M_TTCAN - User Manual - Bosch Semiconductors and Sensors

Chapter 4.
4. TTCAN Operation
4.1 Reference Message
4.1.1 Level 1
Areference message is a data frame characterized by a specific CAN identifier. It is received and
accepted by all nodes except the Time Master (sender of the reference message).
For Level 1 the data length must be at least one; for Level 0,2 the data length must be at least four;
otherwise, the message is not accepted as reference message. The reference message may be
extended by other data up to the sum of eight CAN data bytes. All bits of the identifier except the
three LSBs characterize the message as a reference message. The last three bits specify the
priorities of up to 8 potential time masters. Reserved bits are transmitted as logical 0 and are ignored
by the receivers. The reference message is configured via register TTRMC.
The time master transmits the reference message. If the reference message is disturbed by an error,
it is retransmitted immediately. In case of a retransmission, the transmitted Master_Ref_Mark is
updated. The reference message is sent periodically, but is allowed to stop the periodic transmission
(Next_is_Gap bit) and to initiate transmission event-synchronized at the start of the next basic cycle
by the current time master or by one of the other potential time masters.
The node transmitting the reference message is the current time master. The time master is allowed
to transmit other messages. If the current time master fails, its function is replicated by the potential
time master with the highest priority. Nodes that are neither time master nor potential time master
are time-receiving nodes.
Level 1 operation is configured via TTOCF.OM = “01” and TTOCF.GEN. External clock
synchronization is not available in Level 1.
The information related to the reference message is stored in the first data byte as shown in Table 76
below. Cycle_Count is optional.
Bits 7 6 5 4 3 2 1 0
Table 76
First
Byte
Next_is_Gap res Cycle_Count[5:0]
First byte of Level 1 reference message
M_TTCAN - Revision 3.0.2 - 14.02.2013 97