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MC68HC08GZ32 Datasheet, PDF (58/320 Pages) Freescale Semiconductor, Inc – Microcontrollers
Clock Generator Module (CGM)
Specifications. After choosing N, the actual bus frequency can be determined using equation in 2
above.
4. Select a VCO frequency multiplier, N.
N
=
r
o
u
n
d
⎛
⎜
⎝
-f-V----fC--R--L--C-K--L-D--K--E----S- ⎠⎟⎞
5. Calculate and verify the adequacy of the VCO and bus frequencies fVCLK and fBUS.
fVCLK = (N) × fRCLK
fBUS = (fVCLK) ⁄ 4
6. Select the VCO’s power-of-two range multiplier E, according to Table 4-2.
Table 4-2. Power-of-Two Range Selectors
Frequency Range
E
0 < fVCLK ≤ 8 MHz
0
8 MHz< fVCLK ≤ 16 MHz
1
16 MHz< fVCLK ≤ 32 MHz
2(1)
1. Do not program E to a value of 3.
7. Select a VCO linear range multiplier, L, where fNOM = 71.4 kHz
L = Round
fVCLK
2E x fNOM
8. Calculate and verify the adequacy of the VCO programmed center-of-range frequency, fVRS. The
center-of-range frequency is the midpoint between the minimum and maximum frequencies
attainable by the PLL.
fVRS = (L x 2E) fNOM
9. For proper operation,
fVRS – fVCLK
≤ -f-N----O----M------×-----2---E--
2
10. Verify the choice of N, E, and L by comparing fVCLK to fVRS and fVCLKDES. For proper operation,
fVCLK must be within the application’s tolerance of fVCLKDES, and fVRS must be as close as possible
to fVCLK.
NOTE
Exceeding the recommended maximum bus frequency or VCO frequency
can crash the MCU.
11. Program the PLL registers accordingly:
a. In the VPR bits of the PLL control register (PCTL), program the binary equivalent of E.
b. In the PLL multiplier select register low (PMSL) and the PLL multiplier select register high
(PMSH), program the binary equivalent of N. If using a 1–8 MHz reference, the PMSL register
must be reprogrammed from the reset value before enabling the pll.
c. In the PLL VCO range select register (PMRS), program the binary coded equivalent of L.
MC68HC08GZ32 Data Sheet, Rev. 3
58
Freescale Semiconductor