Atmel SAM9X25, Manual

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11054A–ATARM–27-Jul-11
SAM9X25
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11054A–ATARM–27-Jul-11
SAM9X25
41.8.4.2 Time Triggered Mode
In Time Triggered Mode, basic cycles can be split into several time windows. A basic cycle starts
with a reference message. Each time a window is defined from the reference message, a trans-
mit operation should occur within a pre-defined time window. A mailbox must not win the
arbitration in a previous time window, and it must not be retried if the arbitration is lost in the time
window.
Figure 41-20. Time Triggered Principle
Time Trigger Mode is enabled by setting the TTM field in the CAN_MR register. In Time Trig-
gered Mode, as in Timestamp Mode, the CAN_TIMESTP field captures the values of the internal
counter, but the MTIMESTAMP fields in the CAN_MSRx registers are not active and are read
at 0.
Synchronization by a Reference Message
In Time Triggered Mode, the internal timer counter is automatically reset when a new message
is received in the last mailbox. This reset occurs after the reception of the End Of Frame on the
rising edge of the MRDY signal in the CAN_MSRx register. This allows synchronization of the
internal timer counter with the reception of a reference message and the start a new time
window.
Transmitting within a Time Window
A time mark is defined for each mailbox. It is defined in the 16-bit MTIMEMARK field of the
CAN_MMRx register. At each internal timer clock cycle, the value of the CAN_TIM is compared
with each mailbox time mark. When the internal timer counter reaches the MTIMEMARK value,
an internal timer event for the mailbox is generated for the mailbox.
In Time Triggered Mode, transmit operations are delayed until the internal timer event for the
mailbox. The application prepares a message to be sent by setting the MTCR in the CAN_MCRx
register. The message is not sent until the CAN_TIM value is less than the MTIMEMARK value
defined in the CAN_MMRx register.
If the transmit operation is failed, i.e., the message loses the bus arbitration and the next trans-
mit attempt is delayed until the next internal time trigger event. This prevents overlapping the
next time window, but the message is still pending and is retried in the next time window when
CAN_TIM value equals the MTIMEMARK value. It is also possible to prevent a retry by setting
the DRPT field in the CAN_MR register.
Freezing the Internal Timer Counter
The internal counter can be frozen by setting TIMFRZ in the CAN_MR register. This prevents an
unexpected roll-over when the counter reaches FFFFh. When this occurs, it automatically
freezes until a new reset is issued, either due to a message received in the last mailbox or any
other reset counter operations. The TOVF bit in the CAN_SR register is set when the counter is
Reference
Message
Reference
Message
Global Time
Time Cycle
Time Windows for Messages