STM32L476RGTx Nucleo Blink Code in Assembly[Newbie Ques]

Question

    __initial_sp    EQU 0x20000800
                PRESERVE8 ;8byte alignment of the stack (required by ARM) 
                THUMB    ; will use THUMB2 instructions
                AREA    RESET, DATA, READONLY
__Vectors       DCD     __initial_sp              ; Top of Stack
                DCD     Reset_Handler             ; Reset Handler
                AREA    |.text|, CODE, READONLY 
                ENTRY   
Reset_Handler   PROC    
                LDR r1, =0x4002144C         ;address of RCC_AHBENR register 
                LDR r0, =0x1
                LDR r2, [r1]
                ORRS r2,r0,r2
                STR r2, [r1]

                LDR r1, =0x48000000         ;address of GPIOA_MODER register
                LDR r0, =0x400          ;configure PA5 as output
                LDR r2, [r1]
                ORRS r2,r0,r2
                STR r2, [r1]

                LDR r1, = 0x48000018            ;address of GPIOA_BSRR register 
                LDR r0, =0x20           ;pin 5 on GPIOA set 
                LDR r2, =0x200000       ;pin 5 on GPIOA clear 
loop
                STR r0, [r1]                
                STR r2, [r1]
                B  loop                 
                ENDP
                END

The code flashes onto the MCU(1 warning during compilation) but LED doesn't Blink.While debugging ORRS r2,r0,r2 in the GPIOA_MODER raises the Negative Flag in the PSR and the value is not loaded into GPIOA_MODER.The same happens with BSRR.

The LED is connected to PA5.

Answer 1

I think you got the RCC_AHB2ENR register address wrong.

The base address is given as 0x40021000, the offset is 0x4C. So the address of the register is 0x4002104C and not 0x4002144C.

So in effect you do not enable the clock to the GPIOA, hence nothing will happen (I have not checked those addresses).

Another remark: your loop has no delay, the LED will not blink visibly for humans as it will be switched on and off in the MHz range. It'll look like half on.

Lesson: pay extra attention to magic numbers (always), try to use predefined symbols - then you have to pay extra attention only once.

---

Next error:

The GPIO Mode register of port A has a reset value of 0xABFFFFFF. ORing another 1 into that will not change anything.

You have to reset the setting bits for the pin you want to change first.