NXP Semiconductors PXN2020, Reference Manual, Page: 131 / 1376

System Clock Description
PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor 5-13
as the clock source for the second domain. Selecting the oscillator as the clock source ensures low jitter on
the FlexRay bus.
NOTE
To prevent improper FlexRay behavior, the system clock or the FlexRay
protocol engine clock source must be switched and stable before enabling
the FlexRay module. After it is enabled, the FlexRay module can be
disabled only by asserting RESET.
If the oscillator clock source is selected for the FlexRay interface, then a divided down 4 – 40 MHz XTAL
cannot be selected as the source for the system clock.
5.5.3 API / RTC Clock
Domains
The clock source for the RTC can be selected as one of the following:
• 32 kHz_XTAL
• 4 – 40 MHz XTAL
• 128 kHz_IRC
• 16 MHz_ IRC
If the 32 kHz crystal is used (enabled), it is assumed that the crystal always remains the clock source.
The API/RTC includes a divide of the clock sources. The division of these clock sources can be configured
to have a ÷512 and a ÷32. Both can be bypassed if required.
The 4 – 40 MHz XTAL and 16 MHz IRC can be optionally divided down by ÷1, ÷2, ÷4, ÷8, or ÷16 before
being supplied to the API/RTC. This allows the required wake-up times and resolution to be met since the
÷32 may be too large for some users of the RTC to meet the desired resolution. The 128 kHz IRC can be
divided by ÷1, ÷2, or ÷4. This allows the speed of the clock to be reduced to minimize power consumption
in low power modes and to clock the RTC and 32 kHz.
For accurate RTC support, the 32 kHz_XTAL and 4 – 40 MHz XTAL clocks can be used. The
32 kHz_XTAL is normally used, but there are use cases to have the main 4 – 40 MHz XTAL crystal
oscillator running in low power mode and clocking the RTC.
The maximum 4 – 40 MHz XTAL that can be run in low power mode is an 8 MHz crystal. The oscillator
supports an automatically switched low power output buffer to minimize the current consumption when
the 4 – 40 MHz XTAL is allowed to run. This keeps the overall low power mode current consumption
down.
For low power API, the 128 kHz_IRC is used.
NOTE
To prevent improper real-time clock (RTC) behavior when switching the
system clock source, or before the desired clock source has stabilized, the
RTC must first be disabled by clearing the CRP_RTCC[CNTEN] = 0.