TSS461F ATMEL Corporation, TSS461F Datasheet - Page 14

• Manufacturer Part Number: TSS461F
• Description: VAN Data Link Controller
• Manufacturer: ATMEL Corporation

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TSS461F
Once the bus is free, the module must now, if it is an autonomous module emits a SOF
sequence or, if it is a synchronous access module, wait until it detects a preamble sequence.
Up till this point there can be several modules transmitting on the bus, and there is no possibility
of knowing if this is the case or not. Therefore, the first field in which arbitration can be per-
formed 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 message that is tagged with the lowest identifier
will have priority over the other messages.
However it is possible that two messages transmitted on the bus will have the same identifier.
The TSS461F therefore, continues the arbitration of the bus throughout the whole frame. In
addition, if the identifier in transmission has been programmed for reception as well, it transmits
and receives messages simultaneously, right up till the Frame Check Sequence (FCS). Only
then, if the TSS461F has transmitted the whole message. It discards 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 identifier. 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:
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All the bits in the command field are automatically handled by the TSS461F, so the user doesn’t
need to be concerned for the encoding and decoding of these. The command bits transmitted on
the VAN bus are calculated from the current status of the active message.
After the command field comes the data field. This is just a sequence of bytes transmitted, MSB
first. In the VAN standard the maximum message length is set to 28 bytes, but the TSS461F
handles messages up to 30 bytes.
The next field is the FCS field. This field is a 15 bit CRC checksum defined by the following gen-
erator polynomial g(x) of order 15:
g(x) = x15 + x11 + x10 + x9 + x8 + x7 + x4 + x3 + x2 + 1
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 TSS461F. 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 bit (R/W). This bit indicates the direction of the data in a
frame.
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).
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 included the combination R/W. RTR =
01 is ignored without any IT generation.
7615A–AUTO–02/06