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LPC1768 bootloader?

Topic last updated 05 Aug 2015, by Jonne Valola. 8 replies
Sam A
24 Apr 2013

Hello,

I am trying to plan out how it might be possible to write some sort of bootloader for the LPC1768. The idea is that I could put an updated binary on the SD card I have connected, and then use IAP to have the device reprogram itself.

The key blocker for me is figuring out how to make sure that the bootloader code does not itself get overwritten. I have ported my project to the Keil ARM MDK and have played around with the scatter file to place modules at different spots in the flash. What I would like to do is have the bootloader occupy the first couple blocks in flash and have the actual application use the rest. But I'm really unsure about how to go about loading 2 effectively self-contained programs in flash.

I suppose I could simply write the bootloader, flash that on the chip, and then rely on it to flash the application. But I'm under the impression that code has to be compiled with "knowledge" of where it will be placed in memory since it is possible absolute addressing may be used.

I'm also not sure at all about how to get the bootloader to jump to the application code - obviously I can't use a function call or even a goto if the modules are to be entirely self-contained.

If anyone has any insight into this it would be greatly appreciated! I can certainly help clarify my ideas as well, since I'm sure I haven't been as clear as necessary in my attempted explanation here.

Thanks very much for any help!

Athanassios Mavrogeorgiadis
24 Apr 2013

Hi, what I did for my bootloader is to put the entire bootloader on the last flash sector. Only the vector table of the bootloader is placed in the 0 address. Here are the addresses that I used:

bootloader on 0x050000-0x06FFFF application on 0x001000-0x050000 application vector table on 0x070000-0x077FFF bootloader vector table on 0x000000-0x000FFF

With this way your application fw doesn't need to be compiled specially. But you need the area 0x300-0xfff not to be used from your applicaton. I use the following code for this:

#include <absacc.h>

unsigned char const first_sector[0x1000-0x300] __at (0x00000300) =
{
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};

The only think that the bootload needs to do is to program the application vector on a different flash address (not 0x0000000) and call your application.

NVIC_SetVectorTable(NVIC_VectTab_FLASH, 0x70000);
 call_appl();


__asm call_appl(void)
{
      LDR     R0, =0x70000      ; CHANGE STACK
      LDR     R0, [R0]
      MOV     SP, R0

      LDR     R0, =0x70004      ; CALL APPLICATION
      LDR     R0, [R0]
      BX      R0

      ALIGN
}

Regards Thanassis

Sam A
24 Apr 2013

Hi Thanassis,

Thanks so much for your reply, I really appreciate it!

You definitely know more about this than I do...am I correct in assuming that the vector table is always 0x300 bytes? So in a regular application, 0x000 through 0x300 will always be the vector table? Is that why we don't have to worry about the first 0x300 bytes being used by the application?

Thanks again, Sam

Robert Taylor
25 Apr 2013

You have the right idea Sam. What I've done is have a bootloader sit in the first sector of flash so that on reset the bootloader runs. You can the use the bootloader to read in a new binary off an SD card, use IAP to save the new binary to internal flash and then jump to the new binary.

You're correct that the scatter file for the second binary needs to be changed so that the linker correctly places it in flash. If your bootloader sits in the first sector you can use any address between 0x1000 and 0x7FFFF.

The bootloader should read the second binary from the SD card and then use IAP to write it to flash at the same start address you used in the second binary's scatter file. Once the second binary has been written, you need to change to the vector table offset register (VTOR) to point to the vector table of the new binary, change the main stack pointer (MSP) and then set the program counter (PC) to the start address of the second binary. You can do this in C.

    SCB->VTOR = start_address_of_second_binary;
    // set stack pointer to start address of second binary. The start address of a new binary contains the address of the stack pointer
    __set_MSP(*(uint32_t*) start_address_of_second_binary); 
    // jump to the new binary
    typedef void __attribute__((noreturn)) (*exec)();
    uint32_t *start = (uint32_t*) (start_address_of_second_binary + 4); // address of the reset vector for the new binary
    ((exec)(*start))();
Sam A
25 Apr 2013

Robert, that is a very clear explanation, I really appreciate it. Especially the details and code example regarding how to jump to the application binary from the bootloader. It is nice to know I will be able to do this in C!

One other question I had: if I write the raw binary (.bin) to flash, is that sufficient, or is there some sort of encoding involved that I must handle? I could not find any information regarding this, but in any case I will try the raw binary and see if that works.

Thanks again for your help.

-Sam

Robert Taylor
25 Apr 2013

You don't need to encode the binary.

Sam A
25 Apr 2013

Perfect, thanks!

Robbie King
25 Jan 2015

Sorry to revive this old thread, but this question is directly to @Athanassios...

Do you think you could post a full working example of your approach? This would be SO VERY helpful!

I feel like I am very close, but missing the small details of your implementation...

Best Wishes,

Robbie

Jonne Valola
05 Aug 2015

Robbie King wrote:

Sorry to revive this old thread, but this question is directly to @Athanassios...

Do you think you could post a full working example of your approach? This would be SO VERY helpful!

I feel like I am very close, but missing the small details of your implementation...

Best Wishes,

Robbie

Yea. I'd also like to hear how you can place the application firmware in the memory like that without changing the scatter file. I haven't found any way to implement such a bootloader without making a custom mbed-src in my project ( the situation I want to avoid ).