參數(shù)資料
型號: CR16MCS9VJE8
英文描述: Microcontroller
中文描述: 微控制器
文件頁數(shù): 88/157頁
文件大小: 1256K
代理商: CR16MCS9VJE8
www.national.com
88
Frame Fields
Data and remote frames consist of the following different bit
fields:
— Start of Frame
— Arbitration Field
— Control Field
— Data Field
— CRC Field
— ACK Field
— EOF Field
The
Start of Frame
indicates the beginning of data and re-
mote frames. It consists of a single 'dominant' bit. A node is
only allowed to start transmission when the bus is idle. All
nodes have to synchronize to the leading edge (first edge af-
ter the bus was idle) caused by SOF of the node which starts
transmission first.
The
Arbitration field
consists of the identifier field and the
RTR (Remote Transmission Request) bit. For extended
frames there is also a SRR (Substitute Remote Request) and
a IDE (ID Extension) bit inserted between ID18 and ID17 of
the identifier field. The value of the RTR bit is 'dominant' in a
data frame and 'recessive' in a remote frame.
The
Control field
consists of six bits. For standard frames it
starts with the ID Extension bit (IDE) and a reserved bit
(RB0). For extended frames the control field starts with two
reserved bits (RB1, RB0). These bits are followed by the 4-
bit Data Length Code (DLC).
The CR16CAN receiver accepts all possible combinations of
the reserved bits (RB1, RB0). The transmitter must be con-
figured to send only '0' bits.
The DLC indicates the number of bytes in the data field. It
consists of four bits. The data field can be of length zero. The
admissible number of data bytes for a data frame ranges
from 0 to 8.
The
Data field
consists of the data to be transferred within a
data frame. It can contain 0 to 8 bytes. A remote frame has
no data field.
The
CRC field
consists of the CRC sequence followed by the
CRC delimiter. The CRC sequence is derived by the trans-
mitter from the modulo 2 division of the preceding bit fields,
starting with the SOF up to the end of the data field, excluding
stuff-bits, by the generator polynomial:
The remainder of this division is the CRC sequence transmit-
ted over the bus. On the receiver side, the module divides all
bit fields up to the CRC delimiter excluding stuff-bits, and
checks if the result is zero. This will then be interpreted as a
valid CRC. After the CRC sequence a single ‘recessive’ bit is
transmitted as the CRC delimiter.
The
ACK field
is two bits long and contains the ACK slot and
the ACK delimiter. The ACK slot is filled with a ‘recessive’ bit
by the transmitter. This bit is overwritten with a ‘dominant’ bit
by every receiver that has received a correct CRC sequence.
The second bit of the ACK field is a ‘recessive’ bit called the
acknowledge delimiter.
The
End of Frame field
closes a data and a remote frame.
It consists of seven ‘recessive’ bits.
Data Frame
The structure of a standard and extended data frame is
shown in Figure 46.
A CAN data frame consists of the following fields as previ-
ously described:
— Start of Frame (SOF)
— Arbitration field + Extended Arbitration
— Control field
— Data field
— Cyclic Redundancy Check field (CRC)
— Acknowledgment field (ACK)
— End of Frame (EOF)
Remote Frame
Figure 47 shows the structure of a standard and extended re-
mote frame.
A remote frame is comprised of the following fields sections,
which is the same as a data frame (see Frame Fields on page
88) except for the data field, which is not present.
— Start of Frame (SOF)
— Arbitration field + Extended Arbitration
— Control field
— Cyclic Redundancy Check field (CRC)
— Acknowledgment field (ACK)
— End of Frame (EOF)
Note that the DLC must have the same value as the corre-
sponding data frame to prevent contention on the bus. The
RTR bit is ‘recessive’.
original or destuffed bit
stream
10000011111x
a
a. x = {0,1}
01111100000x
stuffed bit stream
100000
1
1111
0
1
x
011111
0
0000
1
0
x
x
15
+ x
14
+ x
10
+ x
8
+ x
7
+ x
4
+ x
3
+ 1
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