Datasheet

874
SAM9X35 [DATASHEET]
11055E–ATARM–10-Mar-2014
41.7.2.3 Transmit Mailbox
When transmitting a message, the message length and data are written to the transmit mailbox with the correct identifier.
For each transmit mailbox, a priority is assigned. The controller automatically sends the message with the highest priority
first (set with the field PRIOR in CAN_MMRx register).
It is also possible to configure a mailbox in Producer Mode. In this mode, when a remote frame is received, the mailbox
data are sent automatically. By enabling this mode, a producer can be done using only one mailbox instead of two: one
to detect the remote frame and one to send the answer.
41.7.3 Time Management Unit
The CAN Controller integrates a free-running 16-bit internal timer. The counter is driven by the bit clock of the CAN bus
line. It is enabled when the CAN controller is enabled (CANEN set in the CAN_MR register). It is automatically cleared in
the following cases:
 After a reset
 When the CAN controller is in Low-power Mode is enabled (LPM bit set in the CAN_MR and SLEEP bit set in the
CAN_SR)
 After a reset of the CAN controller (CANEN bit in the CAN_MR register)
 In Time-triggered Mode, when a message is accepted by the last mailbox (rising edge of the MRDY signal in the
CAN_MSR
last_mailbox_number
register).
The application can also reset the internal timer by setting TIMRST in the CAN_TCR register. The current value of the
internal timer is always accessible by reading the CAN_TIM register.
When the timer rolls-over from FFFFh to 0000h, TOVF (Timer Overflow) signal in the CAN_SR register is set. TOVF bit in
the CAN_SR register is cleared by reading the CAN_SR register. Depending on the corresponding interrupt mask in the
CAN_IMR register, an interrupt is generated while TOVF is set.
In a CAN network, some CAN devices may have a larger counter. In this case, the application can also decide to freeze
the internal counter when the timer reaches FFFFh and to wait for a restart condition from another device. This feature is
enabled by setting TIMFRZ in the CAN_MR register. The CAN_TIM register is frozen to the FFFFh value. A clear
condition described above restarts the timer. A timer overflow (TOVF) interrupt is triggered.
To monitor the CAN bus activity, the CAN_TIM register is copied to the CAN _TIMESTP register after each start of frame
or end of frame and a TSTP interrupt is triggered. If TEOF bit in the CAN_MR register is set, the value is captured at each
End Of Frame, else it is captured at each Start Of Frame. Depending on the corresponding mask in the CAN_IMR
register, an interrupt is generated while TSTP is set in the CAN_SR. TSTP bit is cleared by reading the CAN_SR register.
The time management unit can operate in one of the two following modes:
 Timestamping mode: The value of the internal timer is captured at each Start Of Frame or each End Of Frame
 Time Triggered mode: A mailbox transfer operation is triggered when the internal timer reaches the mailbox trigger
Timestamping Mode is enabled by clearing TTM field in the CAN_MR register. Time Triggered Mode is enabled by
setting TTM field in the CAN_MR register.
Table 41-5.
Mailbox Object Type Description
Transmit
The message stored in the mailbox data registers will try to win the bus arbitration immediately
or later according to or not the Time Management Unit configuration (see Section 41.7.3).
The application is notified that the message has been sent or aborted.
Producer
The message prepared in the mailbox data registers will be sent after receiving the next remote
frame. This extends transmit mailbox features.