Rainbow Electronics CAN интерфейс - Manual (Page 69)

BOSCH
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Part B - page 67
The total number of TIME QUANTA in a bit time has to be programmable at least from
8 to 25.
Note:
It is often intended that control units do not make use of different oscillators for the local
CPU and its communication device. Therefore the oscillator frequency of a CAN device
tends to be that of the local CPU and is determined by the requirements of the control
unit. In order to derive the desired bitrate, programmability of the bittiming is necessary.
In case of CAN implementations that are designed for use without a local CPU the
bittiming cannot be programmable. On the other hand these devices allow to choose an
external oscillator in such a way that the device is adjusted to the appropriate bit rate
so that the programmability is dispensable for such components.
The position of the sample point, however, should be selected in common for all nodes.
Therefore the bit timing of SLIO devices must be compatible to the following definition
of the bit time:
HARD SYNCHRONIZATION
After a HARD SYNCHRONIZATION the internal bit time is restarted with SYNC_SEG.
Thus HARD SYNCHRONIZATION forces the edge which has caused the HARD
SYNCHRONIZATION to lie within the SYNCHRONIZATION SEGMENT of the
restarted bit time.
RESYNCHRONIZATION JUMP WIDTH
As a result of RESYNCHRONIZATION PHASE_SEG1 may be lengthened or
PHASE_SEG2 may be shortened. The amount of lengthening or shortening of the
PHASE BUFFER SEGMENTs has an upper bound given by the
RESYNCHRONIZATION JUMP WIDTH. The RESYNCHRONIZATION JUMP WIDTH
shall be programmable between 1 and min(4, PHASE_SEG1).
Clocking information may be derived from transitions from one bit value to the other.
The property that only a fixed maximum number of successive bits have the same
value provides the possibility of resynchronizing a bus unit to the bit stream during a
frame. The maximum length between two transitions which can be used for
resynchronization is 29 bit times.
1 Time
Quantum
1 Time
Quantum
4 Time Quanta 4 Time Quanta
1 Bit Time
10 Time Quanta
Sync-
Seg Prop-
Seg
Phase Buffer Seg. 1 Phase Buffer Seg. 2
Bit Timing