MC68HC908MR8 Datasheet, PDF (120/372 Pages) Motorola, Inc – Microcontrollers

Clock Generator Module (CGM)
8.4.2.4 Programming the PLL
This procedure shows how to program the PLL.
NOTE: The round function in the following equations means that the real
number should be rounded to the nearest integer number.
1. Choose the desired bus frequency, f_BUSDES.
2. Calculate the desired VCO frequency (four times the desired bus
frequency).
fVCLKDES = 4 × fBUSDES
3. Choose a practical PLL reference frequency, f_RCLK.
4. Select a VCO frequency multiplier, N.
5. Calculate and verify the adequacy of the VCO and bus
frequencies f_VCLK and f_BUS.
fVCLK = N × fRCLK
fBUS = (fVCLK) ⁄ 4
CAUTION:
6. Select a VCO linear range multiplier, L.
where f_NOM = 4.9152 MHz
7. Calculate and verify the adequacy of the VCO programmed
center-of-range frequency, f_VRS.
f_VRS = (L) f_NOM
8. Verify the choice of N and L by comparing f_VCLK to f_VRS and
f_VCLKDES. For proper operation, f_VCLK must be within the
application’s tolerance of f_VCLKDES, and f_VRS must be as close as
possible to f_VCLK.
Exceeding the recommended maximum bus frequency or VCO
frequency can crash the MCU.
9. 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.
Technical Data
120
Clock Generator Module (CGM)
MC68HC908MR8 — Rev 4.1
Freescale Semiconductor