Chapter 16 Freescale’s Scalable Controller Area Network (S12MSCANV3)
MC9S12XE-Family Reference Manual , Rev. 1.19
654
Freescale Semiconductor
If there is no message transfer ongoing on the CAN bus, the minimum delay will be two additional bus
clocks and three additional CAN clocks. When all parts of the MSCAN are in initialization mode, the
INITAK flag is set. The application software must use INITAK as a handshake indication for the request
(INITRQ) to go into initialization mode.
NOTE
The CPU cannot clear INITRQ before initialization mode (INITRQ = 1 and
INITAK = 1) is active.
16.4.5.6
MSCAN Power Down Mode
The MSCAN is in power down mode (
•
CPU is in stop mode
or
•
CPU is in wait mode and the CSWAI bit is set
When entering the power down mode, the MSCAN immediately stops all ongoing transmissions and
receptions, potentially causing CAN protocol violations. To protect the CAN bus system from fatal
consequences of violations to the above rule, the MSCAN immediately drives the TXCAN pin into a
recessive state.
NOTE
The user is responsible for ensuring that the MSCAN is not active when
power down mode is entered. The recommended procedure is to bring the
MSCAN into Sleep mode before the STOP or WAI instruction (if CSWAI
is set) is executed. Otherwise, the abort of an ongoing message can cause an
error condition and impact other CAN bus devices.
In power down mode, all clocks are stopped and no registers can be accessed. If the MSCAN was not in
sleep mode before power down mode became active, the module performs an internal recovery cycle after
powering up. This causes some fixed delay before the module enters normal mode again.
16.4.5.7
Programmable Wake-Up Function
The MSCAN can be programmed to wake up the MSCAN as soon as CAN bus activity is detected (see
control bit WUPE in
Section 16.3.2.1, “MSCAN Control Register 0 (CANCTL0)
”). The sensitivity to
existing CAN bus action can be modified by applying a low-pass filter function to the RXCAN input line
while in sleep mode (see control bit WUPM in
Section 16.3.2.2, “MSCAN Control Register 1
This feature can be used to protect the MSCAN from wake-up due to short glitches on the CAN bus lines.
Such glitches can result from—for example—electromagnetic interference within noisy environments.
16.4.6
Reset Initialization
The reset state of each individual bit is listed in
Section 16.3.2, “Register Descriptions
,” which details all
the registers and their bit-fields.
Because
of
an
order
from
the
United
States
International
Trade
Commission,
BGA-packaged
product
lines
and
part
numbers
indicated
here
currently
are
not
available
from
Freescale
for
import
or
sale
in
the
United
States
prior
to
September
2010:
S12XE
products
in
208
MAPBGA
packages
