XC886/888CLM
Controller Area Network (MultiCAN) Controller
User’s Manual
15-30
V1.3, 2010-02
MultiCAN, V1.0
15.1.8.2
Frame Transmission
The process of a message object transmission is shown in
. With the copy
of the message object content to be transmitted (identifier, IDE bit, RTR = DIR bit, DLC,
and for data frames also the data field) into the internal transmit buffer of the assigned
CAN node, also several status flags are served and monitored to control consistent data
handling.
The transmission process of a message object starting after the transmit acceptance
filtering is identical for remote and data frames.
MSGVAL, TXRQ, TXEN0, TXEN1
A message can only be transmitted if all four bits in MOSTATn Register MSGVAL
(“Message Valid”), TXRQ (“Transmit Request”), TXEN0 (“Transmit Enable 0”), TXEN1
(“Transmit Enable 1”) are set. Although these bits are equivalent with respect to the
transmission process, they have different semantics:
Table 15-3
Message Transmission Bit Definitions
Bit
Description
MSGVAL
Message Valid
This is the main switch bit of the message object.
TXRQ
Transmit Request
This is the standard transmit request bit. This bit must be set whenever a
message object is to be transmitted. TXRQ is cleared by hardware at the
end of a successful transmission, except when there is new data (indicated
by NEWDAT = 1) to be transmitted.
When bit MOFCRn.STT (“Single Transmit Trial”) is set, TXRQ is already
cleared when the content of the message object is copied into the transmit
frame buffer of the CAN node.
A received remote request (after a remote frame reception) sets bit TXRQ
to request the transmission of the requested data frame.
TXEN0
Transmit Enable 0
This bit can be temporarily cleared by software to suppress the
transmission of this message object when it writes new content to the data
field. This avoids transmission of inconsistent frames that consist of a
mixture of old and new data.
Remote requests are still accepted when TXEN0 = 0, but transmission of
the data frame is suspended until transmission is re-enabled by software
(setting TXEN0).
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