Advance Information
MC68HC08AS20
—
Rev. 4.1
338
Freescale Semiconductor
Valid SOF Symbol
In
Figure 20-9
(4)
, if the active-to-passive received transition
beginning the next data bit (or symbol) occurs between
c
and
d
, the
current symbol would be considered a valid SOF symbol.
Valid BREAK Symbol
In
Figure 20-10
, if the next active-to-passive received transition does
not occur until after
e
, the current symbol will be considered a valid
BREAK symbol. A BREAK symbol should be followed by a
start-of-frame (SOF) symbol beginning the next message to be
transmitted onto the J1850 bus. See
20.5.2 J1850 Frame Format
for
BDLC response to BREAK symbols.
Figure 20-10. J1850 VPW Received BREAK Symbol Times
20.5.5 Message Arbitration
Message arbitration on the J1850 bus is accomplished in a
non-destructive manner, allowing the message with the highest priority
to be transmitted, while any transmitters which lose arbitration simply
stop transmitting and wait for an idle bus to begin transmitting again.
If the BDLC wants to transmit onto the J1850 bus, but detects that
another message is in progress, it waits until the bus is idle. However, if
multiple nodes begin to transmit in the same synchronization window,
message arbitration will occur beginning with the first bit after the SOF
symbol and continue with each bit thereafter. If a write to the BDR (for
instance, to initiate transmission) occurred on or before
104 t
BDLC
from the received rising edge, then the BDLC will transmit
(2) VALID BREAK SYMBOL
240
μ
s
e
ACTIVE
PASSIVE
