How to Enable FPU on ARM Cortex-M7: Simple Steps

To enable the FPU on an ARM Cortex-M7, set bits 20-23 in the CPACR register to enable full access to the floating-point coprocessors. This is done by writing 0xF << 20 to SCB->CPACR. Then, enable the FPU by setting the FPCCR register if needed and ensure proper instruction synchronization.

Syntax

To enable the FPU on ARM Cortex-M7, you modify the CPACR register in the System Control Block (SCB). The key steps are:

• Set bits 20-23 of SCB->CPACR to 0b1111 (full access to CP10 and CP11 coprocessors).
• Optionally configure the Floating Point Context Control Register (FPCCR) for lazy stacking.
• Use __DSB() and __ISB() instructions to ensure changes take effect immediately.

c

SCB->CPACR |= (0xF << 20);  // Enable CP10 and CP11 full access
__DSB();                    // Data Synchronization Barrier
__ISB();                    // Instruction Synchronization Barrier

Example

This example shows how to enable the FPU in a Cortex-M7 startup code. It sets the necessary bits in CPACR and uses barriers to ensure proper synchronization.

c

#include "core_cm7.h"  // CMSIS header for Cortex-M7

void EnableFPU(void) {
    // Enable full access to CP10 and CP11 coprocessors (FPU)
    SCB->CPACR |= (0xF << 20);

    // Ensure all memory accesses complete before continuing
    __DSB();
    // Flush and refill pipeline with updated permissions
    __ISB();
}

int main(void) {
    EnableFPU();
    // Now FPU instructions can be used safely
    return 0;
}

Common Pitfalls

Common mistakes when enabling the FPU include:

• Not setting both CP10 and CP11 bits (bits 20-23) in CPACR, which disables full FPU access.
• Forgetting to use __DSB() and __ISB(), causing the CPU to not recognize the change immediately.
• Trying to use FPU instructions before enabling it, which leads to faults.
• Not enabling the FPU in the startup code before any floating-point operations.

Example of wrong and right way:

c

// Wrong way: Missing synchronization barriers
SCB->CPACR |= (0xF << 20);
// FPU instructions used immediately - may cause fault

// Right way:
SCB->CPACR |= (0xF << 20);
__DSB();
__ISB();
// Now safe to use FPU instructions

Quick Reference

Step	Register/Instruction	Description
1	SCB->CPACR	Set bits 20-23 to 1 to enable CP10 and CP11 full access
2	__DSB()	Data Synchronization Barrier to complete memory operations
3	__ISB()	Instruction Synchronization Barrier to flush pipeline
4	Use FPU instructions	Safe to execute floating-point operations now

Key Takeaways

Enable FPU by setting bits 20-23 in SCB->CPACR to 0xF for full access.

Always use __DSB() and __ISB() after changing CPACR to ensure proper synchronization.

Enable the FPU early in startup before any floating-point instructions run.

Incorrect or missing synchronization can cause faults or undefined behavior.

Check your compiler and startup code to confirm FPU enablement is done correctly.