Information

14.4.3 Run modes
The device contains two different run modes:
• Run
• Very Low-Power Run (VLPR)
14.4.3.1 RUN mode
This is the normal operating mode for the device.
This mode is selected after any reset. When the ARM processor exits reset, it sets up the
stack, program counter (PC), and link register (LR):
• The processor reads the start SP (SP_main) from vector-table offset 0x000
• The processor reads the start PC from vector-table offset 0x004
• LR is set to 0xFFFF_FFFF.
To reduce power in this mode, disable the clocks to unused modules using their
corresponding clock gating control bits in the SIM's registers.
14.4.3.2 Very-Low Power Run (VLPR) mode
In VLPR mode, the on-chip voltage regulator is put into a stop mode regulation state. In
this state, the regulator is designed to supply enough current to the MCU over a reduced
frequency. To further reduce power in this mode, disable the clocks to unused modules
using their corresponding clock gating control bits in the SIM's registers.
Before entering this mode, the following conditions must be met:
• The MCG must be configured in a mode which is supported during VLPR. See the
Power Management details for information about these MCG modes.
• All clock monitors in the MCG must be disabled.
• The maximum frequencies of the system, bus, flash, and core are restricted. See the
Power Management details about which frequencies are supported.
• Mode protection must be set to allow VLP modes, that is, PMPROT[AVLP] is 1.
• PMCTRL[RUNM] is set to 10b to enter VLPR.
• Flash programming/erasing is not allowed.
NOTE
Do not change the clock frequency while in VLPR mode,
because the regulator is slow responding and cannot manage
Functional Description
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
274 Freescale Semiconductor, Inc.