MC68HC05V12 Datasheet, PDF (150/246 Pages) Freescale Semiconductor, Inc – HCMOS Microcontreller Unit

Freescale Semiconductor, Inc.
Byte Data Link Controller – Digital
14.5.4 J1850 VPW Valid/Invalid Bits and Symbols
The timing tolerances for receiving data bits and symbols from the
J1850 bus have been defined to allow for variations in oscillator
frequencies. In many cases, the maximum time allowed to define a data
bit or symbol is equal to the minimum time allowed to define another data
bit or symbol.
Since the minimum resolution of the BDLC for determining what symbol
is being received is equal to a single period of the MUX interface clock
(t_BDLC), an apparent separation in these maximum time/minimum time
concurrences equals one cycle of t_BDLC.
This one clock resolution allows the BDLC to differentiate properly
between the different bits and symbols. This is done without reducing the
valid window for receiving bits and symbols from transmitters onto the
J1850 bus, which has varying oscillator frequencies.
In Huntsinger’s variable pulse width (VPW) modulation bit encoding, the
tolerances for both the passive and active data bits received and the
symbols received are defined with no gaps between definitions. For
example, the maximum length of a passive logic 0 is equal to the
minimum length of a passive logic 1, and the maximum length of an
active logic 0 is equal to the minimum length of a valid SOF symbol.
Invalid Passive Bit
See Figure 14-8(1). If the passive-to-active received transition
beginning the next data bit or symbol occurs between the active-to-
passive transition beginning the current data bit (or symbol) and a, the
current bit would be invalid.
Valid Passive Logic 0
See Figure 14-8(2). If the passive-to-active received transition
beginning the next data bit (or symbol) occurs between a and b, the
current bit would be considered a logic 0.
Valid Passive Logic 1
See Figure 14-8(3). If the passive-to-active received transition
beginning the next data bit (or symbol) occurs between b and c, the
current bit would be considered a logic 1.
Technical Data
Byte Data Link Controller – Digital (BDLC–D)
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MC68HC05V12 — Rev. 2.0