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ARM Cortex-M3 for LM3S (Texas Instruments)

Vector table location

The bootloader itself occupies the location where normally the user program's vector table is located. The vector table in the user program therefore needs to be relocated. The bootloader implementation is such that the user program's vector table must be located at address 0x00002000 instead of the standard 0x00000000.

The vector table relocation can be achieved by changing the ROM start address in the linker descriptor file. Refer to the demo program for an example.

Note that the space needed by the bootloader could be different than 0x2000 (8 kbyte), depending on the selected communication interface and compiler. The demo programs are working examples that you can refer to, to determine the exact vector table relocation.

Because the bootloader remaps the base address of the interrupt vector table, make sure that your program's initialization routine does not set it back to its original l ocation.

Checksum location

A 32-bit checksum value is programmed by the bootloader at the end of a programming session. Upon startup, the bootloader verifies the correctness of this checksum to determine if a valid user program is present and can be started.

The bootloader programs this value at the end of the user program's vector table. Its exact location is configured by macro BOOT_FLASH_VECTOR_TABLE_CS_OFFSET in flash.c. The vector table size can vary based on the microcontroller derivative you are using. Please verify that the default value of BOOT_FLASH_VECTOR_TABLE_CS_OFFSET is correct. If not, you can simply override the value by adding the macro with the correct value to the blt_conf.h configuration header file.

The user program must reserve space for this 32-bit checksum value. To reserve this space, simply enter one extra dummy entry into the interrupt vector table. This reserves space for the checksum at memory address: flash base address + flash reserved for bootloader + FLASH_VECTOR_TABLE_CS_OFFSET. Refer to the demo program for an example.

Stackpointer initialization

On the ARM Cortex-M3 the first entry into the vector table holds the initialization value for the stackpointer, which is automatically loaded into the CPU's stackpointer register upon reset.

After reset, the bootloader gets started first and, if a valid user program is present, it is the bootloaders responsibility to start the user program. This unfortunately means that the automatic initialization of the CPU's stackpointer register does not work for the user program. The user program therefore needs to explicitly set the initial value of the CPU's stackpointer register in the reset interrupt service routine. Refer to the demo program for an example.

manual/ports/armcm3_lm3s.txt · Last modified: 2022/11/07 11:30 by voorburg