Civil Engineering Reference
In-Depth Information
3.3.4
Reception of Messages
The reception of messages in RS-CAN is possible using several methods, but every
method begins in the filtering section of the
multi-reception handler
. Here, the
AFL
determines where a received message will go. For each received message, the AFL
is parsed for a match. If a match is found, the associated AFL entry contains up to
eight storage targets, which can be loaded in parallel with the message. Valid stor-
age targets are reception FIFO units, multi-purpose FIFO units, and a selectable
receive message buffer.
An AFL entry is shown in Fig.
3.7
.
The AFL entry stores
identifier
values (ID) for the standard and extended identi-
fier frame formats of CAN, and associated
mask
flags, where the relevance of each
identifier bit (including remote flag bit
RTR
and extended flag
IDE
) can be masked.
Masked bits will be set as “don't care” for the filtering; this works like a so-called
wildcard.
Reference
DLC
values can be entered for DLC checking. If a received message
matches the ID, but does not have enough data bytes as specified in the DLC speci-
fication of the AFL entry, it will not pass the filter.
Furthermore, the AFL entry contains several pointers:
• A flat RX direction pointer with its associated enable flag (FE). If FE is set, the
message will be stored in the receive message buffer with the number of the
pointer value.
• One or several FIFO direction pointers. Here, each bit represents one of the avai-
lable receive or multi-purpose FIFO units where the message can be stored.
• An additional pointer (PTR) value which is a freely configurable value. This
value will be attached as a property with the message, so that an identification of
the message is possible. This functionality corresponds with the HRH values of
AUTOSAR communication stacks.
3.3.4.1
Reception into a Receive Message Buffer
The receive message buffer stores the whole message including the
PTR
value and
a reception time stamp. Old data within the buffer are overwritten.
The method of storing a message in a receive message buffer is designed to be
used in conjunction with polled message reception, i.e. for non-interruptive infor-
mation, which is read and checked by software at certain intervals.
3.3.4.2
Reception into a FIFO Unit
This kind of reception is used for streamed and interruptive data processing. Sort-
ing into different FIFO units makes it possible to distinguish between higher and
lower priority messages. Every FIFO unit can be configured when its interrupt is
