MC9S08EL32 Datasheet, PDF (224/356 Pages) Freescale Semiconductor, Inc – 8-Bit HCS08 Central Processor Unit (CPU)

Desired Bit Rate:
9,615 bps
External Crystal Frequency: 8.000 MHz
1 Second
104.004 μs
=
9,615 Bits
1 Bit
1 Second
2 Clock Out Period
250 ns
X
=
8,000,000 Clock Out Periods
1 SLIC Clock Period
1 SLIC Clock Period
104.004 μs
1 SLIC Clock Period
416.017 SLIC Clock Periods
X
=
1 Bit
250 ns
1 Bit
Therefore, the closest SLCBT value would be 416 SLIC clocks (SLCBT = 0x01A0).
Figure 12-20. SLCBT Value Calculation Example 4
This resolution affects the sampling accuracy of the SLIC module on receiving bytes, but only as far as
locating the sample point of each bit within a given byte. The best sample point of the bit may be off by as
much as one SLIC clock period from the exact center of the bit, if the proper SLCBT value for the desired
bit rate is an odd number of SLIC clock periods.
Figure 12-21 shows an example of this error. In this example, the user has additionally chosen an incorrect
value of 30 SLIC clocks for the length of one bit time, and a filter prescaler of 1. This makes little
difference in the receive sampling of this particular bit, as the sample point is still within the bit and the
digital filter will catch any noise pulses shorter than 16 filter clocks long.The ideal value of SLCBT would
be 35 SLIC clocks, but the closest available value is 34, placing the sample point at 17 SLIC clocks into
the bit.
The error in the bit time value chosen by the user in the above example will grow throughout the byte, as
the sample point for the next bit will be only 30 SLIC clock cycles later (1 full bit time at this bit rate
setting). The SLIC resynchronizes upon every falling edge received. In a 0x00 data byte, however, there
are no falling edges after the beginning of the start bit. This means that the accumulated error of the
sampling point over the data byte with these settings could be as high as 30 SLIC clock cycles (10 bits x
{2 SLIC clocks due to user error + 1 SLIC clock resolution error}) placing it at the boundary between the
last bit and the stop bit. This could result in missampling and missing a byte framing error on the last bit
on high speed communications when the SLCBT count is relatively low. A properly chosen SLCBT value
would result in a maximum error of 10 SLIC clock counts over a given byte. This is less than one filter
delay time, and will not cause missampling of any of the bits in that byte. At the falling edge of the next
start bit, the SLIC will resynchronize and any accumulated sampling error returns to 0. The sampling error
becomes even less significant at lower speeds, when higher values of SLCBT are used to define a bit time,
as the worst case bit time resolution error is still only one SLIC clock per bit (or maximum of 10 SLIC
clocks per byte).
MC9S08EL32 Series and MC9S08SL16 Series Data Sheet, Rev. 3
226
Freescale Semiconductor