STM32的功能引脚重映射程序不能理解

zhanghuafeng

void GPIO_PinRemapConfig(uint32_t GPIO_Remap, FunctionalState NewState)
{
  uint32_t tmp = 0x00, tmp1 = 0x00, tmpreg = 0x00, tmpmask = 0x00;
  /* Check the parameters */
  assert_param(IS_GPIO_REMAP(GPIO_Remap));
  assert_param(IS_FUNCTIONAL_STATE(NewState)); 
 
  tmpreg = AFIO->MAPR;
 tmpmask = (GPIO_Remap & DBGAFR_POSITION_MASK) >> 0x10;
  tmp = GPIO_Remap & LSB_MASK;
  if ((GPIO_Remap & (DBGAFR_LOCATION_MASK | DBGAFR_NUMBITS_MASK)) == (DBGAFR_LOCATION_MASK | DBGAFR_NUMBITS_MASK))
  {
    tmpreg &= DBGAFR_SWJCFG_MASK;
    AFIO->MAPR &= DBGAFR_SWJCFG_MASK;
  }
  else if ((GPIO_Remap & DBGAFR_NUMBITS_MASK) == DBGAFR_NUMBITS_MASK)
  {
    tmp1 = ((uint32_t)0x03)  0x15)*0x10));
    tmpreg |= ~DBGAFR_SWJCFG_MASK;
  }
  if (NewState != DISABLE)
  {
    tmpreg |= (tmp > 0x15)*0x10));
  }
  AFIO->MAPR = tmpreg;
}//这人程序是开启can脚映射的，但是我不理解里面的俺码比如DBGAFR_LOCATION_MASK ，DBGAFR_NUMBITS_MASKDBGAFR_SWJCFG_MASK，这些掩码是根据什么算出来的？