MC9S08EL32 Series and MC9S08SL16 Series Data Sheet, Rev. 3
228
Freescale Semiconductor
Because perfect conditions are almost impossible to attain, more robust values must be chosen for bit rates.
For reliable communication, it is best to ensure that a bit time is no smaller 2x–3x longer than the filter
delay on the digital receive filter. This is true in LIN or BTM mode and ensures that valid data bits which
have been shortened due to ground shift, asymmetrical rise and fall times, etc., are accepted by the filter
without exception. This would translate to 2x to 3x reduction in the maximum speeds shown in
is at least twice the length of one filter delay value. If system noise is not adequately filtered out it might
be necessary to change the prescaler of the filter and lower the bit rate of the communication. If valid
communications are being absorbed by the filter, corrective action must be taken to ensure that either the
bit rate is reduced or whatever physical fault is causing bit times to shorten is corrected (ground offset,
asymmetrical rise/fall times, insufficient physical layer supply voltage, etc.).
12.6.17 Oscillator Trimming with SLIC
SLCACT can be used as an indicator of LIN bus activity. SLCACT tells the user that the SLIC is currently
processing a message header (therefore synchronizing to the bus) or processing a message frame
(including checksum). Therefore, at idle times between message frames or during a message frame which
has been marked as a “don’t care” by writing IMSG, it is possible to trim the oscillator circuit of the MCU
with no impact to the LIN communications.
It is important to note the exact mechanisms with which the SLIC sets and clears SLCACT. Any falling
edge which successfully passes through the digital receive filter will cause SLCACT to become set. This
might even include noise pulses, if they are of sufficient length to pass through the digital RX filter.
Although in these cases SLCACT is becoming set on a noise spike, it is very probable that noise of this
nature will cause other system issues as well such as corruption of the message frame. The software can
then further qualify if it is appropriate to trim the oscillator.
Table 12-15. Recommended Maximum Bit Rates for BTM Operation Due to Digital Filter
SLIC
Clock
(MHz)
Maximum BTM Bit Rate (kbps)
RXFP =
÷8 RXFP = ÷7 RXFP = ÷6 RXFP = ÷5 RXFP = ÷4 RXFP = ÷3RXFP = ÷2RXFP = ÷1
20
78.125
89.286
104.167
120.000
18
70.313
80.357
93.750
112.500
120.000
16
62.500
71.429
83.333
100.000
120.000
14
54.688
62.500
72.917
87.500
109.375
120.000
12
46.875
53.571
62.500
75.000
93.750
120.000
10
39.063
44.643
52.083
62.500
78.125
104.167
120.000
8
31.250
35.714
41.667
50.000
62.500
83.333
120.000
6
23.438
26.786
31.250
37.500
46.875
62.500
93.750
120.000
4
15.625
17.857
20.833
25.000
31.250
41.667
62.500
120.000
2
7.813
8.929
10.417
12.500
15.625
20.833
31.250
62.500