AN1105 datasheet(29/100 Pages) STMICROELECTRONICS

29 / 100 page

29/100

ST7 pCAN PERIPHERAL DRIVER

Let’s choose, for example, BTR = 3, BS1 = 3, BS2 = 2 (second row).

We have to complete the code:

#define INIT_BRPR 0x03

#define INIT_BTR 0x23

2.2.2.2 Filtering

The driver is able to filter out all messages not needed by the application. First by using hard-

ware filters, and second by using software filters.

Use the hardware filters to eliminate as many identifiers as possible and to avoid further

processing. Use the software filters to eliminate IDs that are unwanted, but that can not

stopped by the hardware.

Hardware Filter Configuration

Let’s define the values of hardware masks and filters. To do this, we must split the IDs into two

groups:

– 0x127, 0x311 and 0x3A5, on one hand; and,

– 0x500 and 0x600 on the other.

Note: When grouping the IDs, try to maximize the number of identical bits inside the same group, thus

minimizing the width of the ID ranges accepted by the cell.

Then let’s find the first filter/mask pair (12 bits to be determined):

– 0x500 is written in binary: 101 0000 0000

– 0x600 is written in binary: 110 0000 0000

In the first three bits, only the most significant bit (MSB) of both identifiers is the same, the fol-

lowing two being different. So, the three MSBs of the mask will be 1 0 0 (match, don’t care,

don’t care) and the three MSBs of the filter will be 1 and then either 1 or 0.

The following eight bits are identically null. So the last eight bits of the filter will be 0, and the

last eight bits of the mask will be 1 (match required).

The least significant bit (LSB) of the filter/mask is used to separate remote frames and data

frames. Here we have a data frame and a remote frame, so it does not have to be checked (1

or 0 into the filter, 0 into the mask).

So we have:

– Filter 1000 0000 0000

– Mask 1001 1111 1110

BTR+1

BS1+BS2+3

AN1105 Datasheet(PDF) 29 Page - STMicroelectronics