MC68HC908GR16A Datasheet, PDF (62/270 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Clock Generator Module (CGM)
4. Select a VCO frequency multiplier, N.
N
=
r
o
u
n
d
⎛
⎜
⎝
-f-^V----f^C--_R--^L--_C-^K--_L-^D--_K--^E----^S- _{⎠⎟⎞}
5. Calculate and verify the adequacy of the VCO and bus frequencies f_VCLK and f_BUS.
f_VCLK = (N) × f_RCLK
f_BUS = (f_VCLK) ⁄ 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 < f_VCLK ≤ 8 MHz
0
8 MHz< f_VCLK ≤ 16 MHz
1
16 MHz< f_VCLK ≤ 32 MHz
2⁽¹⁾
1. Do not program E to a value of 3.
7. Select a VCO linear range multiplier, L, where f_NOM = 71.4 kHz
L = Round
f_VCLK
2^E x f_NOM
8. Calculate and verify the adequacy of the VCO programmed center-of-range frequency, f_VRS. The
center-of-range frequency is the midpoint between the minimum and maximum frequencies
attainable by the PLL.
f_VRS = (L x 2^E) f_NOM
9. For proper operation,
f_VRS – f_VCLK
≤
f_{--N----O----M------}×_-----2_---^E_--
2
10. Verify the choice of N, E, and L by comparing f_VCLK to f_VRS and f_VCLKDES. For proper operation,
f_VCLK must be within the application’s tolerance of f_VCLKDES, and f_VRS must be as close as possible
to f_VCLK.
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.
MC68HC908GR16A Data Sheet, Rev. 1.0
62
Freescale Semiconductor