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Internal Clock Generator Module (ICG)

Data Sheet

MC68HC908RF2 — Rev. 4.0

Internal Clock Generator Module (ICG)

MOTOROLA

The internal clock generator contains:

•

A digitally controlled oscillator

•

A modulo N divider

•

A frequency comparator, which contains voltage and current references, a

frequency to voltage converter, and comparators

•

A digital loop filter

6.3.2.1 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 6.4.4 Quantization Error in

DCO Output.

6.3.2.2 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

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 (fNOM) of

307.2 kHz

±25 percent.

6.3.2.3 Frequency Comparator

The frequency comparator effectively compares the low-frequency base clock

(IBASE) to a nominal frequency, fNOM. First, the frequency 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.

Freescale Semiconductor, Inc.

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