Functional Description
5. Select the VCO’s power-of-two range multiplier E, according to this table:
Frequency Range
E
0
1
2
0 < fVCLK < 9,830,400
9,830,400 ≤ fVCLK < 19,660,800
19,660,800 ≤ fVCLK < 39,321,600
NOTE: Do not program E to a value of 3.
6. Select a VCO linear range multiplier, L, where f
= 125kHz
NOM
f
⎛
⎞
VCLK
--------------------------
E
L = round
⎜
⎝
⎟
⎠
2 × f
NOM
7. Calculate and verify the adequacy of the VCO programmed center-of-range frequency, f
. The
VRS
center-of-range frequency is the midpoint between the minimum and maximum frequencies
attainable by the PLL.
E
f
= (L × 2 )f
VRS
NOM
For proper operation,
E
f
× 2
NOM
--------------------------
f
– f
≤
VCLK
VRS
2
8. Verify the choice of P, R, N, E, and L by comparing f
to f
and f
. For proper
VCLK
VRS
VCLKDES
operation, f
must be within the application’s tolerance of f
, and f
must be as close
VRS
VCLK
VCLKDES
as possible to f
VCLK.
NOTE
Exceeding the recommended maximum bus frequency or VCO frequency
can crash the MCU.
9. Program the PLL registers accordingly:
a. In the PRE bits of the PLL control register (PCTL), program the binary equivalent of P.
b. In the VPR bits of the PLL control register (PCTL), program the binary equivalent of E.
c. In the PLL multiplier select register low (PMSL) and the PLL multiplier select register high
(PMSH), program the binary equivalent of N.
d. In the PLL VCO range select register (PMRS), program the binary coded equivalent of L.
e. In the PLL reference divider select register (PMDS), program the binary coded equivalent of
R.
NOTE
The values for P, E, N, L, and R can only be programmed when the PLL is
off (PLLON = 0).
Table 6-1 provides numeric examples (numbers are in hexadecimal notation):
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor
85
FREESCALE [ Freescale ]
