COP888EB Datasheet, PDF (36/75 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 8k Memory, CAN Interface, 8-Bit A/D, and USART

Frame Formats (Continued)
FIGURE 27. CAN Bus States
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SYNCHRONIZATION
Every receiver starts with a “hard synchronization” on the
falling edge of the SOF bit. One bit time consists of four bit
segments: Synchronization segment, propagation segment,
phase segment 1 and phase segment 2.
A falling edge of the data signal should be in the synchroni-
zation segment. This segment has the fixed length of one
time quanta. To compensate for the various delays within a
network, the propagation segment is used. Its length is pro-
grammable from 1 to 8 time quanta. Phase segment 1 and
phase segment 2 are used to resynchronize during an active
frame. The length of these segments is from 1 to 8 time
quanta long.
Two types of synchronization are supported:
Hard synchronization is done with the falling edge on the
bus while the bus is idle, which is then interpreted as the
SOF. It restarts the internal logic.
Soft synchronization is used to lengthen or shorten the bit
time while a data or remote frame is received. Whenever a
falling edge is detected in the propagation segment or in
phase segment 1, the segment is lengthened by a specific
value, the resynchronization jump width (see Figure 29 ).
If a falling edge lies in the phase segment 2 (as shown in Fig-
ure 29 ) it is shortened by the resynchronization jump width.
Only one resynchronization is allowed during one bit time.
The sample point lies between the two phase segments and
is the point where the received data is supposed to be valid.
The transmission point lies at the end of phase segment 2 to
start a new bit time with the synchronization segment.
1. The resynchronization jump width (RJW) is automati-
cally determined from the programmed value of PS. If a
soft resynchronization is done during phase segment 1
or the propagation segment, then RJW will either be
equal to 4 internal CAN clocks (CKI/(1 + divider)) or the
programmed value of PS, whichever is less. PS2 will
never be shorter than 1 internal CAN clock.
2. (PS1 — BTL settings any PSC setting) The PS1 of the
BTL should always be programmed to values greater
than 1. To allow device resynchronization for positive
and negative phase errors on the bus. (if PS1 is pro-
grammed to one, a bit time could only be lengthened
and never shortened which basically disables half of the
synchronization).
A) Synchronization segment
B) Propagation segment
FIGURE 28. Bit Timing
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