ST92F124 datasheet(335/429 Pages) STMICROELECTRONICS

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CONTROLLER AREA NETWORK (bxCAN)

CONTROLLER AREA NETWORK (Cont’d)

Filter Bank Scale and Mode Configuration

The filter banks are configured by means of the

corresponding CFCRx register. To configure a fil-

ter bank it must be deactivated by clearing the

FACT bit in the CFCR register. The filter scale is

configured by means of the FSC[1:0] bits in the

corresponding CFCR register, refer to Figure 149.

The identifier list or identifier mask mode for the

corresponding Mask/Identifier registers is config-

ured by means of the FMCLx and FMCHx bits in

the CFMR register. The FMCLx bit defines the

mode for the two least significant bytes, and the

FMCHx bit the mode for the two most significant

bytes of filter bank x. Examples:

– If filter bank 1 is configured as two 16-bit filters,

then the FMCL1 bit defines the mode of the

CF1R2 and CF1R3 registers and the FMCH1 bit

defines the mode of the CF1R6 and CF1R7 reg-

isters.

– If filter bank 2 is configured as four 8-bit filters,

then the FMCL2 bit defines the mode of the

CF2R1 and CF2R3 registers and the FMCH2 bit

defines the mode of the CF2R5 and CF2R7 reg-

isters.

Note: In 32-bit configuration, the FMCLx and FM-

CHx bits must have the same value to ensure that

the four Mask/Identifier registers are in the same

mode.

To filter a group of identifiers, configure the Mask/

Identifier registers in mask mode.

To select single identifiers, configure the Mask/

Identifier registers in identifier list mode.

Filters not used by the application should be left

deactivated.

Filter Match Index

Once a message has been received in the FIFO it

is available to the application. Typically application

data are copied into RAM locations. To copy the

data to the right location the application has to

identify the data by means of the identifier. To

avoid this and to ease the access to the RAM loca-

tions, the CAN controller provides a Filter Match

Index.

This index is stored in the mailbox together with

the message according to the filter priority rules.

Thus each received message has its associated

filter match index.

The Filter Match index can be used in two ways:

– Compare the Filter Match index with a list of ex-

pected values.

– Use the Filter Match Index as an index on an ar-

ray to access the data destination location.

For non-masked filters, the software no longer has

to compare the identifier.

If the filter is masked the software reduces the

comparison to the masked bits only.

Filter Priority Rules

Depending on the filter combination it may occur

that an identifier passes successfully through sev-

eral filters. In this case the filter match value stored

in the receive mailbox is chosen according to the

following rules:

– A filter in identifier list mode prevails on an filter

in mask mode.

– A filter with full identifier coverage prevails over

filters covering part of the identifier, e.g. 16-bit fil-

ters prevail over 8-bit filters.

– Filters configured in the same mode and with

identical coverage are prioritized by filter number

and register number. The lower the number the

higher the priority.

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