IOT-GATE-iMX7 and SBC-IOT-iMX7: U-Boot: Firmware Overview
Contents
Introduction
This article describes the firmware for CL-SOM-iMX7 / IOT-GATE-iMX7 / SBC-IOT-iMX7. The firmware consists of two components: Secondary Program Loader (SPL) and U-Boot. SPL is the bootstrap utility invoked by the CPU internal boot ROM code of the NXP i.MX7 SoC. The i.MX7 boot ROM tries to boot U-Boot from either SD card, or SPI flash, as determined by the hardware boot selector (internal jumper E2). SPL performs minimal hardware initializations and loads U-Boot. U-Boot initializes hardware modules necessary for system boot and loads the operating system.
SPL
SPL is responsible for initial clock setup, DDR initialization and boot media controller initialization. It is loaded into the CPU internal SRAM immediately after system reset. After basic hardware setup, SPL loads U-Boot bootloader into the system RAM and transfers control to the U-Boot bootloader.
U-Boot
U-Boot is the bootloader used on the device. It allows flexible selection of operating system boot modes, provides scripting facilities and command line interface through a serial port. U-Boot allows booting operating system from the on-board eMMC flash, removable storage (e.g. USB drive, SD/MMC card), or the network (TFTP/NFS server).
Please consult U-Boot: Quick reference and U-Boot project documentation for U-Boot features and commands description.
U-Boot environment
U-Boot environment resides on the SPI flash, regardless of the boot source. The environment resides at offset 0xC0000 from the start of SPI flash, and its size is 8KiB.
Boot sources
The device can boot from either a micro SD card, or the on-board SPI flash.
The following sections describe the layout of each storage type.
Default storage device mapping
| Offset | Size | Description |
|---|---|---|
| 0x0 (0KB) | 0xc0000 (768KB) | U-Boot |
| 0xc0000 (768KB) | 0x40000 (256KB) | U-Boot environment |
| 0x100000 (1MB) | Remaining SPI flash space | Splash image |
| Offset | Size | Description |
|---|---|---|
| 0x0 (0KB) | 0x980000 (9.5MB) | Kernel (zImage) |
| 0x980000 (9.5MB) | 0x80000 (512KB) | Device Tree blob (.dtb) |
| 0xa00000 (10MB) | Remaining NAND flash space | Root filesystem |
| Offset | Size | Description |
|---|---|---|
| 0x0 (0KB) | 0x400 (1KB) | Reserved/MBR |
| 0x400 (1KB) | 0xffc00 (999KB) | U-Boot |
| 0x100000 (1MB) | Remaining SD-card space | Partitions/Filesystems/Data |
