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Chapter 18

AFCAN Controller

User’s Manual U16580EE3V1UD00

18.14 Time Stamp Function

CAN is an asynchronous, serial protocol. All nodes connected to the CAN bus have a local, autono-
mous clock. As a consequence, the clocks of the nodes have no relation (i.e., the clocks are asynchro-
nous 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.

18.14.1 Time

stamp

function

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 occur-
rence 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)

(TSSEL = 0)

•

EOF event (last bit of end of frame)

(TSSEL = 1)

The TSOUT signal is enabled by setting the TSEN bit of the CnTS register to 1.

Figure 18-34:

Timing diagram of capture signal TSOUT

The TSOUT signal toggles its level upon occurrence of the selected event during data frame reception
(in <~Reference>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.

Caution

The time stamp function using the TSLOCK bit stops toggle of the TSOUT
signal by receiving a data frame in message buffer 0. Therefore, message
buffer 0 must be set as a receive message buffer. Since a receive message
buffer cannot receive a remote frame, toggle of the TSOUT signal cannot be
stopped by reception of a remote frame. Toggle of the TSOUT signal does not
stop when a data frame is received in a message buffer other than message
buffer 0.
For these reasons, a data frame cannot be received in message buffer 0 when
the CAN module is in the normal operation mode with ABT, because message
buffer 0 must be set as a transmit message buffer. In this operation mode,
therefore, the function to stop toggle of the TSOUT signal by the TSLOCK bit
cannot be used.

NEC V850E/PH2, User Manual

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