MPC555
/
MPC556
CAN 2.0B CONTROLLER MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
16-9
quanta clock operates at the S-clock frequency.
provides examples of IMB
clock, CAN bit rate, and S-clock bit timing parameters. Refer to
for more information on the bit timing registers.
16.3.3.1 Configuring the TouCAN Bit Timing
The following considerations must be observed when programming bit timing func-
tions.
• If the programmed PRESDIV value results in a single IMB clock per one time
quantum, then the PSEG2 field in CANCTRL2 register must not be programmed
to zero.
• If the programmed PRESDIV value results in a single IMB clock per one time
quantum, then the information processing time (IPT) equals three time quanta;
otherwise it equals two time quanta. If PSEG2 equals two, then the TouCAN
transmits one time quantum late relative to the scheduled sync segment.
• If the prescaler and bit timing control fields are programmed to values that result
in fewer than 10 IMB clock periods per CAN bit time and the CAN bus loading is
100%, then any time the rising edge of a start-of-frame (SOF) symbol transmitted
by another node occurs during the third bit of the intermission between messag-
es, the TouCAN may not be able to prepare a message buffer for transmission in
time to begin its own transmission and arbitrate against the message which trans-
mitted the early SOF.
• The TouCAN bit time must be programmed to be greater than or equal to nine
IMB clocks, or correct operation is not guaranteed.
16.3.4 Error Counters
The TouCAN has two error counters, the transmit (Tx) error counter and the receive
(Rx) error counter. Refer to
for more information on error
counters.The rules for increasing and decreasing these counters are described in the
CAN protocol, and are fully implemented in the TouCAN. Each counter has the
following features:
• 8-bit up/down-counter
• Increment by eight (Rx error counter also increments by one)
• Decrement by one
• Avoid decrement when equal to zero
Table 16-8 Example IMB Clock, CAN Bit Rate and S-Clock Frequencies
IMB Clock
Frequency
(MHz)
CAN Bit Rate
(MHz)
Possible S-Clock
Frequency (MHz)
Possible Number of
Time Quanta/Bit
PRESDIV Value + 1
25
1
25
25
1
20
1
10, 20
10, 20
2, 1
16
1
8, 16
8, 16
2, 1
25
0.125
1, 1.25, 2.5
8,10, 20
25, 20,10
20
0.125
1, 2, 2.5
8, 16, 20
20, 10, 8
16
0.125
1, 2
8,16
16, 8
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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