XC886/888CLM
Power Supply, Reset and Clock Management
User’s Manual
7-13
V1.3, 2010-02
Power, Reset and Clock, V 1.0
Changing PLL Parameters
To change the PLL parameters, first check if the oscillator is running
(OSC_CON.OSCR = 1). In this case:
1. Select VCO bypass mode (VCOBYP = 1).
2. Program desired NDIV value.
3. Connect oscillator to PLL (OSCDISC = 0).
4. Wait till the LOCK bit has been set.
5. Disable VCO bypass mode.
Select the External Oscillator
To select the external oscillator, the following sequence must be performed:
1. Select the VCO bypass mode (VCOBYP = 1).
2. Disconnect the oscillator from the PLL (OSCDISC = 1).
3. External OSC is powered up by resetting bit XPD.
4. The source of external oscillator is selected by setting bit OSCSS.
5. Wait until the external oscillator is stable
1)
(the delay time should be adjusted
according to different external oscillators).
6. Restart the Oscillator Run Detection by setting bit OSC_CON.ORDRES.
7. Wait for 2048 cycles based on VCO frequency.
If bit OSC_CON.OSCR is set, then continue with the sequence below. Else, repeat the
sequence from step 6.
1. Reprogram the NDIV factor to the required value.
2. Reconnect oscillator to the PLL (OSCDISC = 0).
3. The RESLD bit must be set and the LOCK flag checked. Only if the LOCK flag is set
again, can the VCO bypass mode be deselected and normal operation resumed. If
the LOCK flag is still not set after 200
µ
s (maximum PLL lock-in time with a stable
oscillator; see product data sheet), repeat steps 6, 7 before restarting the lock
detection and checking the LOCK flag.
In order to minimize power consumption while the on-chip oscillator is used, XTAL is
powered down by setting bit XPD. When the external oscillator is used, the on-chip
oscillator can be powered down by setting bit OSCPD.
7.3.2
Clock Source Control
The clock system provides three ways to generate the system clock:
1) A stable oscillation is defined as an amplitude equal or more than 0.4*VDDC. See product data sheet.
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