TSS463-AAR ATMEL [ATMEL Corporation], TSS463-AAR Datasheet - Page 17

• Manufacturer Part Number: TSS463-AAR
• Description: VAN Data Link Controller with Serial Interface
• Manufacturer: ATMEL [ATMEL Corporation]

4205B–AUTO–12/04
tion can be performed is the identifier field. Since the logical zeroes on the bus are
dominant, and all data is transmitted with the most significant bit (MSB) first, the first
module to transmit a logical zero on the bus will be the prioritized module, i.e. the mes-
sage that is tagged with the lowest identifier will have priority over the other messages.
It is possible that two messages transmitted on the bus will have the same identifier. The
TSS463AA, therefore, continues the arbitration of the bus throughout the whole frame.
Moreover, if the identifier in transmission has been programmed for reception as well, it
transmits and receives messages simultaneously, right up until the Frame Check
Sequence (FCS). Only then, if the TSS463AA has transmitted the whole message, it
discard the message received. Arbitration loss in the FCS field is considered as a CRC
error during transmission.
This feature is called full data field arbitration, and it enables the user to extend the iden-
tifier. For instance it can be used to transmit the emitting modules address in the first
bytes of the data field, thus enabling the identifier to specify the contents of the frame
and the data field to specify the source of the information.
The identifier field of the VAN bus frame is always 12 bits long, and it is always followed
by the 4-bit command field:
•
•
•
•
All the bits in the command field are automatically handled by the TSS463AA, so the
user need not to be concerned for encoding and decoding of these bits. The command
bits transmitted on the VAN bus are calculated from the current status of the active
message.
The data field comes after the command field. This is just a sequence of bytes transmit-
ted MSB first. In the VAN standard the maximum message length is set to 28 bytes, but
the TSS463AA handles messages up to 30 bytes.
The next field is the FCS field. This field is a 15 bit CRC checksum defined by the follow-
ing generator polynomial g(x) of order 15:
g(x)= x
The division is done with a rest initialized to 0x7FFF, and an inversion of the CRC bits is
performed before transmission.
The first bit of the command is the extension bit (EXT). This bit is defined by the
user on transmission and is received and retained by the TSS463AA. To conform
with the standard it should be set to 1 (recessive) by the user, else the frame is
ignored without any IT generation.
The second bit is the request ACKnowledge bit (RAK). If this bit is a logical one, the
receiving module must acknowledge the transfer with an in-frame
acknowledgement in the ACK field. If it is set to logical zero, then the ACK field must
contain an acknowledge absent sequence.
The third bit is the Read/Write (R/W). This bit indicates the direction of the data in a
frame.
–
–
Last in the command field is the Remote Transmission Request bit (RTR). This bit is
a logical zero if the frame contains data and a logical one if the frame does not
contain data. In order to conform with the standard, a received frame includes the
combination R/W. RTR = 01 is ignored without any IT generation.
15
If set to zero, it is a "write" message, i.e., data transmitted by one module to
be received by another module.
If it is set to one, it implies a "read" message, i.e. a request that another
module should transmit data to be received by the one that requested the
data (reply request message).
+ x
11
+ x
10
+ x
9
+ x
8
+ x
7
+ x
4
+ x
3
+ x
2
+ 1
TSS463-AA
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