NXP Semiconductors PXN2020, Reference Manual, Page: 121 / 1376

System Clock Description
PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor 5-3
Figure 5-1. PXN20 Available Clock Sources
5.1.3 External High-Frequency Crystal (4 – 40 MHz XTAL)
The 4 – 40 MHz XTAL clock uses an external high frequency crystal to provide the main clock source to
the circuit. The on-chip crystal oscillator has automatic level control and supports 4 MHz to 40 MHz
crystals. It can be used as external square-wave input.
A low power output buffer is selected automatically in Sleep mode to allow crystals of 8 MHz and below
to continue to run and clock the Real Time Clock (RTC) and Autonomous Periodic Interrupts (API).
A crystal in the full 4 – 40 MHz range can be kept enabled in Sleep mode to facilitate fast start up. In this
mode, the crystal oscillator cannot drive any internal modules (e.g.,RTC/ API), as the output driver is
disabled.
5.1.3.1 4 – 40 MHz XTAL Features
• Input frequency range: 4 MHz – 40 MHz
• Automatic level control
• Oscillator input mode 3.3V (V
DDSYN
)
• External crystal mode enabled by default after reset. External square wave input supported, but
crystal bypass mode should be selected in startup code if an external crystal is not used.
• FMPLL reference clock
• Jitter < 0.5%, over one CAN frame
• Duty cycle: 40 – 60%
• Clock source capable of supporting FlexRay communications
4 – 40
OSCCLK
SYSCLKSEL
2 1 0
16 MHz
IRC
PLL
Clock
Switcher
+1,2,4,8,16 SYSCLKDIV
System Clock
Peripheral Clocks
TRIMIRC
3.3 or 5 V
3.3 V
EXTAL
3.3 V
XTAL
3.3 V
3.3 V
EXTAL32
3.3 or 5 V
XTAL32
3.3 or 5 V
128 kHz
IRC
TRIM128IRC
1.2 V
32 kHz XTAL
OSC32KEN
MHz XTAL
(V
DDA
)
(V
RC
)
(V
DDA
)