MC68HC908JW32_09 Datasheet, PDF (63/232 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Functional Description
3. Calculate N:
N
=
rou
nd
⎛
⎜
⎝
R-----f-×_{-R---C-f--V-L---CK---L--×-K----2D---P-E---S--⎠⎟⎞}
4. Calculate and verify the adequacy of the VCO and bus frequencies f_VCLK and f_BUS.
f_VCLK = ^2----P_R----N---(f_RCLK)
f_BUS = ^f--^V--^C-₆--^L--^K-
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_---^E_--
2
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.
MC68HC908JW32 Data Sheet, Rev. 6
Freescale Semiconductor
63