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On-Chip Clock Synthesis

10.4.1 PLL Programming Example

Example 10-1 on page 10-12 provides an example of how to program the PLL to multiply by a factor of 20 and wait for the PLL to stabilize.

Example 10-1. Programming the PLL

;********************

 

 

;* PLL Setup example*

 

 

;* of DSP56824 chip *

 

 

;********************

 

 

PCR0

EQU

$FFF2

; PLL Control Register 0

PCR1

EQU

$FFF3

; PLL Control Register 1

;********************

 

 

;* PLL setup

*

 

 

;* (to increase Phi Clock) *

 

;********************

 

 

MOVEP

#$0180,X:PCR1

; Configure:

 

 

 

; (PLLE) PLL disabled (bypassed)

 

 

 

; -- Oscillator supplies Phi Clock

 

 

 

; (PLLD) PLL Power Down disabled (PLL active)

 

 

 

; -- PLL block active for PLL to attain lock

 

 

 

; (LPST) Low Power Stop disabled

 

 

 

; (PS[2:0]) prescaler clock disabled

 

 

 

; (CS[1:0]) Clockout pin sends Oscillator Clock

MOVEP

#$0260,X:PCR0

; Set Feedback Divider to 1/20

 

 

 

; ...

 

 

 

; insert delay here: wait for PLL lock

 

 

 

; as specified in data sheet

 

 

 

; ...

BFSET

#$4000,X:PCR1

; Enable PLL for Phi Clock

 

 

 

 

10.4.2 Changing the PLL Frequency

To change the output frequency of the PLL (by reprogramming the YD bits) while the PLL output is used by the DSP56800 core (PLLE = 1; PLLD = 0), perform the following sequence of operations:

1.Clear the PLLE bit to switch back to the oscillator clock.

2.Program the YD bits (only after clearing PLLE).

3.Wait until the PLL has locked. See the DSP56824 Technical Data Sheet for settling time specifications.

4.Set the PLLE bit.

NOTE:

Changing the PLL frequency may require changing external filter components and is not recommended. See the DSP56824 Technical Data Sheet for more information on supported PLL frequencies and recommended external filter component values.

10-12

DSP56824 User’s Manual

 

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