MC68HC908GR8 Datasheet, PDF (74/286 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Clock Generator Module (CGMC)
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- ⎠⎟⎞}
–
⎛
integer⎜
⎝
-f-^V----f^C--_R--^L--_C-^K--_L-^D--_K--^E----^S- _{⎠⎟⎞}
⎫
⎬
⎭
4. Select a VCO frequency multiplier, N.
N
=
r
o
un
⎛
d⎜
⎝
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:
Then recalculate N:
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
N
=
ro
un
⎛
d⎜
⎝
-R----f-^×-_R---_C-f--^V-_L---^CK---^L--_×-^K----2^D---_P-^E---^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 < 8 MHz
0
9,830,400 ≤ f_VCLK < 16 MHz
1
19,660,800 ≤ f_VCLK < 32 MHz
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
=
r
ou
⎛
nd⎜
⎝
2----^E--f--V-×^---C--f-_NL^---K-_O^----_M^{---⎠⎟⎞}
MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7
74
Freescale Semiconductor