STM32F745 - Simple LED ON not working with -O2 compiler flag

[CGalv]

Hi,

I am starting to program STM32 devices and I have an STM32F745 chip. Since I want to learn all the details I'm programming it from scratch, using only the definitions from CMSIS.

I have a very simple program where I have 2 LEDs on PB6 and PB7 respectively. Right now I only want to turn them on. I have achieved this compiling the code with "-O0" optimization flag, but when I change it to "-O2" then it doesn't work, the LEDs are off. Here's the code:

#include <stm32f745xx.h>
 
int main()
{
    // Blue LED is on PB7, Red LED is on PB6
    // 1) Enable clock for Port B, since input/output are latched
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOBEN;
 
    // 2) Configure PB6 and PB7 as output. Push-pull is default conf on reset
    // PortB has value 0x0000 0280 on reset
    GPIOB->MODER = 0x0000'5280;
 
    // 3) Wait indefinitely
    while(1){}
}

Compiling with "-O0" optimization flag, the chip behaves as expected and both LEDs are turned on. I get the following assembly:

08000208 <main>:
 8000208:	b480      	push	{r7}
 800020a:	af00      	add	r7, sp, #0
 800020c:	4b05      	ldr	r3, [pc, #20]	; (8000224 <main+0x1c>)
 800020e:	6b1b      	ldr	r3, [r3, #48]	; 0x30
 8000210:	4a04      	ldr	r2, [pc, #16]	; (8000224 <main+0x1c>)
 8000212:	f043 0302 	orr.w	r3, r3, #2
 8000216:	6313      	str	r3, [r2, #48]	; 0x30
 8000218:	4b03      	ldr	r3, [pc, #12]	; (8000228 <main+0x20>)
 800021a:	f44f 42a5 	mov.w	r2, #21120	; 0x5280
 800021e:	601a      	str	r2, [r3, #0]
 8000220:	e7fe      	b.n	8000220 <main+0x18>
 8000222:	bf00      	nop
 8000224:	40023800 	.word	0x40023800
 8000228:	40020400 	.word	0x40020400	

When I compile with "-O2", the LEDs are off. I get the following assembly:

08000208 <main>:
 8000208:	4a04      	ldr	r2, [pc, #16]	; (800021c <main+0x14>)
 800020a:	f44f 40a5 	mov.w	r0, #21120	; 0x5280
 800020e:	4904      	ldr	r1, [pc, #16]	; (8000220 <main+0x18>)
 8000210:	6b13      	ldr	r3, [r2, #48]	; 0x30
 8000212:	f043 0302 	orr.w	r3, r3, #2
 8000216:	6313      	str	r3, [r2, #48]	; 0x30
 8000218:	6008      	str	r0, [r1, #0]
 800021a:	e7fe      	b.n	800021a <main+0x12>
 800021c:	40023800 	.word	0x40023800
 8000220:	40020400 	.word	0x40020400

In both cases I can see that it's storing the correct values in the registries so I don't understand why the LEDs are not being turned on!

Am I missing something obvious?

Thanks!!

[Tesla DeLorean]

There is a documented hazard for writing to a peripheral registers immediately after enabling the clock

[henry.dick]

"both LEDs are turned on."

a pin can be set/cleared for other reasons. it is much more reliable to test code execution by flipping them constantly.

[waclawek.jan]

> There is a documented hazard for writing to a peripheral registers immediately after enabling the clock

RM0385, 5.2.12 Peripheral clock enable register (RCC_AHBxENR, RCC_APBxENRy):

> Caution: Just after enabling the clock for a peripheral, software must wait for a 2 peripheral clock

> cycles delay before accessing the peripheral registers.

[CGalv]

That did the trick, thanks a lot!