C8051F124-GQR Datasheet, PDF (192/350 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F120/1/2/3/4/5/6/7
C8051F130/1/2/3
PLL Frequency
=
Reference
Frequency

-P----L---L----N---
PLLM
14.7.3. Powering on and Initializing the PLL
To set up and use the PLL as the system clock after power-up of the device, the following procedure
should be implemented:
Step 1. Ensure that the reference clock to be used (internal or external) is running and stable.
Step 2. Set the PLLSRC bit (PLL0CN.2) to select the desired clock source for the PLL.
Step 3. Program the Flash read timing bits, FLRT (FLSCL.5–4) to the appropriate value for the
new clock rate (see Section “15. Flash Memory” on page 199).
Step 4. Enable power to the PLL by setting PLLPWR (PLL0CN.0) to ‘1’.
Step 5. Program the PLL0DIV register to produce the divided reference frequency to the PLL.
Step 6. Program the PLLLP3–0 bits (PLL0FLT.3–0) to the appropriate range for the divided
reference frequency.
Step 7. Program the PLLICO1–0 bits (PLL0FLT.5–4) to the appropriate range for the PLL output
frequency.
Step 8. Program the PLL0MUL register to the desired clock multiplication factor.
Step 9. Wait at least 5 µs, to provide a fast frequency lock.
Step 10. Enable the PLL by setting PLLEN (PLL0CN.1) to ‘1’.
Step 11. Poll PLLLCK (PLL0CN.4) until it changes from ‘0’ to ‘1’.
Step 12. Switch the System Clock source to the PLL using the CLKSEL register.
If the PLL characteristics need to be changed when the PLL is already running, the following procedure
should be implemented:
Step 1. The system clock should first be switched to either the internal oscillator or an external
clock source that is running and stable, using the CLKSEL register.
Step 2. Ensure that the reference clock to be used for the new PLL setting (internal or external) is
running and stable.
Step 3. Set the PLLSRC bit (PLL0CN.2) to select the new clock source for the PLL.
Step 4. If moving to a faster frequency, program the Flash read timing bits, FLRT (FLSCL.5–4) to
the appropriate value for the new clock rate (see Section “15. Flash Memory” on
page 199).
Step 5. Disable the PLL by setting PLLEN (PLL0CN.1) to ‘0’.
Step 6. Program the PLL0DIV register to produce the divided reference frequency to the PLL.
Step 7. Program the PLLLP3–0 bits (PLL0FLT.3–0) to the appropriate range for the divided
reference frequency.
Step 8. Program the PLLICO1-0 bits (PLL0FLT.5–4) to the appropriate range for the PLL output
frequency.
Step 9. Program the PLL0MUL register to the desired clock multiplication factor.
Step 10. Enable the PLL by setting PLLEN (PLL0CN.1) to ‘1’.
Step 11. Poll PLLLCK (PLL0CN.4) until it changes from ‘0’ to ‘1’.
Step 12. Switch the System Clock source to the PLL using the CLKSEL register.
Step 13. If moving to a slower frequency, program the Flash read timing bits, FLRT (FLSCL.5–4)
to the appropriate value for the new clock rate (see Section “15. Flash Memory” on
192
Rev. 1.4