Datasheet

Byte Data Link Controller Module
Functional Description
MC9S12DP256 — Revision 1.1
Byte Data Link Controller Module
Rx & Tx Shadow
Registers
Immediately after the Rx Shift Register has completed shifting in a byte
of data, this data is transferred to the Rx Shadow Register and RDRF or
RXIFR is set and interrupt is generated if the interrupt enable bit (IE) in
BCR1 is set. After the transfer takes place, this new data byte in the Rx
Shadow Register is available to the IP Bus interface, and the Rx Shift
Register is ready to shift in the next byte of data. Data in Rx Shadow
Register must be retrieved by the CPU before it is overwritten by new
data from the Rx Shift Register.
Once the Tx Shift Register has completed its shifting operation for the
current byte, the data byte in the Tx Shadow Register is loaded into the
Tx Shift Register. After this transfer takes place, the Tx Shadow Register
is ready to accept new data from the IP Bus.
Digital Loopback
Multiplexer
The Digital Loopback Multiplexer connects the input of the receive digital
filter (See Figure 125) to either the transmit signal out to the pad (TxP)
or the receive signal from the pad (RxP), depending on the state of the
DLOOP bit in DLCBCR2 register.
State Machine All of the functions associated with performing the protocol are executed
or controlled by the State Machine. The State Machine is responsible for
framing, collision detection, arbitration, CRC generation/checking, and
error detection. The following sections describe the BDLC’s actions in a
variety of situations.
4X Mode The BDLC can exist on the same J1850 bus as modules which use a
special 4X (41.6 kbps) mode of J1850 VPW operation. The BDLC
cannot transmit in 4X mode, but can receive messages in 4X mode, if
the RX4X bit is set in BCR2 register. If the RX4X bit is not set in the
BCR2 register, any 4X message on the J1850 bus is treated as noise by
the BDLC and is ignored. Likewise, 4X messages transmitted on the
SAE J1850 bus when the BDLC is in normal mode will be interpreted as
noise on the network by the BDLC.
Receiving a
Message in Block
Mode
Although not a part of the SAE J1850 protocol, the BDLC does allow for
a special “Block Mode” of operation of the receiver. As far as the BDLC
is concerned, a Block Mode message is simply a long J1850 frame that
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