XE8802 Datasheet, PDF (52/193 Pages) Semtech Corporation – Sensing Machine Data Acquisition MCU

XE8802 Sensing Machine Data Acquisition MCU
with ZoomingADC™ and LCD driver
7.5 Clock sources
7.5.1
RC oscillator Configuration
The RC oscillator is always turned on and selected for CPU and system operation at power-on reset, pad NRESET,
and when exiting sleep mode. It can be turned off after the Xtal (quartz oscillator) has been started, after selection
of the external clock or by entering sleep mode.
The RC oscillator has two frequency ranges: sub-MHz (50 kHz to 0.5 MHz) and above-MHz (0.5 MHz to 5 MHz).
Inside a range, the frequency can be tuned by software for coarse and fine adjustment. See registers
RegSysRcTrim1 and RegSysRcTrim2.
Bit EnableRc in register RegSysClock controls the propagation of the RC clock signal and the operation of the
oscillator. The user can stop the RC oscillator by resetting the bit EnableRc. Entering the sleep mode disables the
RC oscillator.
Note: The RC oscillator bias can be maintained while the oscillator is disabled by setting the bit BiasRc in
RegSysClock. This allows a faster restart of the RC oscillator at the cost of increased power consumption (see
section 7.5.3).
7.5.2
RC oscillator frequency tuning
The RC oscillator frequency can be set using the bits in the RegSysRcTrim1 and RegSysRcTrim2 registers.
Figure 7-2 shows the nominal frequency of the RC oscillator as a function of these bits. The absolute value of the
frequency for a given register content may change by ±35% from chip to chip due to the tolerances on the
integrated capacitors and resistors. However, the modification of the frequency as a function of a modification of
the register content is fairly precise. This means that the curves in Figure 7-2 can shift up and down but that the
slope remains unchanged.
The bit RcFreqRange modifies the oscillator frequency by a factor of 10. The upper curve in the figure corresponds
to RcFreqRange=1.
The RcFreqCoarse modifies the frequency of the oscillator by a factor (RcFreqCoarse+1). The figure represents
the frequency for 5 different values of the bits RcFreqCoarse: for each value the frequency is multiplied by 2.
Incrementing the RcFreqFine code, increases the frequency by about 1.4%.
The frequency of the oscillator is therefor given by:
f_RC=f_Rcmin⋅(1+9⋅RcFreqRange)⋅(1+RcFreqCoarse)⋅(1.014)^RcFreqFine
with f_Rcmin the RC oscillator frequency if the registers are all 0.
© Semtech 2006
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