MC68HC908KX8 Datasheet, PDF (125/310 Pages) Motorola, Inc – Microcontrollers

Internal Clock Generator Module (ICG)
Low-Power Modes
7.5.7 Trimming Frequency on the Internal Clock Generator
The unadjusted frequency of the low-frequency base clock (IBASE),
when the comparators in the frequency comparator indicate zero error,
will vary as much as ±25 percent due to process, temperature, and
voltage dependencies. These dependencies are in the voltage and
current references, the offset of the comparators, and the internal
capacitor.
The method of changing the unadjusted operating point is by changing
the size of the capacitor. This capacitor is designed with 639 equally
sized units. Of that number, 384 of these units are always connected.
The remaining 255 units are put in by adjusting the ICG trim factor
(TRIM). The default value for TRIM is $80, or 128 units, making the
default capacitor size 512. Each unit added or removed will adjust the
output frequency by about ±0.195 percent of the unadjusted frequency
(adding to TRIM will decrease frequency). Therefore, the frequency of
IBASE can be changed to ±25 percent of its unadjusted value, which is
enough to cancel the process variability mentioned before.
The best way to trim the internal clock is to use the timer to measure the
width of an input pulse on an input capture pin (this pulse must be
supplied by the application and should be as long or wide as possible).
Considering the prescale value of the timer and the theoretical (zero
error) frequency of the bus (307.2 kHz *N/4), the error can be calculated.
This error, expressed as a percentage, can be divided by 0.195 percent
and the resultant factor added or subtracted from TRIM. This process
should be repeated to eliminate any residual error.
7.6 Low-Power Modes
The WAIT and STOP instructions put the MCU in low power-
consumption standby modes.
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 — Rev. 1.0
MOTOROLA
Internal Clock Generator Module (ICG)
Technical Data
125