MC68HC08AS32 Datasheet, PDF (334/394 Pages) Motorola, Inc – M68HC08 Family of 8-bit microcontroller units (MCUs)

Byte Data Link Controller-Digital (BDLC-D)
Advance Information
334
symbol, if no response is transmitted after an EOD symbol, it
becomes an EOF, and the message is assumed to be completed. The
EOF flag is set upon receiving the EOF symbol.
IFS — Inter-Frame Separation Symbol
The IFS symbol is a 20-µs passive period on the J1850 bus which
allows proper synchronization between nodes during continuous
message transmission. The IFS symbol is transmitted by a node after
the completion of the end-of-frame (EOF) period and, therefore, is
seen as a 300-µs passive period.
When the last byte of a message has been transmitted onto the J1850
bus and the EOF symbol time has expired, all nodes then must wait
for the IFS symbol time to expire before transmitting a start-of-frame
(SOF) symbol, marking the beginning of another message.
However, if the BDLC is waiting for the IFS period to expire before
beginning a transmission and a rising edge is detected before the IFS
time has expired, it will synchronize internally to that edge. If a write
to the BDR register (for instance, to initiate transmission) occurred on
or before 104 • t_BDLC from the received rising edge, then the BDLC
will transmit and arbitrate for the bus. If a CPU write to the BDR
register occurred after 104 • t_BDLC from the detection of the rising
edge, then the BDLC will not transmit, but will wait for the next IFS
period to expire before attempting to transmit the byte.
A rising edge may occur during the IFS period because of varying
clock tolerances and loading of the J1850 bus, causing different
nodes to observe the completion of the IFS period at different times.
To allow for individual clock tolerances, receivers must synchronize to
any SOF occurring during an IFS period.
BREAK — Break
The BDLC cannot transmit a BREAK symbol.
If the BDLC is transmitting at the time a BREAK is detected, it treats
the BREAK as if a transmission error had occurred and halts
transmission.
If the BDLC detects a BREAK symbol while receiving a message, it
treats the BREAK as a reception error and sets the invalid symbol flag
in the BSVR, also ignoring the frame it was receiving. If while
Byte Data Link Controller-Digital (BDLC-D)
MC68HC08AS32 — Rev. 3.0
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