NUC970 Technical Reference Manual
Publication Release Date: Dec. 15, 2015
- 774 -
Revision V1.30
NUC97
0
T
E
CHNIC
A
L
RE
F
E
RE
N
CE
MA
NUA
L
masked to “don’t care” may be overwritten in the Message Object.
Software can read or write each message any time through the Interface Registers and the Message
Handler guarantees data consistency in case of concurrent accesses.
Messages to be transmitted are updated by the application software. If a permanent Message Object
(arbitration and control bits are set during configuration) exists for the message, only the data bytes
are updated and the TxRqst bit with NewDat bit are set to start the transmission. If several transmit
messages are assigned to the same Message Object (when the number of Message Objects is not
sufficient), the whole Message Object has to be configured before the transmission of this message is
requested.
The transmission of any number of Message Objects may be requested at the same time. Message
objects are transmitted subsequently according to their internal priority. Messages may be updated or
set to not valid any time, even when their requested transmission is still pending. The old data will be
discarded when a message is updated before its pending transmission has started.
Depending on the configuration of the Message Object, the transmission of a message may be
requested autonomously by the reception of a remote frame with a matching identifier.
Disabled Automatic Retransmission
5.24.5.3
In accordance with the CAN Specification (see ISO11898, 6.3.3 Recovery Management), the C_CAN
provides means for automatic retransmission of frames that have lost arbitration or have been
disturbed by errors during transmission. The frame transmission service will not be confirmed to the
user before the transmission is successfully completed. This means that, by default, automatic
retransmission is enabled. It can be disabled to enable the C_CAN to work within a Time Triggered
CAN (TTCAN, see ISO11898-1) environment.
The Disabled Automatic Retransmission mode is enabled by setting the Disable Automatic
Retransmission (DAR) bit in the CAN Control Register to one. In this operation mode, the programmer
has to consider the different behavior of bits TxRqst and NewDat in the Control Registers of the
Message Buffers:
•
When a transmission starts, bit TxRqst of the respective Message Buffer is cleared, while bit
NewDat remains set.
•
When the transmission completed successfully, bit NewDat is cleared.
•
When a transmission fails (lost arbitration or error), bit NewDat remains set.
•
To restart the transmission, the software should set the bit TxRqst again.
5.24.6 Test Mode
Test Mode is entered by setting the Test bit in the CAN Control Register. In Test Mode, bits Tx1, Tx0,
LBack, Silent and Basic in the Test Register are writeable. Bit Rx monitors the state of the CAN_RX
pin and therefore is only readable. All Test Register functions are disabled when the Test bit is
cleared.
Silent Mode
5.24.6.1
The CAN Core can be set in Silent Mode by programming the Silent bit in the Test Register to one. In
Silent Mode, the C_CAN is able to receive valid data frames and valid remote frames, but it sends
only recessive bits on the CAN bus and it cannot start a transmission. If the CAN Core is required to
send a dominant bit (ACK bit, Error Frames), the bit is rerouted internally so that the CAN Core