MC68HC908RK2 Datasheet, PDF (134/232 Pages) Motorola, Inc – Microcontroller Unit

Internal Clock Generator Module (ICG)
Fortunately, for an individual part, the process dependencies are
constant. An individual part can operate at approximately ±2 percent
variance from its unadjusted operating point over the entire specification
range of the application. If the unadjusted operating point can be
changed, the entire variance can be limited to ±2 percent.
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. 384 of which 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.
NOTE: It is recommended that the user preserve a copy of the contents of the
ICG trim register (ICGTR) in non-volitale memory.
Address $7FEF is reserved for an optional factory-determined value.
Consult with a local Motorola representative for more information and
availability of this option.
Advance Information
134
Internal Clock Generator Module (ICG)
MC68HC908RK2 — Rev. 4.0
MOTOROLA