPIO number in the device tree!

If you have further doubts or want help checking your current gpioinfo output, just paste it here.

Best regards,
Toradex AI Assistant.