MC68HC908QL4 Datasheet, PDF (165/226 Pages) Motorola, Inc – Microcontrollers

Initialization/Application Information
14.9.15 Byte Transfer Mode Operation
This subsection describes the operation and limitations of the optional UART-like byte transfer mode
(BTM). This mode allows sending and receiving individual bytes, but changes the behavior of the SLCBT
registers (now read/write registers) and locks the SLCDLC to 1 byte data length. The SLCBT value now
becomes the bit time reference for the SLIC, where the software sets the length of one bit time rather than
the SLIC module itself. This is similar to an input capture/output compare (IC/OC) count in a timer module,
where the count value represents the number of SLIC clock counts in one bit time.
Byte transfer mode assumes that the user has a very stable, precise oscillator, resonator, or clock
reference input into the MCU and is therefore not appropriate for use with internal oscillators. There is no
synchronization method available to the user in this mode and the user must tell the SLIC how many clock
counts comprise a bit time. Figure 14-18, Figure 14-19, Figure 14-20, and Figure 14-21 show calculations
to determine the SLCBT value for different settings.
NOTE
It is possible to use the LIN autobauding circuitry in a non-LIN system to
derive the correct bit timing values if system constraints allow. To do this
the SLIC module must be activated in LIN mode (BTM=0) and receive a
break symbol, 0x55 data byte and one additional data byte (at the desired
BTM speed). Upon receiving this sequence of symbols which appears to be
a LIN header, the SLIC module will assert an ID received successfully
interrupt (SLCSV=0x2C). The value in the SLCBT registers will reflect the
bit rate which the 0x55 data character was received and can be saved to
RAM. The user then switches the SLIC into BTM mode and reloads this
value from RAM and the SLIC will be configured to communicate in BTM
mode at the baud rate which the 0x55 data character was sent.
In the example in Figure 14-18, the user should write 0x16, as a write of 0x15 (decimal value of 21) would
automatically revert to 0x14, resulting in transmitted bit times that are 1.33 SLIC clock periods too short
rather than 0.667 SLIC clock periods too long. The optimal choice, which gives the smallest resolution
error, is the closest even number of SLIC clocks to the exact calculated SLCBT value.
There is a trade-off between maximum bit rate and resolution with the SLIC in BTM mode. Faster SLIC
clock speeds improve resolution, but require higher numbers to be written to the SLCBT registers for a
given desired bit rate. It is up to the user to determine what level of resolution is acceptable for the given
application.
Desired Bit Rate:
External Crystal Frequency:
57,600 bps
4.9152 MHz
1 Second
17.36111 ms
=
57,600 Bits
1 Bit
1 Second
4 CGMXCLK Period
813.802 ns
X
=
4,915,200 CGMXCLK Periods
1 SLIC Clock Period
1 SLIC Clock Period
17.36111 ms
1 SLIC Clock Period
21.33 SLIC Clock Periods
X
=
1 Bit
813.802 ns
1 Bit
Therefore, the closest SLCBT value would be 21 SLIC clocks (SLCBT = 0x0015).
Because you can only use even values in SLCBT, the closest acceptable value is 22 (0x0016).
Figure 14-18. SLCBT Value Calculation Example 1
MC68HC908QL4 Data Sheet, Rev. 7
Freescale Semiconductor
165