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參數(shù)資料
| 型號： | ML9620GAZ210 |
| 廠商： | OKI ELECTRIC INDUSTRY CO LTD |
| 元件分類： | 微控制器/微處理器 |
| 英文描述： | 1 CHANNEL(S), 1M bps, LOCAL AREA NETWORK CONTROLLER, PQFP44 |
| 封裝： | 0.80 MM PITCH, PLASTIC, QFP-44 |
| 文件頁數(shù)： | 48/79頁 |
| 文件大?。?/td> | 449K |
| 代理商： | ML9620GAZ210 |
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ML9620 User’s Manual
Chapter 3
Operational Description
3 - 12
3.10 Configuration of the Bit Timing
Even if minor errors in the configuration of the CAN bit timing do not result in immediate failure, the performance
of a CAN network can be reduced significantly.
In many cases, the CAN bit synchronisation will amend a faulty configuration of the CAN bit timing to such a
degree that only occasionally an error frame is generated. In the case of arbitration however, when two or more
CAN nodes simultaneously try to transmit a frame, a misplaced sample point may cause one of the transmitters to
become error passive.
The analysis of such sporadic errors requires a detailed knowledge of the CAN bit synchronisation inside a CAN
node and of the CAN nodes’ interaction on the CAN bus.
3.10.1 Bit Time and Bit Rate
CAN supports bit rates in the range of lower than 1 kBit/s up to 1000 kBit/s.
Each member of the CAN network
has its own clock generator, usually a quartz oscillator.
The timing parameter of the bit time (i.e. the reciprocal
of the bit rate) can be configured individually for each CAN node, creating a common bit rate even though the
CAN node’s oscillator periods (fosc) may be different.
The frequencies of these oscillators are not absolutely stable, small variations are caused by changes in
temperature or voltage and by deteriorating components.
As long as the variations remain inside a specific
oscillator tolerance range (df), the CAN nodes are able to compensate for the different bit rates by resynchronising
to the bit stream.
According to the CAN specification, the bit time is divided into four segments (see figure 3-6).
The
Synchronisation Segment, the Propagation Time Segment, the Phase Buffer Segment 1, and the Phase Buffer
Segment 2.
Each segment consists of a specific, programmable number of time quanta (see Table 3-1).
The
length of the time quantum (tq), which is the basic time unit of the bit time, is defined by the CAN controller’s
system clock fsys and the Baud Rate Prescaler (BRP) : tq = BRP / fsys.
The ML9620’s system clock fsys is the
frequency of its fosc input.
The Synchronisation Segment Sync_Seg is that part of the bit time where edges of the CAN bus level are expected
to occur; the distance between an edge that occurs outside of
Sync_Seg and the Sync_Seg is called the phase
error of that edge.
The Propagation Time Segment Prop_Seg is intended to compensate for the physical delay
times within the CAN network.
The Phase Buffer Segments Phase_Seg1 and Phase_Seg2 surround the Sample
Point. The Synchronisation Jump Width (SJW) defines how far a resynchronisation may move the Sample Point
inside the limits defined by the Phase Buffer Segments to compensate for edge phase errors.
Sync Seg.
Prop. Seg.
Phase Seg. 1
Phase Seg. 2
1Time Quantum (tq)
Sample Point
1 Bit Time
Figure 3-6 Bit Timing
Table 3-1 Parameter of the CAN Bit Time
Parameter
Range
Remark
BRP
[1 .. 32]
defines the length of the time quantum tq
Sync Seg..
1 tq
fixed length, synchronisation of bus input to system clock
Prop. Seg.
[1 .. 8] tq
compensates for the physical delay times
Phase Seg. 1
[1 .. 8] tq
may be lengthened temporarily by synchronisation
Phase Seg. 2
[1 .. 8] tq
may be shortened temporarily by synchronisation
SJW
[1 .. 4] tq
may not be longer than either Phase Buffer Segment
This table describes the minimum programmable ranges required by the CAN protocol
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