Time Stamp Function - NEC V850ES/F 3-L Series User Manual

Chapter 18

18.14 Time Stamp Function

18.14.1 Time stamp function
SOF
TSOUT
Figure 18-34
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CAN is an asynchronous, serial protocol. All nodes connected to the CAN bus
have a local, autonomous clock. As a consequence, the clocks of the nodes
have no relation (i.e., the clocks are asynchronous and may have different
frequencies).
In some applications, however, a common time base over the network
(= global time base) is needed. In order to build up a global time base, a time
stamp function is used. The essential mechanism of a time stamp function is
the capture of timer values triggered by signals on the CAN bus.
The CAN Controller supports the capturing of timer values triggered by a
specific frame. An on-chip 16-bit capture timer unit in a microcontroller system
is used in addition to the CAN Controller. The 16-bit capture timer unit captures
the timer value according to a trigger signal (TSOUT) for capturing that is
output when a data frame is received from the CAN Controller. The CPU can
retrieve the time of occurrence of the capture event, i.e., the time stamp of the
message received from the CAN bus, by reading the captured value. The
TSOUT signal can be selected from the following two event sources and is
specified by the TSSEL bit of the CnTS register.
• SOF event (start of frame)
• EOF event (last bit of end of frame)
The TSOUT signal is enabled by setting the TSEN bit of the CnTS register to
1.
SOF
Timing diagram of capture signal TSOUT
The TSOUT signal toggles its level upon occurrence of the selected event
during data frame reception (in Figure 18-34, the SOF is used as the trigger
event source). To capture a timer value by using the TSOUT signal, the capture
timer unit must detect the capture signal at both the rising edge and falling
edge.
This time stamp function is controlled by the TSLOCK bit of the CnTS register.
When TSLOCK is cleared to 0, the TSOUT signal toggles upon occurrence of
the selected event. If TSLOCK is set to 1, the TSOUT signal toggles upon
occurrence of the selected event, but the toggle is stopped as the TSEN bit is
automatically cleared to 0 as soon as the message storing to the message
buffer 0 starts. This suppresses the subsequent toggle occurrence by the
TSOUT signal, so that the time stamp value toggled last (= captured last) can
be saved as the time stamp value of the time at which the data frame was
received in message buffer 0.
User's Manual U18743EE1V2UM00
(TSSEL = 0)
(TSSEL = 1)
SOF
SOF
CAN Controller (CAN)
t