CAN messages filtering

The filters are hardware, so there is no impact on your processing speed whatsoever. It doesn't make a difference whether you use a mask or set up individual filters. Until you run out of filter banks it doesn't.

Here's a little code snippet I'm using from my personal can filter library. You can use these two functions to calculate the settings for the ID and MASK registers if you want to filter certain ID ranges.

/**
 * Calculates a mask to mask out all numbers outside id_min & id_max as good as possible.
 * This usually leads to masking out all numbers greater than the closest power of 2 of id_max.
 * Still, all combinations are checked.
 *
 * The criteria applied to keep a received frame is as follows: (received ID) AND (ID mask) = (ID arbitration), where AND is a bitwise operator.
 * A way to define a range by using these registers is to fill the MASK one with
 * (~((id_high) - (id_low)))
 * and the ID one with
 * ((id_low) & (id_high)).
 * Consequently, for id_low, it is generally better to choose a value with some LSBs cleared,
 * and for id_high a value that “logically contains�? id_low and with the same LSBs set. Example: the range 0x100-0x3FF
 * will work, but the range 0x100-0x2FF will not because 0x100 is not logically contained in 0x2FF (i.e. 0x100 & 0x2FF = 0).
 *
 * Note: (id_min & id_max) != 0
 *
 * Note 2: id_max-id_min needs to be increased to the next pow(2) - 1
 * This allows too many ids to pass and slightly increases cpu load but that's better than missing some wanted messages!
 *
 * @param id_minMinimum allowed ID (Std or Ext)
 * @param id_maxMax allowed ID
 * @returnRaw 32 bit mask. Not shifted, no IDE or RTR care bits set. In case of error: 0xFFFFFFFF -> id_min is not logically contained in id_max!
 */
uint32_t can_calc_range_mask(uint32_t id_min, uint32_t id_max) {
if (id_min >= id_max) {
return 0xffffffff;
}
/*
 * Round id_max-id_min up to next highest power of 2 - 1
 * -> https://community.st.com/external-link.jspa?url=http%3A%2F%2Fgraphics.stanford.edu%2F%7Eseander%2Fbithacks.html%23RoundUpPowerOf2Float
 */
uint32_t tmp = id_max - id_min;
tmp |= tmp >> 1;
tmp |= tmp >> 2;
tmp |= tmp >> 4;
tmp |= tmp >> 8;
tmp |= tmp >> 16;
id_max = id_min + tmp;
assert_param((id_min & id_max) != 0);
if ((id_min & id_max) == 0) {
// id_min not logically contained in id_max -> problem
/*
 * 0xFFFFFFFF should never be used in mask mode as this filters only ONE id for which list mode should be used.
 * Therefore this value now becomes an error flag.
 */
return 0xffffffff;
} else
return ~((id_max) - (id_min));
}
/**
 * Calculates the ID register for filtering a range of IDs.
 * See can_calc_range_mask for description of algorithm.
 *
 * Note: (id_min & id_max) != 0
 *
 * @param id_minMinimum allowed ID (Std or Ext)
 * @param id_maxMax allowed ID
 * @returnRaw 32 bit ID register value. Not shifted, no IDE or RTR care bits set.
 */
uint32_t can_calc_range_ID(uint32_t id_min, uint32_t id_max) {
assert_param((id_min & id_max) != 0); // catch here for now.
if ((id_min & id_max) == 0 || id_min >= id_max) {
// id_min not logically contained in id_max -> problem
/*
 * 0xFFFFFFFF should never be used in mask mode as this filters only ONE id for which list mode should be used.
 * Therefore this becomes an error flag.
 */
return 0xffffffff;
} else
return ((id_min) & (id_max));
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Enjoy!