32F429IDISCOVERY interference between LEDs [solved]

[Wright.Simon]

Posted on November 24, 2014 at 18:03

Using STM32Cube_FW_F4_V1.3.0 and a GCC 4.7.4 toolchain I built on Mac OS X, the attached demo (based on Projects/STM32F429I-Discovery/Examples/BSP) toggles one of the LEDs every 250 ms and the other every 500 ms.

SystemClock_Config()

is the same as in the original except for checking the status returns of the HAL calls. I’ve set up a tick callback,

volatile
unsigned ticks = 0;
void
HAL_SYSTICK_Callback(
void
)
{
ticks++;
}

and this is checked in the main loop,

while
(1) {
const
unsigned ticks_now = ticks;
if
((ticks_now % 250) == 0) {
BSP_LED_Toggle(LED4);
}
if
((ticks_now % 500) == 0) {
BSP_LED_Toggle(LED3);
}
}

and what happens is that the short-period LED (LED4 here) flickers when the other LED is off and (appears to be) on all the time that the other LED is on.

I can make the LEDs work the way I expected by inserting

HAL_Delay(1)

immediately after the first

BSP_LED_Toggle()

,

if
((ticks_now % 250) == 0) {
BSP_LED_Toggle(LED4);
HAL_Delay(1);
}
if
((ticks_now % 500) == 0) {
BSP_LED_Toggle(LED3);
}

The demonstration also works as expected if I remove the

SystemClock_Config()

call but that doesn’t seem like a good way ahead!

[Wright.Simon]

Posted on November 25, 2014 at 11:48

I thought this might be because of using the non-atomic 

GPIOx->ODR ^= GPIO_Pin 

in 

HAL_GPIO_TogglePin()

, so tried using 

BSP_LED_On()

, 

BSP_LED_Off()

with a local variable to tell which one to call (they use 

HAL_GPIO_WritePin()

, which uses 

GPIOx->BSRRL

, 

GPIOx->BSRRH)

.No luck.The reference manual says that

BSRR

is to be used for atomic updates; looks as though it doesn’t quite do that. Which would make using any GPIO problematic. Which I can’t believe.What am I doing wrong?

[Wright.Simon]

Posted on November 25, 2014 at 18:41

What I should have been doing was to set a flag when the

HAL_SYSTICK_Callback()

saw it was time to do a job and then test/clear the flag in the main loop.

static
int
t_250 = 0;
static
int
t_500 = 0;
void
HAL_SYSTICK_Callback(
void
)
{
static
unsigned ticks = 0;
ticks++;
if
((ticks % 250) == 0) {
t_250 = !0;
}
if
((ticks % 500) == 0) {
t_500 = !0;
}
}

[...]

while
(1) {
if
(t_500) {
BSP_LED_Toggle(LED3);
t_500 = 0;
}
if
(t_250) {
BSP_LED_Toggle(LED4);
t_250 = 0;
}
}

Mind you,

BSP_LED_Toggle()

, and by extension toggling any GPIO pin, definitely isn’t thread-safe.

[Tesla DeLorean]

Posted on November 25, 2014 at 18:50

The reference manual says that

BSRR

is to be used for atomic updates. It can if it's used as a 32-bit register as intended.