How to use the fpu in stm32f4xx ?

Ooops, I was paraphrasing the problem in the code.  A 16-bit signed int has a value from elsewhere in the code and that Watch window reflects that.  But once it's recast as a float and placed in a float variable, it is zero.

Looks like it throws a Hard Fault on a basic constant assign myFloat= 1234.5;

Make sure the FPU is enabled either in Reset_Handler or SystemInit()

Reset_Handler PROC
EXPORT Reset_Handler [WEAK]
IMPORT SystemInit
IMPORT __main
;FPU settings
LDR R0, =0xE000ED88 ; Enable CP10,CP11
LDR R1,[R0]
ORR R1,R1,#(0xF << 20)
STR R1,[R0]
LDR R0, =SystemInit
BLX R0
LDR R0, =__main
BX R0
ENDP

void SystemInit(void)
{
/* FPU settings ------------------------------------------------------------*/
#if (__FPU_PRESENT == 1) && (__FPU_USED == 1)
SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2)); /* set CP10 and CP11 Full Access */
#endif

// STM32 USART2 (Tx PA.2, Rx PA.3) STM32F4 Discovery - sourcer32@gmail.com
#include <
stdio.h
>
#include <
stdlib.h
>
#include ''stm32f4_discovery.h''
/**************************************************************************************/
void RCC_Configuration(void)
{
/* --------------------------- System Clocks Configuration -----------------*/
/* USART2 clock enable */
RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART2, ENABLE);
/* GPIOA clock enable */
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);
}
/**************************************************************************************/
void GPIO_Configuration(void)
{
GPIO_InitTypeDef GPIO_InitStructure;
/*-------------------------- GPIO Configuration ----------------------------*/
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_2 | GPIO_Pin_3;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_Init(GPIOA, &GPIO_InitStructure);
/* Connect USART pins to AF */
GPIO_PinAFConfig(GPIOA, GPIO_PinSource2, GPIO_AF_USART2); // USART2_TX
GPIO_PinAFConfig(GPIOA, GPIO_PinSource3, GPIO_AF_USART2); // USART2_RX
}
/**************************************************************************************/
void USART2_Configuration(void)
{
USART_InitTypeDef USART_InitStructure;
/* USARTx configuration ------------------------------------------------------*/
/* USARTx configured as follow:
- BaudRate = 115200 baud
- Word Length = 8 Bits
- One Stop Bit
- No parity
- Hardware flow control disabled (RTS and CTS signals)
- Receive and transmit enabled
*/
USART_InitStructure.USART_BaudRate = 115200;
USART_InitStructure.USART_WordLength = USART_WordLength_8b;
USART_InitStructure.USART_StopBits = USART_StopBits_1;
USART_InitStructure.USART_Parity = USART_Parity_No;
USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
USART_Init(USART2, &USART_InitStructure);
USART_Cmd(USART2, ENABLE);
}
/**************************************************************************************/
int main(void)
{
RCC_Configuration();
GPIO_Configuration();
USART2_Configuration();
{
int16_t myInt=0x1234;
float myFloat;
myFloat=(float)myInt; //gets zero
printf(''myFloat %f
'', myFloat);
}
while(1); // Don't want to exit
}
//******************************************************************************
// Hosting of stdio functionality through USART2
//******************************************************************************
#include <
rt_misc.h
>
#pragma import(__use_no_semihosting_swi)
struct __FILE { int handle; /* Add whatever you need here */ };
FILE __stdout;
FILE __stdin;
int fputc(int ch, FILE *f)
{
static int last;
if ((ch == (int)'
') && (last != (int)'
'))
{
last = (int)'
';
while(USART_GetFlagStatus(USART2, USART_FLAG_TXE) == RESET);
USART_SendData(USART2, last);
}
else
last = ch;
while(USART_GetFlagStatus(USART2, USART_FLAG_TXE) == RESET);
USART_SendData(USART2, ch);
return(ch);
}
int fgetc(FILE *f)
{
char ch;
while(USART_GetFlagStatus(USART2, USART_FLAG_RXNE) == RESET);
ch = USART_ReceiveData(USART2);
return((int)ch);
}
int ferror(FILE *f)
{
/* Your implementation of ferror */
return EOF;
}
void _ttywrch(int ch)
{
static int last;
if ((ch == (int)'
') && (last != (int)'
'))
{
last = (int)'
';
while(USART_GetFlagStatus(USART2, USART_FLAG_TXE) == RESET);
USART_SendData(USART2, last);
}
else
last = ch;
while(USART_GetFlagStatus(USART2, USART_FLAG_TXE) == RESET);
USART_SendData(USART2, ch);
}
void _sys_exit(int return_code)
{
label: goto label; /* endless loop */
}
/**************************************************************************************/
#ifdef USE_FULL_ASSERT
/**
* @brief Reports the name of the source file and the source line number
* where the assert_param error has occurred.
* @param file: pointer to the source file name
* @param line: assert_param error line source number
* @retval None
*/
void assert_failed(uint8_t* file, uint32_t line)
{
/* User can add his own implementation to report the file name and line number,
ex: printf(''Wrong parameters value: file %s on line %d

'', file, line) */
/* Infinite loop */
while (1)
{
}
}
#endif
/**
* @}
*/

myFloat 46000000 

Well you nailed it.