MC68HC08AS32 Datasheet, PDF (95/280 Pages) Motorola, Inc – M68HC08 Family of 8-bit microcontroller units (MCUs)

4. Calculate the VCO frequency, f_VCLK.
f_VCLK = N × f_RCLK
Example: f_VCLK = 8 × 4 MHz = 32 MHz
5. Calculate the bus frequency, f_BUS, and compare f_BUS with f_BUSDES. If the
calculated f_BUS is not within the tolerance limits of your application, select
another f_BUSDES or another f_RCLK.
f_BUS
=
f--_{V----C---L---K-}
4
Example:
f_BUS
=
3----2-----M-----H----z--
4
=
8 MHz
6. Using the value 4.9152 MHz for f_NOM, calculate the VCO linear range
multiplier, L. The linear range multiplier controls the frequency range of the
PLL.
L = round_ -ff-^V-_N--^C-_O---^L-_M-^K--_
Example:
L
=
------3----2-----M-----H----z-------
4.9152 MHz
=
7
NOTE:
7. Calculate the VCO center-of-range frequency, f_VRS. The center-of-range
frequency is the midpoint between the minimum and maximum frequencies
attainable by the PLL.
f_VRS = L × f_NOM
Example: f_VRS = 7 × 4.9152 MHz = 34.4 MHz
Exceeding the recommended maximum bus frequency or VCO frequency can
crash the MCU.
For proper operation,
f_VRS – f_VCLK
≤
f_{--N----O----M--}
2
8. Program the PLL registers accordingly:
a. In the upper four bits of the PLL programming register (PPG), program
the binary equivalent of N.
b. In the lower four bits of the PLL programming register (PPG), program
the binary equivalent of L.
MC68HC08AS32 — Rev. 4.1
Freescale Semiconductor
Data Sheet
95