參數(shù)資料
型號： MC68HC05V7CFNR2
廠商： MOTOROLA INC
元件分類： 微控制器/微處理器
英文描述： 8-BIT, MROM, 2.1 MHz, MICROCONTROLLER, PQCC68
封裝： PLASTIC, LCC-68
文件頁數(shù)： 41/170頁
文件大小： 589K
代理商： MC68HC05V7CFNR2
SECTION 15: MESSAGE DATA LINK CONTROLLER
MOTOROLA
Page 121
MC68HC05V7 Specification Rev. 1.0
15.5.4.10
Valid SOF Symbol
If the active to passive transition beginning the next data bit or symbol occurs between
c
and
d, the current symbol would be considered a valid SOF symbol. See Figure 15-14(4).
15.5.4.11
Valid BREAK Symbol
If the next active to passive transition does not occur until after
d, the current symbol will
be considered a valid BREAK symbol. Following the BREAK symbol, an IFS period must
be observed, after which normal communication can resume on the J1805 bus. See Figure
15.5.5
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 MDLC wishes to transmit onto the J1850 bus, but detects that another message is in
progress, it must wait 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.
The VPW symbols and J1850 bus electrical characteristics are carefully chosen so that a
logic zero (active or passive type) will always dominate over a logic one (active or passive
type) simultaneously transmitted. Hence logic zeroes are said to be ’dominant’ and logic
ones are said to be ’recessive’. Whenever a node detects a dominant bit when it transmits
a recessive bit, it loses arbitration, and immediately stops transmitting. This is known as
’bitwise arbitration’.
Figure 15-15:
J1850 VPW Bitwise Arbitration
Transmitter A
Transmitter B
J1850 Bus
SOF
Data
Bit 1
Data
Bit 4
Data
Bit 5
"0"
Transmitter A detects
an active state on
the bus, and stops
transmitting
Transmitter B wins
Passive
Active
Passive
Active
Passive
Active
"0"
"1"
Data
Bit 2
"1"
Data
Bit 3
"0"
"1"
arbitration and
continues
transmitting
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