MC68HC08AS20 Datasheet, PDF (331/386 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

NOTE: The J1850 protocol BREAK symbol is not related to the HC08 Break
Module (See Section 11. Break Module (Break).)
IDLE — Idle Bus
An idle condition exists on the bus during any passive period after
expiration of the IFS period (for example, > 300 µs). Any node sensing
an idle bus condition can begin transmission immediately.
20.5.3 J1850 VPW Symbols
Huntsinger’s variable pulse width modulation (VPW) is an encoding
technique in which each bit is defined by the time between successive
transitions and by the level of the bus between transitions, (for instance,
active or passive). Active and passive bits are used alternately. This
encoding technique is used to reduce the number of bus transitions for
a given bit rate.
Each logic 1 or logic 0 contains a single transition and can be at either
the active or passive level and one of two lengths, either 64 µs or 128 µs
(t_NOM at 10.4 kbps baud rate), depending upon the encoding of the
previous bit. The start-of-frame (SOF), end-of-data (EOD), end-of-frame
(EOF), and inter-frame separation (IFS) symbols always will be encoded
at an assigned level and length. See Figure 20-6.
Each message will begin with an SOF symbol, an active symbol, and,
therefore, each data byte (including the CRC byte) will begin with a
passive bit, regardless of whether it is a logic 1 or a logic 0.
All VPW bit lengths stated in the following descriptions are typical values
at a 10.4-kbps bit rate. EOF, EOD, IFS, and IDLE, however, are not
driven J1850 bus states. They are passive bus periods observed by
each node’s CPU.
MC68HC08AS20 — Rev. 4.1
Freescale Semiconductor
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