How to enable FPU in MK22FX512AVLQ12

[vaibhavi_padwal]

Hi,

I am using MK22FX512AVLQ12 microcontroller in my custom board design. I want to do some operations such as pow(), multiplication, division which involve uint64_t as well as float operands using FPU of Cortex M4 of controller. The Macro __FPU_PRESENT is 1 in MK22FA12.h file. Also I have defined  __FPU_PRESENT in project settings. But Macro  __FPU_USED is 0 in core_cm4.h by the conditions defined there.

After referring to link I have enabled all the settings for HardABI in project settings. But after viewing the disassembly code , I find it is still using some of the software library functions.

For eg.

float  l_f_DifferenceCurr;

uint64_t l_ui64_DiffMultiplier = 0U;

 l_ui64_DiffMultiplier = (uint64_t)(l_f_DifferenceCurr * (float)MF_ONE_LAKH);

Please refer the attached screenshot of disassembly for Above line

I have included arm_math.h file and math.h file in the code.

Please suggest the appropriate steps to use FPU for float operations.

Thanks & Regards,

Vaibhavi

[ErichStyger]

Hi @vaibhavi_padwal ,

this is a float to unsigned 64bit integral conversion. Your FPU is able to do single precision operations (add, sub, mul, ...) and float to intergral 32bit conversion. Because you want to use a 64bit integral, a software conversion has to be used.

So the question is: do you really need 64bit? Then yes, this is what you get.

If you are ok with 32bits, then you can use

l_ui64_DiffMultiplier = (uint32_t)(l_f_DifferenceCurr * (float)MF_ONE_LAKH);

and you get something like this:

 10:	4b0a      	ldr	r3, [pc, #40]	; (3c <main+0x3c>)
  12:	edd3 7a00 	vldr	s15, [r3]
  16:	eeb0 7a08 	vmov.f32	s14, #8	; 0x40400000  3.0
  1a:	ee67 7a87 	vmul.f32	s15, s15, s14
  1e:	eefc 7ae7 	vcvt.u32.f32	s15, s15
  22:	ee17 3a90 	vmov	r3, s15
  26:	461a      	mov	r2, r3
  28:	f04f 0300 	mov.w	r3, #0
  2c:	4904      	ldr	r1, [pc, #16]	; (40 <main+0x40>)
  2e:	e9c1 2300 	strd	r2, r3, [r1]

Somewhat related: you might need to upgrade to a processor with double precision FPU too (ARM Cortex-M7 for example) if you want a more powerful FPU.

Keep in mind that there are two things: one thing is to tell the compiler to create instructions matching the FPU in the hardware (this is done through the compiler switches, do not mess with the #defines unless you know exactly what you are doing). The other thing is to enable the FPU in the firmware as usually it is disabled by default, see https://mcuoneclipse.com/2016/10/29/tutorial-building-freertos-applications-for-arm-cortex-m4-on-i-m...

If you are new to FPU on ARM Cortex-M4: have a read at https://mcuoneclipse.com/2019/03/29/be-aware-floating-point-operations-on-arm-cortex-m4f/ 

I hope this helps,

Erich