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
LDR r0, =0x40000000 ;PERIPH_BASE ADD r0, #0x20000 ;AHBPERIPH_BASE ADD r0, #0x0400 ;GPIOA_BASE ADD r0, #0x10
Note that there are way more efficient ways to get the address of the register.
Maybe you should spend some time looking at the code that the Keil compiler generates, with the processor instruction set document as cross-reference.
The documentation tells what an instruction does, while the compiler output will give an indication of how you use the instructions practically.
There are more efficient ways to do everything but since I am just learning assembly, this will do for now. When you say "code that the Keil compiler generates" do you mean go into debug mode and look at disassembly?
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.
Can be a single line
LDR r0, =0x40000000+0x20000+0x0400+0x10 ;PERIPH_BASE+AHBPERIPH_BASE+GPIOA_BASE
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.