MC68HC908GP32_08 Datasheet, PDF (66/266 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Clock Generator Module (CGM)
When the tolerance on the bus frequency is tight, choose f_RCLK to an integer divisor of f_BUSDES,
and R = 1. If f_RCLK cannot meet this requirement, use the following equation to solve for R with
practical choices of f_RCLK, and choose the f_RCLK that gives the lowest R.
R
=
round
R_M
A
X
×
⎧⎛
⎨⎜
⎩⎝
f--_{V----fC--R--L--C-K--L-D--K--E----S- ⎠⎟⎞}
–
i
nte
ge
⎛
r⎜
⎝
f--_{V----fC--R--L--C-K--L-D--K--E----S- ⎠⎟⎞}
⎫
⎬
⎭
4. Select a VCO frequency multiplier, N.
N
=
r
ou
⎛
nd⎜
⎝
R------×_{-----ff--VR----CC---L-L--K-K---D----E---S--⎠⎟⎞}
Reduce N/R to the lowest possible R.
5. If N is < N_max, use P = 0. If N > N_max, choose P using this table:
Current N Value
P
0 < N ≤ Nmax
0
Nmax < N ≤ Nmax × 2
1
Nmax × 2 < N ≤ Nmax × 4
2
Nmax × 4 < N ≤ Nmax × 8
3
Then recalculate N:
N
=
r
oun
⎛
d⎜
⎝
-R---_f--^×_R----_C-f--^V-_L---^C_K---^L---_×^K----^D_2----^E_P---^S--_{⎠⎟⎞}
6. Calculate and verify the adequacy of the VCO and bus frequencies f_VCLK and f_BUS.
f_VCLK = (2^P × N ⁄ R) × f_RCLK
f_BUS = (f_VCLK) ⁄ 4
7. 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.
8. Select a VCO linear range multiplier, L, where f_NOM = 38.4 kHz
L
=
ro
u
nd
⎛
⎜
⎝
2----^E--f--V-×^---C--f-_NL^----KO----_M^{---⎠⎟⎞}
MC68HC908GP32 Data Sheet, Rev. 10
66
Freescale Semiconductor