S12 Clock, Reset and Power Management Unit (S12CPMU)
7.3.2.6
S12CPMU Clock Select Register (CPMUCLKS)
This register controls S12CPMU clock selection.
0x0039
7
6
5
4
3
2
1
0
R
W
0
0
RTI
OSCSEL
COP
OSCSEL
PLLSEL
PSTP
PRE
PCE
Reset
1
0
0
0
0
0
0
0
= Unimplemented or Reserved
Figure 7-9. S12CPMU Clock Select Register (CPMUCLKS)
Read: Anytime
Write:
1. Only possible when PROT=0 (CPMUPROT register).
2. All bits anytime in Special Modes.
3. PLLSEL, PSTP, PRE, PCE, RTIOSCSEL: Anytime in Normal Mode.
4. COPOSCSEL: Anytime in normal mode until CPMUCOP write once has taken place.
If COPOSCSEL was cleared by UPOSC=0 (entering Full Stop Mode with COPOSCSEL=1
or insufficient OSCCLK quality), then COPOSCSEL can be set once again.
After writing CPMUCLKS register, it is strongly recommended to read
back CPMUCLKS register to make sure that write of PLLSEL,
RTIOSCSEL and COPOSCSEL was successful.
Table 7-5. CPMUCLKS Descriptions
Description
Field
7
PLL Select Bit
PLLSEL
This bit selects the PLLCLK as source of the System Clocks (Core Clock and Bus Clock).
PLLSEL can only be set to 0, if UPOSC=1.
UPOSC= 0 sets the PLLSEL bit.
Entering Full Stop Mode sets the PLLSEL bit.
0 System clocks are derived from OSCCLK if oscillator is up (UPOSC=1, fbus = fosc / 2.
1 System clocks are derived from PLLCLK, fbus = fPLL / 2.
6
Pseudo Stop Bit
PSTP
This bit controls the functionality of the oscillator during Stop Mode.
0 Oscillator is disabled in Stop Mode (Full Stop Mode).
1 Oscillator continues to run in Stop Mode (Pseudo Stop Mode), option to run RTI and COP.
Note: Pseudo Stop Mode allows for faster STOP recovery and reduces the mechanical stress and aging of the
resonator in case of frequent STOP conditions at the expense of a slightly increased power consumption.
Note: When starting up the external oscillator (either by programming OSCE bit to 1 or on exit from Full Stop
Mode with OSCE bit is already 1) the software must wait for a minimum time equivalent to the startup-time
of the external oscillator tUPOSC before entering Pseudo Stop Mode.
S12P-Family Reference Manual, Rev. 1.13
Freescale Semiconductor
211
FREESCALE [ Freescale ]
