ARM Cortex M4 hard fault - floating point

Question

When I run my program, which just calculates a sine wave:

for(i = 0; i < ADS1299_SIGNAL_WINDOW; i++){
    TEST[i] = (float32_t)(10.0f * (float32_t)(arm_sin_f32((float32_t)(3.14f * i/ADS1299_SIGNAL_WINDOW))));
}

The compiler generates the following line, which results in a hard fault:

800702a:    ed2d 8b04     vpush    {d8-d9}

What is happening? For reference, here are my flags for the compiler:

SETTINGS="-g -nostartfiles -mthumb -mthumb-interwork -march=armv7e-m -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard -fsingle-precision-constant -fdata-sections -ffunction-sections -O3 -Wl,-T,../STM32F407VG_FLASH.ld"
DECLARE="-DARM_MATH_CM4 -D__FPU_PRESENT=1 -D__FPU_USED"
....  -larm_cortexM4lf_math

Answer 1

The problem is that you're doing both the CPACR enable, and some floating-point operations in the same scope. Because code in main uses floating-point registers, the compiler (being well-behaved and respecting the ABI), will emit code to preserve those registers on entry to main. Before any other code in main executes. Including the write to CAPCR which makes them accessible. Oops.

To avoid that, either enable FP in the CPACR before entry to main in a reset handler (if your toolchain allows), or simply do all FP operations in another function, and ensure main itself doesn't touch any FP registers.

It would also be wise (if you haven't already) to ensure you have a DSB; ISB synchronisation sequence after the CPACR write. Otherwise, you could potentially still get a fault from any stale FP instuctions already in the pipeline.

Answer 2

If you use the FPU, the stack should be aligned to 8 bytes boundaries. If you are using an RTOS, check the thread stack initialization code. If you are running on pure bare metal, check the startup code for the stack setup.

Answer 3

I think the problem is that the FPU is not enabled. I've got same problem with Nordic Semiconductors SDK examples on Keil 4. In the Keil IDE the check box for FPU enable is marked, but in the SystemInit code it is conditional compilation like this:

void SystemInit(void) {
#if (__FPU_USED == 1)
    SCB->CPACR |= (3UL << 20) | (3UL << 22);
    __DSB();
    __ISB();
#endif
}

But I think the Keil 4 IDE does not set this __FPU_USED to 1 and on the VPUSH instruction I've got a HardFault because the FPU is not enabled.

I think you need to enable the FPU in SystemInit and the problem then will be solved.