AM335x U-Boot User's Guide.pdf
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AM335x U-Boot User's Guide
1
AM335x U-Boot User's Guide
AM335x U-Boot User's Guide
Linux PSP
U-Boot
In AM335x the ROM code serves as the bootstrap loader, sometimes referred to as the Initial Program Loader (IPL)
or the Primary Program Loader (PPL). The booting is completed in two consecutive stages by U-Boot
[1]
binaries.
The binary for the 1st U-Boot stage is referred to as the Secondary Program Loader (SPL) or the MLO. The binary
for the 2nd U-Boot stage is simply referred to as U-Boot. SPL is a non-interactive loader and is a specially built
version of U-Boot. It is built concurrently when building U-Boot.
The ROM code can load the SPL image from any of the following devices
€€ Memory devices non XIP (NAND/SDMMC)
The image should have the Image header. The image header is of length 8 byte which has the load address(Entry
point) and the size of the image to be copied. RBL would copy the image, whose size is given by the length field in
the image header, from the device and loads into the internal memory address specified in the load address field of
Image header.
€€ Peripheral devices (UART)
RBL loads the image to the internal memory address 0x402f0400 and executes it. No Image Header present.
Two stage U-Boot design
This section gives an overview of the two stage U-Boot approach adopted for AM335X.
The size of the internal RAM in AM335X is 128KB out of which 18KB at the end is used by the ROM code. Also, 1
KB at the start (0x402f0000 - 0x402f0400) is secure and it cannot be accessed This places a limit of 109KB on the
size of the U-Boot binary which the ROM code can transfer to the internal RAM and use as an initial stack before
initialization of DRAM.
Since it is not possible to squeeze in all the functionality that is normally expected from U-Boot in < 110KB (after
setting aside some space for stack, heap etc) a two stage approach has been adopted. Initial stage initalize only the
required boot devices (NAND, MMC, I2C etc); 2nd full stage initall all other devices (ethernet, timers, clocks etc).
The 1st binary is generated MLO and the 2nd stage is generated as u-boot.img.
NOTE
*When using memory boot (NAND) a header needs to be attached to the SPL binary indicating the load address and
the size of the image. SPI boot additionally requires endian conversion before flashing the image.
€€ When using peripheral boot (UART) there can be no header as the load address is fixed.
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