MC68HC08AS20 Datasheet, PDF (97/386 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

• Software must wait a given time, t_AL, after entering tracking mode
before selecting the PLL as the clock source to CGMOUT
(BCS = 1).
• The LOCK bit is disabled.
• CPU interrupts from the CGM are disabled.
8.4.2.4 Programming the PLL
Use this procedure to program the PLL:
1. Choose the desired bus frequency, f_BUSDES.
Example: f_BUSDES = 8 MHz
2. Calculate the desired VCO frequency, f_VCLKDES.
f_VCLKDES = 4 × f_BUSDES
Example: f_VCLKDES = 4 × 8 MHz = 32 MHz
NOTE:
3. Using a reference frequency, f_RCLK, equal to the crystal frequency,
calculate the VCO frequency multiplier, N.
The round function means that the result is rounded to the nearest
integer.
N = round_ ^f--V---f^C--_R--^L-_C-^K--_L-^D--_K-^E---^S-_
Example: N = ^{3----2-----M-----H----z--} = 8
4 MHz
4. Calculate the VCO frequency, f_VCLK.
f_VCLK = N × f_RCLK
Example: f_VCLK = 8 × 4 MHz = 32 MHz
MC68HC08AS20 — Rev. 4.1
Freescale Semiconductor
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