Freescale Semiconductor, Inc.
Internal Clock Generator Module (ICG)
8.4.2.1 Digitally Controlled Oscillator
The digitally controlled oscillator (DCO) is an inaccurate oscillator which
generates the internal clock (ICLK). The clock period of ICLK is
dependent on the digital loop filter outputs (DSTG[7:0] and DDIV[3:0]).
Because there is only a limited number of bits in DDIV and DSTG, the
precision of the output (ICLK) is restricted to a long-term precision of
approximately ±0.202 percent to ±0.368 percent when measured over
several cycles (of the desired frequency). Additionally, since the
propagation delays of the devices used in the DCO ring oscillator are a
measurable fraction of the bus clock period, reaching the long-term
precision may require alternately running faster and slower than desired,
making the worst case cycle-to-cycle frequency variation ±6.45 percent
to ±11.8 percent (of the desired frequency). The valid values of
DDIV:DSTG range from $000 to $9FF. For more information on the
quantization error in the DCO, see 8.5.4 Quantization Error in DCO
Output.
8.4.2.2 Modulo N Divider
The modulo N divider creates the low-frequency base clock (IBASE) by
dividing the internal clock (ICLK) by the ICG multiplier factor (N)
contained in the ICG multiplier register (ICGMR). When N is
programmed to a $01 or $00, the divider is disabled and ICLK is passed
through to IBASE undivided. When the internal clock generator is stable,
the frequency of IBASE will be equal to the nominal frequency (f
) of
NOM
307.2 kHz ±25 percent.
8.4.2.3 Frequency Comparator
The frequency comparator effectively compares the low-frequency base
clock (IBASE) to a nominal frequency, f . First, the frequency
NOM
comparator converts IBASE to a voltage by charging a known capacitor
with a current reference for a period dependent on IBASE. This voltage
is compared to a voltage reference with comparators, whose outputs are
fed to the digital loop filter. The dependence of these outputs on the
capacitor size, current reference, and voltage reference causes up to
±25 percent error in f
.
NOM
Advance Information
112
MC68HC908RFRK2
MOTOROLA
Internal Clock Generator Module (ICG)
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