NXP Semiconductors MKW01Z128, Reference Manual

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System Considerations
MKW01xxRM Reference Manual, Rev. 3, 04/2016
4-12 Freescale Semiconductor, Inc.
NOTE
In the following sub-sections, it is assumed that the MCU GPIO is
connected to drive the transceiver reset.
4.4.4.1 Single crystal with ClkOut driving MCU EXTAL input
The single crystal (transceiver) with ClkOut driving the MCU EXTAL input (external clock) is the most
common configuration for low cost and excellent frequency accuracy. The ClkOut frequency is
programmable but setting the divide ratio to 1 resulting in a 32 MHz CLKOUT (with a 32 MHz crystal) is
recommended to drive the MCU external source.
In this configuration, clock start-up from a reset condition involves:
• MCU reset is released and MCU starts on internal 20 MHz clock, which is derived internally via
the FLL from the slow IRC 32 kHz clock.
• Initialization software should reset and then release reset to the transceiver (MCU still running on
start-up clock)
• Wait for transceiver start-up.
• Program ClkOut to desired frequency (clkout to 000, divide by 1, recommended)
• Wait for the ClkOut source plus PLL to lock, and then switch MCU clock to external source
If the transceiver is forced to a low power condition, the MCU can revert to the internal oscillator and FLL.
4.4.4.2 Single Crystal with MCU Using Internal Clock Only
The single crystal (transceiver) with the MCU using internal clock only has no real advantage over using
the ClkOut output, except for slightly lower power.
In this configuration, clock start-up from a reset condition involves:
• MCU reset is released and MCU starts on internal 20 MHz clock, which is derived internally via
the FLL from the slow IRC 32 kHz clock
• Initialization software should assert reset and then release reset to the transceiver (MCU still
running on start-up clock)
• Program MCG to desired clock rate if the default is not the preferred choice
• Program transceiver to disable ClkOut.
The MCU does not have a high accuracy time base when using the internal reference.
4.4.4.3 Dual Crystal Operation
The transceiver crystal can be augmented by the use of a second crystal on the MCU. The typical
application would use a 32.768 kHz crystal that would allow an accurate time base in the MCU for long
power down delays. The obvious disadvantage of this configuration is additional cost.
In this configuration, clock start-up from a reset condition involves:
• MCU reset is released and MCU starts on internal 20 MHz clock, which is derived internally via
the FLL from the slow IRC 32 kHz clock