MPC555
/
MPC556
CLOCKS AND POWER CONTROL
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
8-15
The switching from state three to state four is accomplished by clear-
ing the STBUC and LOCSS bits. If the switching is done when the
PLL is not locked, the system clock will not oscillate until lock condi-
tion is met.
The default value of the LME bit is determined by MODCK[1:3] during assertion of
the PORESET line. The configuration modes are shown in
8.8 Low-Power Modes
The LPM and other bits in the PLPRCR are encoded to provide one normal operating
mode and four low-power modes. In normal and doze modes the system can be in
high state with frequency defined by the DFNH bits, or in
the low state with frequency
defined by the DFNL bits. The normal-high operating mode is the state out of reset.
This is also the state of the bits after the low-power mode exit signal arrives.
There are four low-power modes:
• Doze mode
• Sleep mode
• Deep-sleep mode
• Power-down mode
8.8.1 Entering a Low-Power Mode
Low-power modes are enabled by setting the POW bit in the MSR and clearing the
LPML (low-power mode lock) bit in the PLPRCR. Once enabled, a low-power mode is
entered by setting the LPM bits to the appropriate value. This can be done only in one
of the normal modes. The user cannot change the LPM or CSRC bits when the MCU
is in doze mode.
summarizes the control bit descriptions for the different clock power modes.
Table 8-3 Status of Clock Source
STATE
PORESET
HRESET
LME
LOCS
(status)
LOCSS
(sticky)
STBUC
BUCS
Chip
Clock
Source
1
0
0
1
0
0
0
1
BUCLK
2
1
0
1
0/1
0
0
1
BUCLK
3
1
NOTES:
1. At least one of the two bits, LOCSS or BUCS, must be asserted (one) in this state.
2. X = don’t care.
1
1
1
x
2
0/1
0/1
1
BUCLK
4
1
0
0/1
0
x
2
0
0
Oscillator
5
1
1
0/1
0
x
2
0
0
Oscillator
6
1
0
1
0/1
1
0/1
1
BUCLK
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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