MC68HC908RK2 Datasheet, PDF (71/158 Pages) Motorola, Inc – Microcontroller Unit

Usage Notes
6.4.3 Clock Monitor Interrupts
The clock monitor circuit can be used to recover from perilous situations such as crystal loss. To use the
clock monitor effectively, these notes should be observed:
• Enable the clock monitor and clock monitor interrupts.
• The first statement in the clock monitor interrupt service routine should be a read to the ICG control
register (ICGCR) to verify that the clock monitor flag (CMF) is set. This is also the first step in
clearing the CMF bit.
• Never use BSET or BCLR instructions on the ICGCR, as this may inadvertently clear CMF. Only
use the BRSET and BRCLR instructions to check the CMF bit and not to check any other bits in
the ICGCR.
• When the clock monitor detects inactivity on the selected clock source (defined by the CS bit of the
ICG control register), the inactive clock is deselected automatically and the remaining active clock
is selected as the source for CGMXCLK. The interrupt service routine can use the state of the CS
bit to check which clock is inactive.
• When the clock monitor detects inactivity, the application may have been subjected to extreme
conditions which may have affected other circuits. The clock monitor interrupt service routine
should take any appropriate precautions.
6.4.4 Quantization Error in DCO Output
The digitally controlled oscillator (DCO) is comprised of three major sub-blocks:
1. Binary weighted divider
2. Variable-delay ring oscillator
3. Ring oscillator fine-adjust circuit
Each of these blocks affects the clock period of the internal clock (ICLK). Since these blocks are controlled
by the digital loop filter (DLF) outputs DDIV and DSTG, the output of the DCO can change only in
quantized steps as the DLF increments or decrements its output. The following sections describe how
each block will affect the output frequency.
6.4.4.1 Digitally Controlled Oscillator
The digitally controlled oscillator (DCO) is an inaccurate oscillator which generates the internal clock
(ICLK), whose clock period is dependent on the digital loop filter outputs (DSTG[7:0] and DDIV[3:0]).
Because of the digital nature of the DCO, the clock period of ICLK will change in quantized steps. This
will create a clock period difference or quantization error (Q-ERR) from one cycle to the next. Over several
cycles or for longer periods, this error is divided out until it reaches a minimum error of 0.202% to 0.368%.
The dependence of this error on the DDIV[3:0] value and the number of cycles the error is measured over
is shown in Table 6-3.
MC68HC908RK2 Data Sheet, Rev. 5.1
Freescale Semiconductor
71