70353C Datasheet, PDF (55/76 Pages) Microchip Technology – Section 21. Enhanced Controller Area Network

Section 21. Enhanced Controller Area Network (ECAN™)
21.7.5 DeviceNet™ Filtering
The DeviceNet filtering feature is based on the CAN Specification 2.0A protocol, in which up to 18
bits of the data field can be compared with the EID of the message acceptance filter in addition
to the SID.
The DeviceNet feature is enabled or disabled by the DeviceNet Filter Bit Number (DNCNT<4:0>)
control bits in ECAN Control Register 2 (CiCTRL2<4:0>). The value specified in the DNCNT field
determines the number of data bits to be used for comparison with the EID bits of the message
acceptance filter. If the DNCNT<4:0> bits (CiCTRL2<4:0>) are cleared, the DeviceNet feature is
disabled.
For a message to be accepted, the 11-bit SID must match the SID<10:0> bits in the message
acceptance filter and the first ‘n’ data bits in the message should match the EID<17:0> bits in the
message acceptance filter. For example, as illustrated in Figure 21-16, the first 18 data bits of the
received message are compared with the corresponding identifier bits (EID<17:0>) of the
message acceptance filter.
Note:
The DeviceNet filtering feature will function only when all of the following are true:
• The IDE bit (ECAN Message Buffer Word 0<0>) = 0, which means the
message is a standard ID message
• The EXIDE bit (CiRXFn<3>) = 0
• The MIDE bit (CiRXMnSID<3> = 1
• The value of the DNCNT<4:0> bits (CiCTRL2<4:0>) is non-zero
Figure 21-16: ECAN™ Operation with DeviceNet™ Filtering
S
IDENTIFIER
O
11 Bits
F
STANDARD MESSAGE DATA FRAME
DATA BYTE 0 DATA BYTE 1 DATA BYTE 2
EOF IFS
7 Bits 3 Bits
21
MESSAGE SID<10:0>
Data Byte 0
Data Byte 1
Data Byte 2
SID10 SID9
SID0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
Accept/Reject Message
SID10 SID9
SID0
MESSAGE ACCEPTANCE FILTER
SID<10:0>
EID17 EID16
EID10 EID9 EID8
EID2 EID1 EID0
MESSAGE ACCEPTANCE FILTER
EID<0:17>
Note 1:
2:
The DeviceNet™ filtering configuration shown for EID bits is DNCNT<4:0> = 10010.
Any message acceptance filter used for DeviceNet filtering must have its MIDE bit set (CiRXMnSID<3> = 1) and EXIDE bit cleared
(CiRXFnSID<3> = 0).
© 2008-2011 Microchip Technology Inc.
DS70353C-page 21-55