TC1796
Peripheral Units (Vol. 2 of 2)
Controller Area Network (MultiCAN) Controller
User’s Manual
22-13
V2.0, 2007-07
MultiCAN, V2.0
as sending every basic cycle, sending every second basic cycle, or sending only once
within the matrix cycle.
22.1.6.5 Generation of the Network Time Unit (NTU)
The granularity of any timing information within TTCAN is the NTU. The cycle time is
measured in NTU that is based on the nominal CAN bit time (TTCAN level 1), or on the
Time Quantum
t
Q
(TTCAN level 2), or on the CAN input frequency. In level 2, the node’s
local relation between the physical oscillator of a TTCAN controller and the system wide
NTU must be established. A clock signal in the node provides the system clock to a
frequency divider. This frequency divider generates the system-wide NTU while a node
local TUR takes care for the correct relationship between the system clock and NTU.
NTU now can be used to calculate a local time and to build the global time.
22.1.6.6 Global Time Generation and Drift Correction
The node sending the reference message is the time master in the TTCAN network. The
reference message includes the time master’s (by definition correct) global time value
for frame synchronization. All nodes take a snap-shot of their time values at the SOF bit
sample point of the received reference message. After reference message reception,
each node can calculate its local offset that indicates the difference between the master
global time value and the own local time value. In the current basic cycle, the node can
compute the global time for the next basic cycle by:
global time = local time + local offset
This equation ensures that all nodes have a consistent global time. Due to slightly
different clock drifts of the different nodes in a CAN network, a mechanism is established
that guarantees that local and global time run in parallel. This mechanism is the
continuous update of the TUR. The initial value of TUR is locally known in the node by
the node clock signal specification. During node operation, the node locally measures
the length between two successive reference messages in number of oscillator periods
and in local time. The quotient of these two values gives the actual TUR. The achievable
precision determines a reasonable choice of the NTU value in physical seconds.