
Initialization/Application Information
MC68HC908QL4  MC68HC908QL3  MC68HC908QL2 Data Sheet, Rev. 4
Freescale Semiconductor
171
signal to become a logic level 0. Furthermore, the counter is prevented from underflowing and can only 
be incremented from this state.
The data latch will retain its value until the counter next reaches the opposite end point, signifying a 
definite transition of the SLCRX signal.
14.9.17.2  Digital Filter Performance
The performance of the digital filter is best described in the time domain rather than the frequency domain.
If the signal on the SLCRX signal transitions, then there will be a delay before that transition appears at 
the filtered Rx data output signal. This delay will be between 15 and 16 clock periods, depending on where 
the transition occurs with respect to the sampling points. This ‘filter delay’ is not an issue for SLIC 
operation, as there is no need for message arbitration.
The effect of random noise on the SLCRX signal depends on the characteristics of the noise itself. Narrow 
noise pulses on the SLCRX signal will be completely ignored if they are shorter than the filter delay. This 
provides a degree of low-pass filtering. 
Figure 14-23
 shows the configuration of the digital receive filter 
and the consequential effect on the filter delay. This filter delay value indicates that for a particular setup, 
only pulses of which are greater than the filter delay will pass the filter.
For example, if the frequency of the SLIC clock (f
SLIC
) is 3.2 MHz, then the period (t
SLIC
) is of the SLIC 
clock is 313 ns. With the default receive filter prescaler setting of division by 3, the resulting maximum 
filter delay in the absence of noise will be 15.00 
μ
s. By simply changing the prescaler of the receive filter, 
the user can then select alternatively 5
 μ
s, 10
 μ
s, or 20
 μ
s as a minimum filter delay according to the 
systems requirements.
If noise occurs during a symbol transition, the detection of that transition may be delayed by an amount 
equal to the length of the noise burst. This is just a reflection of the uncertainty of where the transition is 
truly occurring within the noise. 
NOTE
The user must always account for the worst case bit timing of their LIN bus 
when configuring the digital receive filter, especially if running at faster 
speeds. Ground offset and other physical layer conditions can cause 
shortening of bits as seen at the digital receive pin, for example. If these 
shortened bit lengths are less than the filter delay, the bits will be 
interpreted by the filter as noise and will be blocked, even though the 
nominal bit timing might be greater than the filter delay.