HCS08 datasheet(100/290 Pages) MOTOROLA

100 / 290 page

Internal Clock Generator (ICG) Module

MC9S08GB/GT

100

MOTOROLA

6.4.4 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 (fBus).

The clock scheme will be FLL engaged, internal (FEI). So

fICGOUT = (fIRG / 7) * P * N / R ; P = 64, fIRG = 243 kHz

Solving for N / R gives:

N / R = 10.8 MHz /(243/7 kHz * 64) = 4.86 ; We can choose N = 10 and R = 2.

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

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 requested (bit is really a don’t care)

Bits 4:3 CLKS

FLL engaged, internal 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 = $31 (%00110001)

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

001

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

ICGTRM = $xx

Bit 7:0 TRIM

Only need to write when trimming internal oscillator; done in separate

operation (see example #4)

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