|
MC68HC908GR16 Datasheet, PDF (64/276 Pages) Motorola, Inc – Microcontrollers | |||
|
◁ |
Clock Generator Module (CGM)
When the tolerance on the bus frequency is tight, choose fRCLK to an integer divisor of fBUSDES,
and R = 1. If fRCLK cannot meet this requirement, use the following equation to solve for R with
practical choices of fRCLK, and choose the fRCLK that gives the lowest R.
R
=
round
RM
A
X
Ã
â§
â¨
â©
â
â
â
f--V----fC--R--L--C-K--L-D--K--E----S- â ââ
â
â
integerâ
â
-f-V----fC--R--L--C-K--L-D--K--E----S- â ââ
â«
â¬
â
4. Select a VCO frequency multiplier, N.
N
=
r
o
un
â
dâ
â
R------Ã-----ff--VR----CC---L-L--K-K---D----E---S--â ââ
Reduce N/R to the lowest possible R.
5. If N is < Nmax, use P = 0. If N > Nmax, choose P using this table:
Current N Value
P
0 < N ⤠Nmax
0
Nmax < N ⤠Nmax à 2
1
Nmax à 2 < N ⤠Nmax à 4
2
Nmax à 4 < N ⤠Nmax à 8
3
Then recalculate N:
N
=
ro
un
â
dâ
â
-R----f-Ã-R---C-f--V-L---CK---L--Ã-K----2D---P-E---S--â ââ
6. Calculate and verify the adequacy of the VCO and bus frequencies fVCLK and fBUS.
fVCLK = (2P Ã N â R) Ã fRCLK
fBUS = (fVCLK) â 4
7. Select the VCOâs power-of-two range multiplier E, according to this table:
Frequency Range
E(1)
0 < fVCLK < 8 MHz
0
8 MHz ⤠fVCLK < 16 MHz
1
16 MHz ⤠fVCLK < 32 MHz
2
1. Do not program E to a value of 3.
8. Select a VCO linear range multiplier, L, where fNOM = 38.4 kHz
L
=
ro
un
dn
â
â
â
2----E--f--V-Ã---C--f-NL----KO----M---â ââ
MC68HC908GR16 Data Sheet, Rev. 5.0
64
Freescale Semiconductor
|
▷ |