MC68HC908AT32 Datasheet, PDF (340/378 Pages) Freescale Semiconductor, Inc – Microcontrollers

Byte Data Link Controller-Digital (BDLC-D)
Valid EOF and IFS Symbols
In Figure 28-8(1), if the passive-to-active received transition beginning the SOF symbol of the next
message occurs between a and b, the current symbol will be considered a valid end-of-frame (EOF)
symbol.
See Figure 28-8(2). If the passive-to-active received transition beginning the SOF symbol of the next
message occurs between c and d, the current symbol will be considered a valid EOF symbol followed
by a valid inter-frame separation symbol (IFS). 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, any time 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
In Figure 28-8(2), if the passive-to-active received transition beginning the start-of-frame (SOF) symbol
of the next message does not occur before d, the bus is considered to be idle, and any node wishing
to transmit a message may do so immediately.
200 µs
128 µs
64 µs
ACTIVE
(1) INVALID ACTIVE BIT
PASSIVE
a
ACTIVE
(2) VALID ACTIVE LOGIC 1
PASSIVE
a
b
ACTIVE
(3) VALID ACTIVE LOGIC 0
PASSIVE
ACTIVE
b
c
(4) VALID SOF SYMBOL
PASSIVE
c
d
Figure 28-9. J1850 VPW Received Active Symbol Times
Invalid Active Bit
In Figure 28-9(1), if the active-to-passive received transition beginning the next data bit (or symbol)
occurs between the passive-to-active transition beginning the current data bit (or symbol) and a, the
current bit would be invalid.
Valid Active Logic 1
In Figure 28-9(2), if the active-to-passive received transition beginning the next data bit (or symbol)
occurs between a and b, the current bit would be considered a logic 1.
MC68HC908AT32 Data Sheet, Rev. 3.1
340
Freescale Semiconductor