Interrupts
register disables the PLL and clears the BCS bit in the PLL control register (PCTL). (See 6.3.8 Base
Clock Selector Circuit and 6.3.7 Special Programming Exceptions.). Reset initializes the register to
$40 for a default range multiply value of 64.
NOTE
The VCO range select bits have built-in protection such that they cannot be
written when the PLL is on (PLLON = 1) and such that the VCO clock
cannot be selected as the source of the base clock (BCS = 1) if the VCO
range select bits are all clear.
The PLL VCO range select register must be programmed correctly.
Incorrect programming can result in failure of the PLL to achieve lock.
6.5.5 PLL Reference Divider Select Register
The PLL reference divider select register (PMDS) contains the programming information for the modulo
reference divider.
Address:
$003B
Bit 7
0
6
0
5
0
4
0
3
RDS3
0
2
RDS2
0
1
RDS1
0
Bit 0
RDS0
1
Read:
Write:
Reset:
0
0
0
0
= Unimplemented
Figure 6-9. PLL Reference Divider Select Register (PMDS)
RDS[3:0] — Reference Divider Select Bits
These read/write bits control the modulo reference divider that selects the reference division factor, R.
(See 6.3.3 PLL Circuits and 6.3.6 Programming the PLL.) RDS[3:0] cannot be written when the PLLON
bit in the PCTL is set. A value of $00 in the reference divider select register configures the reference
divider the same as a value of $01. (See 6.3.7 Special Programming Exceptions.) Reset initializes the
register to $01 for a default divide value of 1.
NOTE
The reference divider select bits have built-in protection such that they
cannot be written when the PLL is on (PLLON = 1).
NOTE
The default divide value of 1 is recommended for all applications.
6.6 Interrupts
When the AUTO bit is set in the PLL bandwidth control register (PBWC), the PLL can generate a CPU
interrupt request every time the LOCK bit changes state. The PLLIE bit in the PLL control register (PCTL)
enables CPU interrupts from the PLL. PLLF, the interrupt flag in the PCTL, becomes set whether
interrupts are enabled or not. When the AUTO bit is clear, CPU interrupts from the PLL are disabled and
PLLF reads as logic 0.
Software should read the LOCK bit after a PLL interrupt request to see if the request was due to an entry
into lock or an exit from lock. When the PLL enters lock, the divided VCO clock, CGMPCLK, divided by
two can be selected as the CGMOUT source by setting BCS in the PCTL. When the PLL exits lock, the
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor
93
FREESCALE [ Freescale ]
