MC68HC908KX8 Datasheet, PDF (105/310 Pages) Motorola, Inc – Microcontrollers

Internal Clock Generator Module (ICG)
Functional Description
7.4.2.1 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 of only a limited number of bits in DDIV and DSTG, the
precision of the output (ICLK) is restricted to a 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 7.5.4 Quantization Error in DCO Output.
7.4.2.2 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_NOM) of
307.2 kHz ± 25 percent.
7.4.2.3 Frequency Comparator
The frequency comparator effectively compares the low-frequency base
clock (IBASE) to a nominal frequency, f_NOM. 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.
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 — Rev. 1.0
MOTOROLA
Internal Clock Generator Module (ICG)
Technical Data
105