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Power Management
9-8
Freescale Semiconductor
9.3.1
Peripheral Shut Down
All peripherals, except for the SCM and crossbar switch, may have the software remove their input clocks
individually to reduce power consumption. See
Section 9.2.4, “Peripheral Power Management Registers
for more information. A peripheral may be disabled at any time and remains
disabled during any low-power mode of operation.
9.3.2
Limp mode
The device may also boot into a low-frequency limp mode, in which the PLL is bypassed and the device
runs from a factor of the input clock (EXTAL). In this mode, EXTAL feeds a counter that divides the input
clock by 2
n
, where
n
is the value of the programmable counter field, CDR[LPDIV]. The programmed
value of the divider may be changed without glitches or otherwise negative affects to the system. While in
this mode, the PLL is placed in bypass mode to reduce overall system-power consumption.
Limp mode may be entered and exited by writing to MISCCR[LIMP].
While in this mode, a 2:1 ratio maintains between the core and the primary bus clock. Because they do not
function at speeds as low as the minimum input clock frequency, the SDRAM controller, USB On-to-Go,
FECs, PCI controller, and ATA controller are not functional in limp mode.
9.3.3
Low-Power Modes
The system enters a low-power mode by executing a STOP instruction. The low-power mode the device
actually enters (stop, wait, or doze) depends on the setting of the WCR[LPMD] bits. Entry into any of these
modes idles the CPU with no cycles active, powers down the system, and stops all internal clocks
appropriately. During stop mode, the system clock is stopped low.
A wake-up event is required to exit a low-power mode and return to run mode. Wake-up events consist of
any of these conditions:
•
Any type of reset
•
Any valid, enabled interrupt request
Exiting from low-power mode via an interrupt request requires:
•
An interrupt request whose priority is higher than the value programmed in the WCR[PRILVL].
•
An interrupt request whose priority is higher than the value programmed in the interrupt priority
mask (I) field of the core’s status register.
•
An interrupt request from a source not masked in the interrupt controller’s interrupt mask register.
•
An interrupt request which has been enabled at the module of the interrupt’s origin.
9.3.3.1
Run Mode
Run mode is the normal system operating mode. Current consumption in this mode is related directly to
the system clock frequency.
Summary of Contents for MCF54455
Page 33: ...xxviii Freescale Semiconductor ...
Page 67: ...Freescale Semiconductor 1 ...
Page 125: ...Freescale Semiconductor 1 ...
Page 145: ...Enhanced Multiply Accumulate Unit EMAC 5 21 Freescale Semiconductor ...
Page 173: ...Cache 6 28 Freescale Semiconductor ...
Page 179: ...Static RAM SRAM 7 6 Freescale Semiconductor ...
Page 207: ...Power Management 9 16 Freescale Semiconductor ...
Page 323: ...Reset Controller Module 13 8 Freescale Semiconductor ...
Page 389: ...Pin Multiplexing and Control 16 44 Freescale Semiconductor ...
Page 575: ...PCI Bus Controller 22 58 Freescale Semiconductor ...
Page 600: ...Advanced Technology Attachment ATA Freescale Semiconductor 23 25 ...
Page 601: ...Freescale Semiconductor 1 ...
Page 842: ...I2 C Interface Freescale Semiconductor 33 16 ...
Page 843: ...Freescale Semiconductor 1 ...
Page 921: ...Revision History A 6 Freescale Semiconductor ...