
Phase-Locked Loop and Power Control
3-4
MC68328 DRAGONBALL PROCESSOR USER’S MANUAL
MOTOROLA
of 225 (decimal), every divisor is available to fine-tune the VCO in 32 kHz steps. The formula
for the dual-modulus divider is:
Divisor = 14 (P + 1) + Q + 1
Where:
1 <= Q <= 14
P >= Q + 1
Below the value of 225, some divisors are not allowed as the P and Q relationships cannot
be met.
3.3.3 Normal Startup
When the MC68328 processor is awakened from sleep mode by a system interrupt, the PLL
achieves lock within a few milliseconds. The crystal oscillator is always on after initial pow-
erup, so the crystal startup time is not a factor. The master clock starts operation after the
PLL achieves lock.
3.3.4 Change of Frequency
To change the VCO frequency, use the sequence below. As the system clock is disabled
while the PLL loses then reacquires lock, disable all peripheral devices before making
changes to the frequency.
This fragment assumes all peripherals have been disabled and the CPU is operating at the
highest possible frequency (SYSCLK SEL = 7). FREQSEL is the address of the frequency-
select register. NEWFREQ is the new frequency value (P and Q values) to be programmed.
lea #$FFF202,A0
point to the Freq Sel Register
move.w #NEWFREQ,D1
prepare the new frequency (Q and P)
WAIT
move.w (A0),D0
get the contents of the register
bpl.w WAIT
wait for CLK32 to go high
move.w D1,(A0)
load the new frequency
WAIT1
move.w (A0),D0
the program will wait in WAIT1
bmi.w WAIT1
or WAIT2 during the period when the
WAIT2
move.w (A0),D0
PLL loses then reacquires lock
bpl.w WAIT2
* at this point, the PLL will have reacquired lock and SYSCLK will
* be stable at the new frequency and the program can continue
Normally, the master frequency will be changed only during the bootup sequence. While it
is possible to dynamically control the master frequency, it is recommended that the fre-
quency be set to its permanent value at bootup (or use the default).
3.3.5 PLL Shutdown
The procedure for PLL shut down to place the system in sleep mode is similar to changes
made to the frequency. The difference is that the system can be awakened only by an inter-
rupt or reset. While there are different approaches, the simplest is to synchronize the soft-
Содержание DragonBall MC68328
Страница 5: ...vi MC68328 DRAGONBALL PROCESSOR USER S MANUAL MOTOROLA ...
Страница 25: ...Overview 1 14 MC68328 DRAGONBALL PROCESSOR USER S MANUAL MOTOROLA ...
Страница 45: ...System Integration Module 2 20 MC68328 DRAGONBALL PROCESSOR USER S MANUAL MOTOROLA ...
Страница 103: ...Parallel Ports 7 16 MC68328 DRAGONBALL PROCESSOR USER S MANUAL MOTOROLA ...
Страница 117: ...UART 8 14 MC68328 DRAGONBALL PROCESSOR USER S MANUAL MOTOROLA ...
Страница 127: ...SPI Master 10 6 MC68328 DRAGONBALL PROCESSOR USER S MANUAL MOTOROLA ...
Страница 137: ...Electrical Characteristics PRELIMINARY MOTOROLA MC68328 DRAGONBALL PROCESSOR USER S MANUAL 13 4 ...