Fast pin toggling on STMH723

[Mazz]

I have been trying to see how fast can I toggle the pins on STM32H7 MCU. I'm using a nucleo-H723zg board. 

the clock configs are at the highest (550Mhz for CPU, 275 for APB and AHB), the pins are configured as very high speed. 
But still with a simple while loop such as 

 

while (1) {

GPIOE->BSRR = (uint32_t) GPIO_PIN_2;

GPIOE->BSRR = (uint32_t) GPIO_PIN_2 << 16U;

}

the frequency of the PWM I see on the oscilloscope is 7.3 MHz max. Ofcourse the PWM is not the final goal
but I wanted to see how fast this mcu can implemnet the bit toggling.
Am I doing something wrong here?

 

[LCE]

That seems very slow indeed.

Any interrupts active?

In case you need fast PWM, use a timer.

[Tesla DeLorean]

Not really a great test, way the bus is constructed apt to be slow.

Use a TIM, they have an actual mode for this..

Perhaps use an address bit of an FMC bus? And use M2M mode DMA

[Mazz]

I don't just need a pwm. it was more of a test because I suspected that every instruction is taking longer than I expected. 

originally I realized this while trying to implement a custom protocol and realized every instruction seem to add a considerable amount of overhead. 

for example a simple

 GPIOE->BSRR = (uint32_t) GPIO_PIN_2;

adding 60ns. Also not getting accurate delay because 

CYCCNT_reg = DWT->CYCCNT; 

adding extra 20 or 30ns.

[Uwe Bonnes]

You did not use a scope to measure timing?

[Uwe Bonnes]

What is GPIO High time, what is low time? Low time include the loop. Run many such sequences in a loop! Is Icache Otherwise the loop will include wait state. And what is optimization setting?

[Mazz]

The high and low time were almost the same. maybe 45% high 55% low. changed the optimization from none (-O0) to optimize for speed (-Ofast) and no change. 

[AScha.3]

From my tests :

20 MHz can get from H743 , at 480MHz , -O2, : 25ns hi or lo time ;

compared to cpu with "better" = direct access to port/bus : H563 , at 250MHz :

 4ns pin access, 16ns with while-loop : 82MHz output by pin toggling - not bad , i would say...

see:

https://community.st.com/t5/stm32-mcus-products/which-is-best-stm32-mcu-for-fast-driving-of-gpio/m-p/632729#M233727

 

[waclawek.jan]

As @Tesla DeLorean said above, in 'H7, GPIO access time from processor is dominated by traversal through the many buses and their interfaces. 

JW

[PGump.1]

Hi,

Try removing the unnecessary casting, from

GPIOE->BSRR = (uint32_t) GPIO_PIN_2 << 16U;

to

GPIOE->BSRR = GPIO_PIN_2 << 16;

I hope this helps.

Kind regards
Pedro