NXP Semiconductors MKW01Z128, Reference Manual

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System Considerations
MKW01xxRM Reference Manual, Rev. 3, 04/2016
Freescale Semiconductor, Inc. 4-11
approximation, use 10 pF as an estimate of combined pin and PCB capacitance for each oscillator pin
(EXTAL and XTAL).
When using the oscillator in low frequency range modes, the external components Cx and Cy are not
required.
When using the oscillator in low power modes, the external R
F
is not required and should not be used.
4.4.3.3 MCU Internal Clock Source
The MKW01 has an internal reference clock (nominally 32 kHz) that is used in a number of ways:
• Default MCU clock out of reset - the internal ICS clock module defaults to the internal oscillator
enabled and the FLL engaged. The resulting default CPU clock frequency is a nominal 20 MHz
(10 MHz bus clock).
• Programmable system clock - either used with or without the FLL, the internal oscillator can
remain as the normal RUN frequency source
— Nominal maximum CPU clock of 48 MHz
— Total trimmed frequency deviation of +/-2% maximum
• Wake-up clock for low power modes - the internal oscillator can be enabled to clock the RTC to
provide a wake-up timer from Stop2 or Stop3
4.4.3.4 LPO 1 kHz Oscillator
The LPO is independent of the ICS module and can be used to clock the RTC or COP. Its period can vary
greatly from 0.7–1.3 ms.
4.4.4 System Clock Configurations
Because of the multiple clock configurations of the MCU, the availability of external clock source pins of
both the transceiver and the MCU, and the ClkOut output from the transceiver, there are a number of
variations for MKW01Z128 system clock configurations. Key considerations for any system clock
configuration are:
• The transceiver 32 MHz source (typically the reference crystal oscillator) must always be present.
• Battery-operated application requirements for low power can impact the choices for MCU clock
source.
• The system clock configuration will impact system initialization procedures.
• Software requirements can impact MCU processor and bus speed - The user must be aware of the
performance requirements for the MCU. The CPU clock is always 2X the internal bus speed, and
the application software may impact the required system clock rate.
As far as external connections are concerned, there are three possibilities which are covered in the
following sections.