Can-bus Filter and Mask Logic

问题

I want to understand below code, why the filter 0x100 between 1xFF address. Mask value is 1110000000000000 (16 bit) and filter value is 10000000000000. That mean my ids could be 100100000000000(16bit),10000100000000(16 bit) etc. (100.. side must be constant all ids.

But the 0x100 value is 100000000 (9 bit) how this possible I can not understand.How it compares 9-bit and 16-bit id ?

HAL_StatusTypeDef CAN_Filter_Init(CAN_HandleTypeDef *hcanxx)
{
  CAN_FilterTypeDef can_filter_config;
  //100 to 1FF in other words block all messages after 0x0200
  can_filter_config.FilterActivation = CAN_FILTER_ENABLE;
  can_filter_config.FilterBank = 0;
  can_filter_config.FilterIdHigh = 0x100 << 5;
  // can_filter_config.FilterIdHigh = 0x000;

  can_filter_config.FilterIdLow = 0x0000;
  can_filter_config.FilterMode = CAN_FILTERMODE_IDMASK;
  can_filter_config.FilterScale = CAN_FILTERSCALE_32BIT;
  can_filter_config.FilterMaskIdHigh = 0x700 << 5;
  // can_filter_config.FilterMaskIdHigh = 0X000;
  can_filter_config.FilterMaskIdLow = 0x0000;
  can_filter_config.FilterFIFOAssignment = CAN_FILTER_FIFO0;
  can_filter_config.SlaveStartFilterBank = 0;

  if (HAL_CAN_ConfigFilter(hcanxx, &can_filter_config) != HAL_OK)
  {
    Error_Handler();
  }
  return HAL_OK;
}

回答1:

The CAN bus has 11-bit identifiers (base frame format) and 29-bit identifiers (extended frame format).

So if FilterIdHigh is 16 bits, and the 11-bit identifier is in the upper bits of the register, then you need to left shift the identifier by 5 bits to put it into the 11 MSBs of the register. That is why the code has the left shift (<< 5) for both the FilterIdHigh and the FilterMaskIdHigh.

The 0x100 value isn't a 9-bit value, it's an 11-bit value 001 0000 0000.
The mask 0x700 value is also an 11-bit value 111 0000 0000.

As a result, the upper three bits of the 11-bit identifier must be 001, and any identifier of the form
001 xxxx xxxx will pass the filter. So identifiers from 0x100 thru 0x1ff will pass the filter.

来源：https://stackoverflow.com/questions/57686650/can-bus-filter-and-mask-logic

标签

can-bus