TC1796
System Units (Vol. 1 of 2)
Clock System and Control
User’s Manual
3-18
V2.0, 2007-07
Clock, V2.0
3.2.2.4
Changing PLL Parameters
There are some restrictions that must be regarded when PLL parameters in register
PLL_CLC are modified.
•
Only one parameter should be changed with a PLL_CLC register write operation
•
SYSFS can be changed without precautions
•
VCOBYP can be changed without precautions
•
PDIV and KDIV can be switched at any time in VCO Bypass Mode. However, the
maximum operating frequency of the TC1796 must not be exceeded.
•
Before changing VCOSEL, the VCO Bypass Mode must be selected.
•
Before deselecting the VCO Bypass Mode, PLL lock detection must be restarted
(RESLD bit set) and then the LOCK flag must be checked for the PLL lock condition.
When LOCK is set, the VCO Bypass Mode can be deselected again.
Note: PDIV and NDIV can also be switched in PLL Mode. When changing NDIV, the
VCO clock
f
VCO
may exceed the target frequency until the PLL becomes locked.
After changing PDIV or NDIV, wait for the PLL lock condition. This procedure is
typically used for increasing the VCO clock step-by-step.
3.2.2.5
Setting up the PLL after Reset
After reset, the system clock will be running at the VCO base frequency
f
VCObase
divided
by factor K. Now the following actions must be executed next:
1. Wait until the oscillator is running (OSC_CON.OSCR = 1)
2. Selection of the VCO Bypass Mode (PLL_CLC.VCOBYP = 1)
3. Selecting the VCO band by programming PLL_CON.VCOSEL
4. Program the desired P, N and K values (PDIV, NDIV, and KDIV bit fields of register
PLL_CLC)
5. Connect the oscillator to the PLL, default after reset (PLL_CLC.OSCDISC = 0)
6. Wait until the PLL becomes locked (PLL_CLC.LOCK = 1)
7. Disable the VCO Bypass Mode (PLL_CLC.VCOBYP = 0)
Table 3-6
Reset Values of Register PLL_CLC
Reset
BYPASS
Function
Reset Values
Power-on
reset
0
The system is driven by the PLL clock
based on the VCO base frequency.
0163 0F80
H
1
The system is driven directly by the
oscillator clock output.
2163 0F80
H
Other resets X
Register content remains unmodified
UUUU UUUU
H
1)
1) U = unchanged to previously programmed values.