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41.3.1 Power, clocking, and reset

The RTC is an always powered block that remains active in all low power modes and is

RTC registers retain their state during chip power-down and that the RTC time counter

remains operational.

The time counter within the RTC is clocked by a 32.768 kHz clock and can supply this

clock to other peripherals. The 32.768 kHz clock can only be sourced from an external

crystal using the oscillator that is part of the RTC module.

The RTC includes its own analog POR block, which generates a VBAT power-on-reset

signal whenever the RTC module is powered up and initializes all RTC registers to their

default state. A software reset bit can also initialize all RTC registers. The RTC also

monitors the chip power supply and electrically isolates itself when the rest of the chip is

powered down.

Any attempt to access an RTC register, except the access control registers, when VBAT

is powered down, when the RTC is electrically isolated, or when VBAT POR is asserted,

will result in a bus error.

41.3.1.1 Oscillator control

The 32.768 kHz crystal oscillator is disabled at VBAT POR and must be enabled by

software. After enabling the cystal oscillator, wait the oscillator startup time before

setting SR[TCE] or using the oscillator clock external to the RTC.

The crystal oscillator includes tunable capacitors that can be configured by software. Do

not change the capacitance unless the oscillator is disabled.

41.3.1.2 Software reset

Writing one to the CR[SWR] forces the equivalent of a VBAT POR to the rest of the

RTC module. The CR[SWR] is not affected by the software reset and must be cleared by

software. The access control registers are not affected by either VBAT POR or the

software reset; they are reset by the chip reset.

Functional description

K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012

1024

Preliminary

Freescale Semiconductor, Inc.

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