MC908AP16CFAE Datasheet, PDF (85/324 Pages) Freescale Semiconductor, Inc – Table of Contents

Functional Description
5. Select the VCO’s power-of-two range multiplier E, according to this table:
Frequency Range
E
0 < f_VCLK < 9,830,400
0
9,830,400 ≤ f_VCLK < 19,660,800
1
19,660,800 ≤ f_VCLK < 39,321,600
2
NOTE: Do not program E to a value of 3.
6. Select a VCO linear range multiplier, L, where f_NOM = 125kHz
L
=
ro
und
⎛
⎜
⎝
2----^E--f--V-×^---C--f-_NL^----KO----_M^{---⎠⎟⎞}
7. Calculate and verify the adequacy of the VCO programmed center-of-range frequency, f_VRS. The
center-of-range frequency is the midpoint between the minimum and maximum frequencies
attainable by the PLL.
f_VRS = (L × 2^E)f_NOM
For proper operation,
f_VRS – f_VCLK ≤ ^f--^N----^O----^M--₂----^×-----²---^E--
8. Verify the choice of P, R, N, E, 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.
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