Freescale Semiconductor MC68HC908MR32 Manual Do Utilizador
Functional Description
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
Freescale Semiconductor
61
The PLL also may operate in manual mode (AUTO = 0). Manual mode is used by systems that do not
require an indicator of the lock condition for proper operation. Such systems typically operate well below
f
require an indicator of the lock condition for proper operation. Such systems typically operate well below
f
BUSMAX
and require fast startup. These conditions apply when in manual mode:
•
ACQ is a writable control bit that controls the mode of the filter. Before turning on the PLL in manual
mode, the ACQ bit must be clear.
mode, the ACQ bit must be clear.
•
Before entering tracking mode (ACQ = 1), software must wait a given time, t
ACQ
), after turning on the PLL by setting PLLON in the PLL control
register (PCTL).
•
Software must wait a given time, t
AL
, after entering tracking mode before selecting the PLL as the
clock source to CGMOUT (BCS = 1).
•
The LOCK bit is disabled.
•
CPU interrupts from the CGM are disabled.
4.3.2.4 Programming the PLL
Use this 9-step procedure to program the PLL.
lists the variables used and their meaning.
1.
Choose the desired bus frequency, f
BUSDES
.
Example: f
BUSDES
= 8 MHz
2.
Calculate the desired VCO frequency, f
VCLKDES
.
f
VCLKDES
= 4 x f
BUSDES
Example: f
VCLKDES
= 4 x 8 MHz = 32 MHz
3.
Using a reference frequency, f
RCLK
, equal to the crystal frequency, calculate the VCO frequency
multiplier, N. Round the result to the nearest integer.
4.
Calculate the VCO frequency, f
VCLK
.
Table 4-1. Variable Definitions
Variable
Definition
f
BUSDES
Desired bus clock frequency
f
VCLKDES
Desired VCO clock frequency
f
RCLK
Chosen reference crystal frequency
f
VCLK
Calculated VCO clock frequency
f
BUS
Calculated bus clock frequency
f
NOM
Nominal VCO center frequency
f
VRS
Shifted FCO center frequency
f
VCLKDES
f
RCLK
N =
Example: N =
32 MHz
4 MHz
= 8 MHz
f
VCLK
= N x f
RCLK
Example: f
VCLK
= 8 x 4 MHz = 32 MHz