HCS08 datasheet(98/290 Pages) MOTOROLA

98 / 290 page

Internal Clock Generator (ICG) Module

MC9S08GB/GT

MOTOROLA

6.4.3 Example #2: External Crystal = 4 MHz, Bus Frequency = 20 MHz

In this example, the FLL will be used (in FEE mode) to multiply the external 4 MHz oscillator up to

40-MHz to achieve 20 MHz bus frequency.

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 (fBus).

During reset initialization software, the clock scheme will be set to FLL engaged, external (FEE). So

fICGOUT = fext * P * N / R ; P = 1, fext = 4.00 MHz

Solving for N / R gives:

N / R = 40 MHz /(4 MHz * 1) = 10 ; We can choose N = 10 and R = 1

The values needed in each register to set up the desired operation are:

ICGC1 = $78

(%01111000)

Bit 7

Unimplemented or reserved, always reads zero

Bit 6

RANGE

Configures oscillator for high-frequency range; FLL prescale factor is 1

Bit 5

REFS

Requests an oscillator

Bits 4:3 CLKS

FLL engaged, external reference clock mode

Bit 2

OSCSTEN 0

Disables the oscillator

Bit 1

Reserved for Motorola’s internal use; always write zero

Bit 0

Unimplemented or reserved, always reads zero

ICGC2 = $30

(%00110000)

Bit 7

LOLRE

Generates an interrupt request on loss of lock

Bit 6:4 MFD

011

Sets the MFD multiplication factor to 10

Bit 3

LOCRE

Generates an interrupt request on loss of clock

Bit 2:0 RFD

000

Sets the RFD division factor to

÷1

ICGS1 = $xx

This is read only except for clearing interrupt flag

ICGS2 = $xx

This is read only. Should read DCOS before performing any time critical tasks

ICGFLTLU/L = $xx

Not used in this example

ICGTRM

Not used in this example

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