Datasheet

Byte Data Link
MC9S12DP256 — Revision 1.1
Byte Data Link Controller Module
Transmitting A Message
The design of the BDLC module enables the user to easily handle
message reception and message transmission separately. This can
greatly simplify the communication software, as all received messages
can be handled virtually the same, regardless of their origin.
This chapter will therefore describe only the steps necessary for
transmitting a message, and will not address the resulting reception of
that message by the BDLC. Message reception is described in
Receiving A Message. Later sections will deal with transmitting and
receiving In-Frame Responses on the SAE J1850 bus.
BDLC Transmission
Control Bits
There is only one BDLC control bit which is used when transmitting a
message onto the SAE J1850 bus. This bit, the Transmit End of Data
(TEOD) bit, is set by the user to indicate to the BDLC that the last byte
of that part of the message frame has been loaded into the DLCBDR.
The TEOD bit, located in DLCBCR2, is also used when transmitting an
In-Frame Response (IFR), but that usage is described in Transmitting
An In-Frame Response (IFR). Setting the TEOD bit indicates to the
BDLC that the last byte written to the BDLC Data Register is the final
byte to be transmitted, and that following this byte a CRC byte and EOD
symbol should be transmitted automatically. Setting the TEOD bit will
also inhibit any further TDRE interrupts until TEOD is cleared. The
TEOD bit will be cleared on the rising edge of the first bit of the
transmitted CRC byte, or if an error or loss of arbitration is detected on
the bus.
BDLC Data
Register
The BDLC Data Register is a double-buffered register which is used for
handling the transmitted and received message bytes.Bytes to be
transmitted onto the SAE J1850 bus are written to the DLCBDR, and
bytes received from the bus by the BDLC are read from the DLCBDR.
Since this register is double buffered, bytes written into it cannot be read
by the CPU. If this is attempted, the byte which is read will be the last
byte placed in the DLCBDR by the BDLC, not the last byte written to the
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