How exFAT + FAT32 Partition Configuration Enables Large SD Cards on ATOMCam

ATOMCam devices use a dual-partition scheme—FAT32 for the bootloader and exFAT for data—to bypass the 2 TB filesystem limit and support very large SD cards.

The atomcam_tools project solves a critical hardware limitation in ATOMCam firmware: the boot ROM (u‑boot) can only read FATFS (FAT12/16/32) partitions. To support modern SD cards larger than 2 TB—where FAT32 is impossible—the tools implement an exFAT + FAT32 partition configuration that reserves a small FAT32 slice for boot files while using exFAT for the remaining capacity.

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Why ATOMCam Needs a Dual-Partition Layout

The camera’s bootloader is hardcoded to recognize only FATFS volumes. When the device powers on, u‑boot searches the SD card for factory boot images (e.g., factory_t31_…) and expects to find them on a FAT32-formatted partition.

However, FAT32 has a maximum volume size of 2 TB (and practical limits near 16 TB with non-standard cluster sizes). Modern SD cards now exceed this threshold. The solution is to split the card into two partitions:

1. A small FAT32 partition for the bootloader.
2. A large exFAT partition for the tools and data, leveraging exFAT’s theoretical limit of 128 PiB.

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How the exFAT + FAT32 Partition Configuration Works

The atomcam_tools repository implements this layout through kernel patches and library overrides that ensure the camera boots correctly while accessing the full SD card capacity.

The FAT32 Boot Partition

The first partition must be formatted as FAT32 and contain the factory boot files required by u‑boot. In libcallback/mmc_mount.c, the tools override the SDK’s mount behavior to ensure this partition is presented as /media/mmc, the path the camera firmware expects.

The code intercepts local_sdk_device_open and related calls to prevent the firmware from remounting or reformatting the card, preserving the dual-partition layout.

The exFAT Data Partition

The second partition holds the atomcam_tools runtime: rootfs_hack.squashfs, SSH keys (authorized_keys), hostname files, and user data. Because the Linux kernel in the buildroot environment includes the exFAT driver (added via patches/kernel/linux-fs-exfat.patch), the system mounts this partition natively.

This configuration removes the 2 TB ceiling. Users can insert 4 TB, 8 TB, or larger cards, with the only practical limit being the SD card hardware itself.

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Technical Implementation in atomcam_tools

The repository contains specific files that enforce the exFAT + FAT32 partition configuration and prevent the camera from breaking it.

Kernel exFAT Support

The file patches/kernel/linux-fs-exfat.patch adds the exFAT filesystem driver to the buildroot kernel. Without this patch, the Linux runtime would not recognize the second partition, making the tools inaccessible.

Mount Override Mechanisms

In libcallback/mmc_mount.c, the project overrides local_sdk_device_open to redirect the SDK’s mount attempts. This ensures the FAT32 boot partition remains mounted at /media/mmc and the camera does not attempt to unmount or reformat it.

Format Protection

The file libcallback/mmc_format.c stubs out local_sdk_device_mmc_format. This prevents the camera’s stock firmware from reformatting the SD card into a single FAT32 partition, which would destroy the exFAT data partition and the tools installed there.

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Setting Up the Dual-Partition SD Card

To use the exFAT + FAT32 partition configuration, prepare the SD card on a Linux PC before inserting it into the camera.

1. Create the Partition Layout

Use sfdisk to create a GPT partition table with a small FAT32 boot partition and a large exFAT data partition:

# Replace /dev/sdX with your actual SD card device

sudo sfdisk /dev/sdX <<EOF
label: gpt
size=64M,type=ef00,name="BOOT"
size=,type=0700,name="TOOLS"
EOF

2. Format the Partitions

Format the first partition as FAT32 for the bootloader and the second as exFAT for the tools:

# Format boot partition as FAT32

sudo mkfs.vfat -F 32 /dev/sdX1

# Format data partition as exFAT (requires exfatprogs)

sudo mkfs.exfat /dev/sdX2

3. Install the Boot Files

Copy the factory boot images required by u‑boot to the FAT32 partition:

mkdir /tmp/boot
sudo mount /dev/sdX1 /tmp/boot
sudo cp factory_t31_* /tmp/boot/
sudo umount /tmp/boot

4. Install atomcam_tools

Extract the tools release package onto the exFAT partition:

mkdir /tmp/tools
sudo mount -t exfat /dev/sdX2 /tmp/tools
unzip atomcam_tools.zip -d /tmp/tools
sudo umount /tmp/tools

Insert the card into the ATOMCam. The device boots from the FAT32 partition and automatically accesses the exFAT partition for all tool operations, supporting SD cards well beyond 2 TB.

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Summary

• Hardware limitation: ATOMCam’s u‑boot bootloader only recognizes FATFS partitions, requiring a FAT32 boot slice.
• Capacity solution: The exFAT + FAT32 partition configuration uses a small FAT32 partition for boot files and a large exFAT partition for data, bypassing the 2 TB FAT32 limit.
• Kernel support: The patches/kernel/linux-fs-exfat.patch adds exFAT drivers to the buildroot kernel.
• Protection mechanisms: libcallback/mmc_mount.c and libcallback/mmc_format.c override SDK functions to prevent the camera from reformatting or remounting the card incorrectly.
• User setup: Partitions must be created on a PC using tools like sfdisk and mkfs.exfat before the card is inserted into the camera.

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Frequently Asked Questions

Can I use a single FAT32 partition for SD cards smaller than 32 GB?

Yes, for cards under 32 GB you can use a single FAT32 partition. However, the exFAT + FAT32 partition configuration is still recommended because it prevents the camera’s stock firmware from reformatting the card and deleting your tools. The override functions in libcallback/mmc_format.c specifically protect against this behavior regardless of card size.

Why doesn't the camera automatically format the exFAT partition?

The camera’s native SDK includes functions like local_sdk_device_mmc_format that would normally reformat the SD card into a single FAT32 volume. The atomcam_tools project intercepts these calls in libcallback/mmc_format.c and stubs them out, returning success without performing any format operation. This preserves the dual-partition layout you created on your PC.

What is the maximum SD card size supported with this configuration?

The exFAT + FAT32 partition configuration theoretically supports SD cards up to 128 PiB (pebibytes), which is the architectural limit of the exFAT filesystem. In practice, the limit is determined by the SD card hardware and the Linux kernel’s block device handling. The system has been tested with multi-terabyte cards, far exceeding the 2 TB limit imposed by FAT32.

Is the exFAT driver included in the stock ATOMCam firmware?

No, the stock firmware does not include exFAT support. The atomcam_tools project adds this capability by applying patches/kernel/linux-fs-exfat.patch to the buildroot kernel during compilation. This patch introduces the exFAT filesystem driver, allowing the Linux runtime to mount and read the second partition after the bootloader has handed control to the kernel.

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