STM32F4 Discovery Multi-dimensional Array Problem

[kriscicluna]

Posted on February 07, 2018 at 09:08

I have a control algorithm which operates within a Timer Interrupt of 200 us. Basically my code generates an output and compares it to a look up table. When I have a single array defined:

const float Adm_Re_array[720] = {...values...}

And search for a match in the LUT which has the minimum distance from the array using this code:

for (i=1;i<=8;i=i++)

{

k = act_element+i-4;

if (k < 0) k = k + 720;

if (k >(719)) k = k - 720;

dist_Re = Adm_Re-Adm_Re_array[k];

dist_T = (dist_Re*dist_Re);

if ((dist_T <= dist_T_min)||(dist_T_min == -1))

{

dist_T_min = dist_T;

act_element = k;

}

}

My control algorithm works well and I have used only 124 us of the 200 us.

When I change from a single dimension to a multi-dimensional array of form:

const float Adm_Re_array[7][720] = {...values...}

And search for a match in the LUT which has the minimum distance from the array using this code:

for (i=1;i<=8;i=i++)

{

k = act_element+i-4;

if (k < 0) k = k + 720; 

if (k >(719)) k = k - 720 

dist_Re = Adm_Re-Adm_Re_array[4][k];

dist_T = (dist_Re*dist_Re);

if ((dist_T <= dist_T_min)||(dist_T_min == -1))

{

dist_T_min = dist_T;

act_element = k;

}

}

My control code goes unstable while the loop time does not change significantly from the 124 us as I am only looking at one row of the multidimensional array in my search. Are there any known issues with the STM32 using multidimensional arrays similar to what I am observing. 

Regards,

K

[AvaTar]

Posted on February 07, 2018 at 09:18

You are not on a PC here, with virtually unlimited ressources.

Can you tell where this array is located (stack or global) ?

My control code goes unstable ...

What exactly does this mean ?

Are there any known issues with the STM32 using multidimensional arrays similar to what I am observing.

One thing has nothing to do with the other.

Organizing memory for your variables is the sole responsibility of the compiler (toolchain).

BTW, I am pretty sure your algorithm could use integer types and scaled math instead of floating point.

[kriscicluna]

Posted on February 07, 2018 at 09:36

I am generating a sinusoidal current (controlled by the microcontroller) and comparing it to the fixed LUT in memory. When I compare it to a single dimension array the LUT and real time current are exactly the same. When I introduce multi-dimensional arrays the generated sinusoidal current (real-time) is not controlled according to the reference (hence not stable).

Yes I can easily use integer and scale accordingly. The size of the array is constant hence i defined it const float. Could you direct me towards documentation on how to optimize my array definitions?

[AvaTar]

Posted on February 07, 2018 at 10:16

When I introduce multi-dimensional arrays the generated sinusoidal current (real-time) is not controlled according to the reference (hence not stable).

Still not sure what you mean with unstable.

Deviation too high, ringing, or does your code take too long ?

Yes I can easily use integer and scale accordingly. The size of the array is constant hence i defined it const float.

Do you use the FPU at all in your project ?

Adding floating point code does not mean the FPU is actually used.

And have you realized that single precision floating point has only 24 bit resolution ?

Algorithms designed with double in mind tend to become unstable with float.

if (k < 0) k = k + 720; 

if (k >(719)) k = k - 720

I would replace that with something like

k = (uint32_t) (k%720);

Generally, the involved datatypes and some surrounding code would clarify things, too.

As a general advice, you can test your algorithms extensively with proper datatypes on a PC before.

Instrumentation, debugging and validation of results is much easier.

We use a (not so cheap) testing tool and a simulation environment for that purpose.