Datasheet

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
Table 12-14. Recommended maximum bit rates are shown in Table 12-15, and ensure that a single bit time
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 89.286 104.167 120.000 120.000 120.000 120.000 120.000
18 70.313 80.357 93.750 112.500 120.000 120.000 120.000 120.000
16 62.500 71.429 83.333 100.000 120.000 120.000 120.000 120.000
14 54.688 62.500 72.917 87.500 109.375 120.000 120.000 120.000
12 46.875 53.571 62.500 75.000 93.750 120.000 120.000 120.000
10 39.063 44.643 52.083 62.500 78.125 104.167 120.000 120.000
8 31.250 35.714 41.667 50.000 62.500 83.333 120.000 120.000
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