NXP Semiconductors K22F series, Reference Manual, Page: 1064 / 1407

42.3 Functional description
42.3.1 Power, clocking, and reset
The RTC is an always powered block that remains active in all low power modes and is
powered by the battery power supply (VBAT). The battery power supply ensures that the
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.
NOTE
An attempt to access an RTC register, except the access control
registers, results in a bus error when:
• VBAT is powered down,
• the RTC is electrically isolated, or
• VBAT POR is asserted.
To determine if the VBAT domain is active, software can use the ADC to measure
VBAT via an internal connection. See the chip-specific ADC information to find the
ADC input channel assignments.
42.3.1.1 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.
Functional description
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
1064 NXP Semiconductors