MC68HC908QL4 Datasheet, PDF (152/226 Pages) Motorola, Inc – Microcontrollers

Slave LIN Interface Controller (SLIC) Module
The SLIC clock is the same as the CPU bus clock. The module is designed to provide better than 1% bit
rate accuracy at the lowest value of the SLIC clock frequency and the accuracy improves as the SLIC
clock frequency is increased. For this reason, it is advantageous to choose the fastest SLIC clock which
is still within the acceptable operating range of the SLIC. Because the SLIC may be used with MCUs with
internal oscillators, the tolerance of the oscillator must be taken into account to ensure that SLIC clock
frequency does not exceed the bounds of the SLIC clock operating range. This is especially important if
the user wishes to use the oscillator untrimmed, where process variations might result in MCU frequency
offsets of ±25%.
The acceptable range of SLIC clock frequencies is 2–8 MHz to guarantee LIN operations with greater than
1.5% accuracy across the 1–20 kbps range of LIN bit rates. The user must ensure that the fastest possible
SLIC clock frequency never exceeds 8 MHz or that the slowest possible SLIC clock never falls below 2
MHz under worst case conditions. This would include, for example, oscillator frequency variations due to
untrimmed oscillator tolerance, temperature variation, or supply voltage variation.
To initialize the SLIC module into LIN operating mode, the user must perform the following steps prior to
needing to receive any LIN message traffic. These steps assume the MCU has been reset either by a
power-on reset (POR) or any other MCU reset mechanism.
The steps for SLIC Initialization for LIN operation are:
1. Write SLCC1 to clear INITREQ.
2. When INITACK = 0, write SLCC1 & SLCC2 with desired values for:
a. SLCWCM — Wait clock mode (default = leaving SLIC clock running when in CPU wait).
3. Write SLCP to set up prescalers for:
a. RXFP — Digital receive filter clock prescaler (default = SLIC divided by 3).
4. Enable the SLIC module by writing SLCC2:
a. SLCE = 1 to place SLIC module into run mode.
b. BTM = 0 to disable byte transfer mode (default).
5. Write SLCC1 to enable SLIC interrupts (if desired).
14.9.6.2 Byte Transfer Mode Initialization
Bit rate synchronization is handled automatically in LIN mode, using the synchronization data contained
in each LIN message to derive the desired bit rate. In byte transfer mode (BTM = 1); however, the user
must set up the bit rate for communications using the clock selection, prescaler, and SLCBT.
More information on byte transfer mode is described in 14.9.15 Byte Transfer Mode Operation, including
the performance parameters on recommended maximum speeds, bit time resolution, and oscillator
tolerance requirements.
After the desired settings of prescalers and bit time are determined, the SLIC Initialization for BTM
operation is virtually identical to that of LIN operation.
The steps are:
1. Write SLCC1 to clear INITREQ.
2. When INITACK = 0, write SLCC2 with desired values for:
a. SLCWCM — Wait clock mode (default = leaving SLIC clock running when in CPU wait).
3. Write SLCICP to set up prescalers for:
a. RXFP — Digital receive filter clock prescaler (default = SLIC divided by 3).
MC68HC908QL4 Data Sheet, Rev. 7
152
Freescale Semiconductor