Freescale Semiconductor MPC5200B ユーザーズマニュアル
MPC5200B Users Guide, Rev. 1
20-46
Freescale Semiconductor
Functional Description
Because of the BDLC module’s architecture, it can both transmit and receive messages of unlimited length. The CRC calculations, both for
transmitting and receiving, are not limited to eight bytes, but will instead be calculated and verified using all bytes in the message, regardless
of the number. All control bits, including TEOD and IMSG, also work in an identical manner, regardless of the length of the message.
transmitting and receiving, are not limited to eight bytes, but will instead be calculated and verified using all bytes in the message, regardless
of the number. All control bits, including TEOD and IMSG, also work in an identical manner, regardless of the length of the message.
To transmit or receive these “Block Mode” messages, no extra BDLC module control functions must be performed. The user simply transmits
or receives as many bytes as desired in one message frame, and the BDLC module will operate just as if a message of normal length was being
used.
or receives as many bytes as desired in one message frame, and the BDLC module will operate just as if a message of normal length was being
used.
20.8.8.2
Transmitting Or Receiving A Message In 4X Mode
In a diagnostic or production environment large amounts of data may need to be downloaded across the network to a component or module.
This data is often sent in a large “Block Mode” message (see above) which violates the SAE J1850 limit for message length. In order to speed
up the downloading of these large blocks of data, they are sometimes transmitted at four times (4X) the normal bit rate for the Variable Pulse
Width modulation version of SAE J1850. This higher speed transmission, nominally 41.6kbps, allows these large blocks to be transmitted
much more quickly.
This data is often sent in a large “Block Mode” message (see above) which violates the SAE J1850 limit for message length. In order to speed
up the downloading of these large blocks of data, they are sometimes transmitted at four times (4X) the normal bit rate for the Variable Pulse
Width modulation version of SAE J1850. This higher speed transmission, nominally 41.6kbps, allows these large blocks to be transmitted
much more quickly.
The BDLC module is designed to receive and transmit messages at this higher speed. By setting the 4XE bit in BDLC Control Register 2, the
user can command the BDLC module to transmit and receive any message over the network at a 4X rate.
user can command the BDLC module to transmit and receive any message over the network at a 4X rate.
If the BDLC module is placed in this 4X mode, messages transmitted at the normal bit rate will not be received correctly. Likewise, 4X
messages transmitted on the SAE J1850 bus when the BDLC module is in normal mode will be interpreted as noise on the network by the
BDLC module. For more information on the 4XE bit, refer to Section • 4X Mode.
messages transmitted on the SAE J1850 bus when the BDLC module is in normal mode will be interpreted as noise on the network by the
BDLC module. For more information on the 4XE bit, refer to Section • 4X Mode.