Boot Flow

The GSRD boot flow includes the following stages:

1. BootROM
2. Preloader
3. U-Boot
4. Linux

The BootROM and the Preloader stages are needed for all the applications in which the Cyclone V or Arria V SoC are used. They are shown in blue in the above figure. The U-boot and Linux are used by the GSRD, but a custom application may have the Preloader loading a bare-metal application directly for example.

The following table presents a short description of the different boot stages:

Stage	Description
BootROM	Performs minimal configuration and loads Preloader into 64KB OCRAM
Preloader	Configures clocking, IOCSR, pinmuxing, DDRAM and loads U-boot into DDRAM
U-boot	Configures FPGA, loads Linux kernel
Linux	Runs the end application

For more information about SoC FPGA booting please refer to the following documents:

• Cyclone V Booting and Configuration
• Arria V Booting and Configuration

BootROM

Upon power on, the processor executes the BootROM code which resides inside on-chip ROM.

The main role of the BootROM is to initialize all required hardware components in order to boot up the next stage boot software, the Preloader.

The BootROM fetches the Preloader binaries from serial NOR flash, NAND flash or SD/MMC flash memory based on the BSEL pins.

Instead of running the Preloader from Flash, the BootROM can:

• Run Preloader from FPGA memory,
• Run code from RAM - this option can be used only on Warm reset

The BootROM uses the CSEL pins to determine the clocks to be used.

Preloader

The Preloader used by the GSRD is based on the SPL (Second Program Loader) framework, which is part of the U-Boot. Because of this, the Preloader and U-Boot share most of the source code, such as drivers. This gives the Preloader access to the variety of well tested drivers available for U-Boot.

The main functions of the Preloader are:

• Initializing the SDRAM interface including calibration and SDRAM’s PLL configuration
• Fetching the subsequent bootloader image from NAND, SD/MMC or serial NOR flash.
• Storing the boot image to SDRAM and pass control over to subsequent bootloader

In addition to the above, the Preloader:

• Configures the SOCFPGA HPS I/Os through Scan Manager
• Configures pin muxing through System Manager (configurable by Qsys)
• Re-configures the PLLs based on user generated settings (configurable by Preloader Generator)
• Releases all/certain peripherals from reset through Reset Manager (configurable by Qsys)
• Initializes the required flash controller (either NAND, SD/MMC or QSPI) based on boot option

Bootloader

The bootloader used by GSRD is U-Boot.

U-Boot is an open source, primary boot loader used in embedded devices. U-Boot is licensed under GPL as it is an open source framework.

For more details about U-Boot, please refer to http://www.denx.de/wiki/U-Boot/Documentation.

The main functions of U-Boot are:

• Setting up the OS environment
• Fetching the OS image from NAND, SD/MMC, serial NOR flash, Ethernet through TFTP, USB mass storage
• Storing the boot image to SDRAM and pass control over to subsequent bootloader
• Providing a console that can be used for user operations such as modifying the Device Tree Blob (DTB) and boot arguments.

Booting Examples

• Booting from QSPI Flash
• Booting from FPGA