Board Users Guide

Functional Description
MPC5200B Users Guide, Rev. 1
Freescale Semiconductor 20-23
Figure 20-7. J1850 VPW EOF and IFS Symbols
• Valid EOF & IFS Symbol
In Figure 20-7(1), if the passive to active transition beginning the SOF symbol of the next message occurs between T
rv4(Min)
and
T
rv4(Max)
, the current symbol will be considered a valid EOF symbol.
If the passive to active transition beginning the SOF symbol of the next message occurs after T
rv5(Min)
, the current symbol will be
considered a valid EOF symbol followed by a valid IFS symbol. See Figure 20-7(2). All nodes must wait until a valid IFS symbol
time has expired before beginning transmission. However, due to variations in clock frequencies and bus loading, some nodes may
recognize a valid IFS symbol before others, and immediately begin transmitting. Therefore, anytime a node waiting to transmit
detects a passive to active transition once a valid EOF has been detected, it should immediately begin transmission, initiating the
arbitration process.
• Idle Bus
If the passive to active transition beginning the SOF symbol of the next message does not occur before T
tv5(Min)
, the bus is
considered to be idle, and any node wishing to transmit a message may do so immediately.
(2) Valid EOF+
IFS Symbol
280µs
300µs
T
rv4(Min)
T
rv4(Max)
(1) Valid EOF
Symbol
Active
Passive
Active
Passive
T
rv5(Min)