MC9S08AW16CFUE Datasheet, PDF (153/324 Pages) Freescale Semiconductor, Inc – MC9S08AW60 Features

Chapter 8 Internal Clock Generator (S08ICGV4)
8.5.4 Example #3: No External Crystal Connection, 5.4 MHz Bus
Frequency
In this example, the FLL will be used (in FEI mode) to multiply the internal 243 kHz (approximate)
reference clock up to 10.8 MHz to achieve 5.4 MHz bus frequency. This system will also use the trim
function to fine tune the frequency based on an external reference signal.
After the MCU is released from reset, the ICG is in self-clocked mode (SCM) and supplies approximately
8 MHz on ICGOUT which corresponds to a 4 MHz bus frequency (f_Bus).
The clock scheme will be FLL engaged, internal (FEI). So
f_ICGOUT = (f_IRG / 7) * P * N / R ; P = 64, f_IRG = 243 kHz
Solving for N / R gives:
Eqn. 8-5
N / R = 10.8 MHz /(243/7 kHz * 64) = 4.86 ; We can choose N = 10 and R = 2.
Eqn. 8-6
A trim procedure will be required to hone the frequency to exactly 5.4 MHz. An example of the trim
procedure is shown in example #4.
The values needed in each register to set up the desired operation are:
ICGC1 = $28 (%00101000)
Bit 7 HGO
0
Bit 6 RANGE 0
Bit 5 REFS
1
Bits 4:3 CLKS 01
Bit 2 OSCSTEN 0
Bit 1 LOCD 0
Bit 0
0
Configures oscillator for low power
Configures oscillator for low-frequency range; FLL prescale factor is 64
Oscillator using crystal or resonator requested (bit is really a don’t care)
FLL engaged, internal reference clock mode
Disables the oscillator
Loss-of-clock enabled
Unimplemented or reserved, always reads zero
ICGC2 = $31 (%00110001)
Bit 7 LOLRE 0
Bit 6:4 MFD
011
Bit 3 LOCRE 0
Bit 2:0 RFD
001
Generates an interrupt request on loss of lock
Sets the MFD multiplication factor to 10
Generates an interrupt request on loss of clock
Sets the RFD division factor to ÷2
ICGS1 = $xx
This is read only except for clearing interrupt flag
ICGS2 = $xx
This is read only; good idea to read this before performing time critical operations
ICGFLTLU/L = $xx
Not used in this example
MC9S08AW60 Data Sheet, Rev 2
Freescale Semiconductor
153