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Q: How can I change the Periodic Interrupt Timer (PIT) "tick" time for the SYSCALL timing
functions?
A: Change the Periodic Interrupt Timer .PITR parameter to alter the "tick" count. This
parameter's value is placed into the PITR register by the PWR_INI routine.
Q: How can I change the default RS-232 communications parameters, such as baud rate, parity,
number of data bits, stop bits, and XON/XOFF flow control?
A: Use the Console Default Table for SCI (.BAUD, .PARITY, .DATA, .STOP, .XON_ENB,
.XON, and .XOFF) to change these parameters.
Q: How can I change the crystal frequency? Can I use an external clock?
A: Change the FCRYSTAL parameter to alter the crystal frequency for the on-board Voltage
Controlled Oscillator (VCO). To use an external clock, the FEXTAL parameter must be set
to the external clock frequency and the MODCLK* line must be held low during reset.
CPU32Bug monitors the MODCLK* signal after reset to determine which parameter to use
when calculating SCI baud rates.
Q: Why do certain baud rates fail to work after I change the crystal frequency or use an external
clock?
A: There is an integral relationship between the system clock rate (F
rates, as per Section 5.6.3.1 SCI CONTROL REGISTER 0 (SCCR0), in the MC68332 User's
Manual, MC68332UM/AD (or in the previous MC68332 System Integration Module User's
Manual, SIM32UM/AD), as defined by the following equation:
where SCBR equals {1, 2, 3, ..., 8191}. For a specific baud rate to function, the difference
between the Nominal Baud Rate and the Actual Baud Rate as typified by Table 5-13, should
be kept within 3% for reliable operation. Reliable communication also depends greatly upon
the ability of the communications hardware at the other end, i.e., a modern VLSI UART
device, such as found in the IBM-PC, might tollerate baud rate error differences up to 5%.
In summary, all baud rates may not be available depending upon the system clock rate used.
M68CPU32BUG REV 1
SCI baud = System Clock/(32 x SCBR)
C-20
USER CUSTOMIZATION
) and QSCI baud
SYSTEM
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