if(FLASH->CR&FLASH_CR_LOCK)
  {
    FLASH->KEYR=FLASH_KEY1;
    FLASH->KEYR=FLASH_KEY2;//проверял, разблокируется успешно
  }
  FLASH->CR|= FLASH_CR_PG;
  *addr=(uint16_t)data;
  while(FLASH->SR&FLASH_SR_BSY);//вот здесь всё зависает
  FLASH->CR &= ~FLASH_CR_PG;
  FLASH->CR |= FLASH_CR_LOCK;

FLASH_Unlock();
FLASH_ProgramHalfWord(0x08004000, (uint16_t)0x1234);
FLASH_Lock();

void save_param_to_flash(uint32_t first_address_page)
{
   //unlock flash
    FLASH->KEYR=0x45670123;
    FLASH->KEYR=0xCDEF89AB;
    //set programing bit
    FLASH->CR |= FLASH_CR_PG;

    //save time
    *((uint16_t*)first_address_page) = time;
    while((FLASH->SR & FLASH_SR_BSY) != 0){;}

    //save frequency
    *((uint16_t*)first_address_page+2) = frequency;
    while((FLASH->SR & FLASH_SR_BSY) != 0){;}

    //save time_on
    *((uint16_t*)first_address_page+4) = percent;
    while((FLASH->SR & FLASH_SR_BSY) != 0){;}

    //save mode
    *((uint16_t*)first_address_page+6) = mode;
    while((FLASH->SR & FLASH_SR_BSY) != 0){;}

    //save current
    *((uint16_t*)first_address_page+8) = current;

    FLASH->CR &= ~FLASH_CR_PG;

    //lock flash
    FLASH->CR |= FLASH_CR_LOCK;
}

*((uint32_t*)0x08004000)=0x1234;