Freescale Semiconductor MPC5200B - Manual (Page 695)

MPC5200B Users Guide, Rev. 1
20-16 Freescale Semiconductor
Functional Description
READ: any time
WRITE: ignored in normal and emulation modes
Register functionality is modified in special test mode.
IDLE Idle (Bit 0)
This bit indicates when the BDLC module is idle.
1 = BDLC module has received IFS and no data is being transmitted or received.
0 = BDLC module is either transmitting or receiving data.
NOTE
BDLC module is only idle after receiving IFS. The IDLE bit is 0 during reset since the BDLC module
needs to wait for an IFS before becoming idle. Noise on the bus will be filtered and the IDLE bit will
remain unchanged.
20.8 Functional Description
20.8.1 General
The BDLC module is a serial communication module which allows the user to send and receive messages across a Society of Automotive
Engineers (SAE) J1850 serial communication network
. The user’s software handles each transmitted or received message on a byte-by-byte
basis, while the BDLC performs all of the network access, arbitration, message framing and error detection duties.
20.8.1.1 J1850 Frame Format
As noted above and in Section 20.2, Features , the BDLC module communicates across an SAE J1850 network. As such, all messages
transmitted on the J1850 bus are structured using the format below. The following sections describe this format and it’s meanings.
Figure 20-4. J1850 Bus Message Format (VPW)
SAE J1850 states that each message has a maximum length of 101 bit times or 12 bytes (excluding SOF, EOD, NB and EOF).
• SOF - Start of Frame Symbol
All messages transmitted onto the J1850 bus must begin with an long active SOF symbol. This indicates to any listeners on the
J1850 bus the start of a new message transmission. The SOF symbol is not used in the CRC calculation.
• Data - In Message Data Bytes
The data bytes contained in the message include the message priority/type, message I.D. byte, and any actual data being transmitted
to the receiving node. See SAE J1850 - Class B Data Communications Network Interface, for more information about 1 and 3 Byte
Headers.
Table 20-12. BDLC Status Register
msb 0 1 2 3 4 5 6 7 lsb
R 0 0 0 0 0 0 0 IDLE
W Unimplemented Reserved Unimplemented
RESET: 0 0 0 0 0 0 0 0
= Unimplemented
SOF
E
O
D
EOF
Priority Message
Data
n
CRC IFR
I
F
S
Idle Idle
ID (Data1) (Data0)
Optional