How long is __NOP()

[AD_716]

Hello,

How long does the __NOP() function last?
Is it the time of a cycle? Or 1/F = cycle time?
My MCU is an STM32F401RE, and I understand that the maximum frequency is 84 Mhz.
So 1/84 Mhz = 11.90 ns, is that the time of a cycle?

In my case, if I loop 14 times on __NOP(), it takes 1.24us or 1240 ns (measured with an oscilloscope).
But if I do 14 x 11.90 ns, I get 167 ns...
Does anyone have an explanation?

Thank you in advance for your valuable answers

Sincerely, Antoine
Code:

 

 __disable_irq();
 
 for (int i = 0; i < (nLed * (nBytePerLed * nBit8)); i++) 
 {
 int j = 0;
 Data = 1;
 if (data[i])
 {// ONE
 for (; j < 8; j++) {__nop();} //......i = 8: 660ns
 Data = 0;
 for (; j < 14; j++) {__nop();}//......i = 14: 1.24us
 } 
 else 
 {// ZERO
 for (; j < 3; j++) {__nop();}//.......i = 3: 380ns
 Data = 0;
 for (; j < 13; j++) {__nop();}//......i = 3: 1.20us
 }
 }
 
 __enable_irq();

[AScha.3]

the nop will need one clock (i assume - to be 100% sure, look at the manual at ARM for this core).

to see, what you producing with the C compiler, first look at the assembler output !

and most important: all depends on the optimizer settings - what you set here?

[Nikita91]

What you are measuring is loop time, not NOP time.

Look at the generated code in the debug/xxx.list file

Replace the loops with the corresponding series of NOP...

[AD_716]

Good evening,

Thank you for your answer,
I understand better why my calculations are wrong. Thanks for the tip.
I'll take a look at the Debug file, but it's new to me! It's in the BUILD folder of the project?

What do you mean by: "Replace the loops with the corresponding series of NOP..."

Thanks in advance

Best regards,
Antoine

[KnarfB]

Looks like you want to do some WS2812B... LED driver. There are much more efficient methods for that, like DMA, SPI. Doing such small periods by bit-banging easily leads into a dead-end.

hth

KnarfB

[AD_716]

Good evening,

Thank you for your answer,
Yes, I want to drive WS2812. This function is taken from NeoPixel, I simply adapted the __NOP() time loop to make it work on STM32F4.

I can't use SPI or UART because I'm already busy with other functions.
I've seen that others use DMA but I haven't found any concrete examples, and never in the examples is a pin assigned to drive the LEDs. I don't understand how to implement DMA without CubeProgrammer, I code with Mbed studio.

Do you have any examples? Any advice, as my search for DMA is going nowhere...

Thanks in advance

Best regards,
Antoine

[KnarfB]

DMA: https://controllerstech.com/interface-ws2812-with-stm32/ Don't use mbed.

hth

KnarfB

 

[Nikita91]

Replace 

 for (; j < 8; j++) {__nop();} //......i = 8: 660ns

by

__nop();
__nop();
__nop();
__nop();
__nop();
__nop();
__nop();
__nop();

I don't use Mbed studio

 

[AD_716]

Thanks, I'll give it a try!

Sincerely Antoine