CHAPTER 7-2: VBAT Domain(A)
260 FM4 Peripheral Manual, Doc. No. 002-04856 Rev. *E
2.3 32 kHz Oscillation Circuit
The 32 kHz oscillation circuit is an oscillation circuit exclusively for the crystal oscillator for the clock, and
creates the subclock.
Overview of Functions of 32 kHz Oscillation Circuit
The 32 kHz oscillation circuit has the following functions.
− Oscillation current switch function
− Oscillation boost function
− Clock generator cooperative operation function
Oscillation current switch function
The amplifier circuit of the 32 kHz oscillation circuit is driven by a constant current source.
The current value of the constant current source can be controlled by the value of the CCS Register.
Oscillation boost function
In the case of the crystal oscillator for the clock, it takes longer time for the oscillation frequency to
stabilize.
The oscillation stabilization time can be shortened by increasing the current supplied to the amplifier
circuit for a certain period of time after the start of oscillation.
During the period from the start of oscillation to the end of the oscillation boost time set in the BOOST
Register, the current the constant current source supplies is the current value set in the CCB Register.
After the above period has elapsed, the current the constant current source supplies switches to the
current value set in the CCS Register.
If a current value larger than the one set in the CCS Register is set in the CCB Register, the oscillation
boost function can work effectively.
If not using the oscillation boost function, set in the CCB Register a value same as the one set in the CCS
Register.
Clock generator cooperative operation function
The SOSCNTL bit in the WTOSCCNT Register enables or disables the cooperative operation between
the 32 kHz oscillation circuit and the clock generator belonging to the CPU Domain.
With the cooperative operation enabled, the 32 kHz oscillation of this circuit stops when the CPU transits
to stop mode or deep standby stop mode.
If the cooperative operation is disabled, the 32 kHz oscillation of this circuit does not stop regardless of
the operation mode of the CPU.
Notes:
− The 32 kHz oscillation circuit in the VBAT Domain does not have the oscillation stabilization wait
function or the clock failure detection function. Enable the clock cooperative function and use the
clock failure detection function of the CPU Domain.
− The appropriate amount of current flowing to the amplifier circuit varies depending on the
characteristic (ESR) and load capacitance (CL) of the oscillator connected to the VBAT Domain.
Select an appropriate amount of current by performing a matching evaluation between the VBAT
Domain and the crystal oscillator.
− If using the RTC with a backup power supply for the VBAT Domain instead of the VCC power
supply, disable the cooperative operation with the clock generator.
− After the 32 kHz oscillation of this circuit has started, do not update the CCB Register or the CCS
Register.
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