s908gz60cfa Freescale Semiconductor, Inc, s908gz60cfa Datasheet - Page 84

• Manufacturer Part Number: s908gz60cfa
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Clock Generator Module (CGM)
PLLON — PLL On Bit
BCS — Base Clock Select Bit
VPR1 and VPR0 — VCO Power-of-Two Range Select Bits
84
This read/write bit activates the PLL and enables the VCO clock, CGMVCLK. PLLON cannot be
cleared if the VCO clock is driving the base clock, CGMOUT (BCS = 1). (See
Circuit.) Reset sets this bit so that the loop can stabilize as the MCU is powering up.
This read/write bit selects either the crystal oscillator output, CGMXCLK, or the VCO clock,
CGMVCLK, as the source of the CGM output, CGMOUT. CGMOUT frequency is one-half the
frequency of the selected clock. BCS cannot be set while the PLLON bit is clear. After toggling BCS,
it may take up to three CGMXCLK and three CGMVCLK cycles to complete the transition from one
source clock to the other. During the transition, CGMOUT is held in stasis. (See
Selector
These read/write bits control the VCO’s hardware power-of-two range multiplier E that, in conjunction
with L controls the hardware center-of-range frequency, f
PLLON bit is set. Reset clears these bits. (See
4.5.5 PLL VCO Range Select
1 = PLL on
0 = PLL off
1 = CGMVCLK divided by two drives CGMOUT
0 = CGMXCLK divided by two drives CGMOUT
Circuit.) Reset clears the BCS bit.
Do not inadvertently clear the PLLF bit. Any read or read-modify-write
operation on the PLL control register clears the PLLF bit.
PLLON and BCS have built-in protection that prevents the base clock
selector circuit from selecting the VCO clock as the source of the base clock
if the PLL is off. Therefore, PLLON cannot be cleared when BCS is set, and
BCS cannot be set when PLLON is clear. If the PLL is off (PLLON = 0),
selecting CGMVCLK requires two writes to the PLL control register. (See
4.3.8 Base Clock Selector
Verify that the value of the VPR1 and VPR0 bits in the PCTL register are
appropriate for the given reference and VCO clock frequencies before
enabling the PLL. See
on selecting the proper value for these control bits.
MC68HC908GZ60 • MC68HC908GZ48 • MC68HC908GZ32 Data Sheet, Rev. 6
1. Do not program E to a value of 3.
VPR1 and VPR0
Table 4-4. VPR1 and VPR0 Programming
00
01
10
Register.)
4.3.6 Programming the PLL
Circuit.).
2
NOTE
NOTE
NOTE
E
0
1
(1)
4.3.3 PLL
VRS
Circuits,
VCO Power-of-Two
Range Multiplier
. VPR1:VPR0 cannot be written when the
for detailed instructions
1
2
4
4.3.6 Programming the
4.3.8 Base Clock Selector
Freescale Semiconductor
4.3.8 Base Clock
PLL, and