HCS08 datasheet(96/290 Pages) MOTOROLA

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Internal Clock Generator (ICG) Module

MC9S08GB/GT

MOTOROLA

6.4.2 Example #1: External Crystal = 32 kHz, Bus Frequency = 4.19 MHz

In this example, the FLL will be used (in FEE mode) to multiply the external 32 kHz oscillator up to

8.38-MHz to achieve 4.19 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).

The clock scheme will be FLL engaged, external (FEE). So

fICGOUT = fext * P * N / R ; P = 64, fext = 32 kHz

Solving for N / R gives:

N / R = 8.38 MHz /(32 kHz * 64) = 4 ; we can choose N = 4 and R =1

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

ICGC1 = $38 (%00111000)

Bit 7

Unimplemented or reserved, always reads zero

Bit 6

RANGE

Configures oscillator for low-frequency range; FLL prescale factor is 64

Bit 5

REFS

Oscillator using crystal or resonator is requested

Bits 4:3 CLKS

FLL engaged, external reference clock mode

Bit 2

OSCSTEN 0

Oscillator disabled

Bit 1

Reserved for Motorola’s internal use; always write zero

Bit 0

Unimplemented or reserved, always reads zero

ICGC2 = $00 (%00000000)

Bit 7

LOLRE

Generates an interrupt request on loss of lock

Bits 6:4 MFD

000

Sets the MFD multiplication factor to 4

Bit 3

LOCRE

Generates an interrupt request on loss of clock

Bits 2:0 RFD

000

Sets the RFD division factor to

÷2

ICGS1 = $xx

This is read only except for clearing interrupt flag

ICGS2 = $xx

This is read only; should read DCOS = 1 before performing any time critical tasks

ICGFLTLU/L = $xx

Only needed in self-clocked mode; FLT will be adjusted by loop to give 8.38 MHz DCO clock

Bits 15:12 unused

0000

Bits 11:0 FLT

No need for user initialization

ICGTRM = $xx

Bits 7:0

TRIM

Only need to write when trimming internal oscillator; not used when external

crystal is clock source

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