MC68HC908KX2 MOTOROLA [Motorola, Inc], MC68HC908KX2 Datasheet - Page 105

• Manufacturer Part Number: MC68HC908KX2
• Description: Microcontrollers
• Manufacturer: MOTOROLA [Motorola, Inc]

7.4.2.1 Digitally Controlled Oscillator
7.4.2.2 Modulo N Divider
7.4.2.3 Frequency Comparator
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 — Rev. 1.0
MOTOROLA
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 of only a limited number of bits in DDIV and DSTG, the
precision of the output (ICLK) is restricted to a precision of approximately
(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
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
307.2 kHz
The frequency comparator effectively compares the low-frequency base
clock (IBASE) to a nominal frequency, f
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
0.202 percent to 0.368 percent when measured over several cycles
25 percent error in f
Internal Clock Generator Module (ICG)
7.5.4 Quantization Error in DCO
25 percent.
NOM
.
Internal Clock Generator Module (ICG)
NOM
. First, the frequency
Output.
Functional Description
Technical Data
NOM
) of
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