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Reading GPIO_IDR assembly

camilo ochoa

Hello, I am using a discovery board with an STM32l152. The user push button is ported to PA0. I need to read from the IDR bit on the GPIOA when the button is pushed. I read that it can only be accessed via word only. I have already set up the moder, otyper, etc correctly and checked it against the debugger. Below is the offset address of GPIOA_IDR.

   LDR r0, =0x40000000 ;PERIPH_BASE
   ADD r0, #0x20000      ;AHBPERIPH_BASE
   ADD r0, #0x0400               ;GPIOA_BASE
   ADD r0, #0x10


   if((GPIOA-IDR & 0x1) == 1) turn on LED

I would like to do the the C statement above in assembly. Thank you very much

Parents
  • 0 in reply to camilo ochoa

    If you have a commercial license, you can ask the compiler to produce a list file.

    With the demo version, you'll have to settle for stepping in the debugger.

    If you are learning assembler, then one of the things you need to learn quickly is how to assign an arbitrary pointer - or integer - to a register. If the core can't perform a 32-bit immediate load, then you'll have to consider doing an indirect load, or combine two 16-bit loads.

    Often, it's a good idea to load a base pointer with specific enough address that the processor supports the relative offset directly when doing the following operations. So you basically ends up with something like (in C)

    port_1->reg1 = xx;
port_1->reg2 = yy;


    or in array notation

    port_1[offset_reg1] = xx;
port_1[offset_reg2] = yy;

    Remember that the assembler can add together multiple constants at build time, so you don't need your processor to add the numbers at run-time.

Reply
  • 0 in reply to camilo ochoa

    If you have a commercial license, you can ask the compiler to produce a list file.

    With the demo version, you'll have to settle for stepping in the debugger.

    If you are learning assembler, then one of the things you need to learn quickly is how to assign an arbitrary pointer - or integer - to a register. If the core can't perform a 32-bit immediate load, then you'll have to consider doing an indirect load, or combine two 16-bit loads.

    Often, it's a good idea to load a base pointer with specific enough address that the processor supports the relative offset directly when doing the following operations. So you basically ends up with something like (in C)

    port_1->reg1 = xx;
port_1->reg2 = yy;


    or in array notation

    port_1[offset_reg1] = xx;
port_1[offset_reg2] = yy;

    Remember that the assembler can add together multiple constants at build time, so you don't need your processor to add the numbers at run-time.

Children
  • 0 in reply to edPer Westermark 

      LDR r0, =0x40000000 ;PERIPH_BASE
   ADD r0, #0x20000      ;AHBPERIPH_BASE
   ADD r0, #0x0400               ;GPIOA_BASE
   ADD r0, #0x10

    Can be a single line

      LDR r0, =0x40000000+0x20000+0x0400+0x10 ;PERIPH_BASE+AHBPERIPH_BASE+GPIOA_BASE

  • 0 in reply to Harib Suntar

    And your C line

       if((GPIOA-IDR & 0x1) == 1) turn on LED

    can be something like:

            LDR r0, GPIOA_ADDR

        LDR  R1,[R0]
        TST  R1,#0x00000001
        BEQ  SkipIt
        ;TURN ON LED

SkipIt: ;Carry on

    ARM assembler isn't the easiest but it can be good fun and not many programmers bother with it because they think they can do everything in C and don't think they need to understand the low level stuff.