MC908GR8CDWE Datasheet, PDF (107/408 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Freescale Semiconductor, Inc.
Clock Generator Module (CGMC)
Functional Description
allows. See Electrical Specifications. Choose the reference
divider, R = 1. After choosing N and P, the actual bus frequency
can be determined using equation in 2 above.
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_MAX
×






f--_{V----fC--R--L--C-K--L-D--K--E----S- }
–
i
nte
ge

r

-f-^V----f^C--_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
ou
nd



-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
MC68HC908GR8 — Rev 4.0
MOTOROLA
Clock Generator Module (CGMC)
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Technical Data
107