mcf51ac256a Freescale Semiconductor, Inc, mcf51ac256a Datasheet - Page 388

• Manufacturer Part Number: mcf51ac256a
• Description: Mcf51ac Flexis
• Manufacturer: Freescale Semiconductor, Inc

Multipurpose Clock Generator (MCGV3)
frequency calculations using RDIV, BDIV, and VDIV settings for each clock mode. The bus frequency is
equal to MCGOUT divided by 2.
This section will include 3 mode switching examples using an 8 MHz external crystal. If using an external
clock source less than 1 MHz, the MCG should not be configured for any of the PLL modes (PEE and
PBE).
16.5.3.1
In this example, the MCG will move through the proper operational modes from FEI to PEE mode until
the 8 MHz crystal reference frequency is set to achieve a bus frequency of 16 MHz. Because the MCG is
in FEI mode out of reset, this example also shows how to initialize the MCG for PEE mode out of reset.
First, the code sequence will be described. Then a flowchart will be included which illustrates the
sequence.
16-20
1. First, FEI must transition to FBE mode:
1
FEI (FLL engaged internal)
FEE (FLL engaged external)
FBE (FLL bypassed external)
FBI (FLL bypassed internal)
PEE (PLL engaged external)
PBE (PLL bypassed external)
BLPI (Bypassed low power internal)
BLPE (Bypassed low power external)
a) MCGC2 = 0x36 (%00110110)
R is the reference divider selected by the RDIV bits, B is the bus frequency divider selected by the BDIV bits,
F is the FLL factor selected by the DRS[1:0] and DMX32 bits, and M is the multiplier selected by the VDIV
bits.
– BDIV (bits 7 and 6) set to %00, or divide-by-1
– RANGE (bit 5) set to 1 because the frequency of 8 MHz is within the high frequency range
– HGO (bit 4) set to 1 to configure the crystal oscillator for high gain operation
– EREFS (bit 2) set to 1, because a crystal is being used
– ERCLKEN (bit 1) set to 1 to ensure the external reference clock is active
Example 1: Moving from FEI to PEE Mode: External Crystal = 8 MHz, Bus
Frequency = 16 MHz
Clock Mode
MCF51AC256 ColdFire Integrated Microcontroller Reference Manual, Rev. 5
Table 16-12. MCGOUT Frequency Calculation Options
(f
(f
f
f
[(f
f
f
f
ext
int
ext
int
ext
int
ext
ext
/ B
/ B
/ B
/ B
/ B
* F ) / B
/ R *F) / B
/ R) * M] / B
f
MCGOUT
1
Typical f
immediately after reset.
f
31.25 kHz to 39.0625 kHz
f
31.25 kHz to 39.0625 kHz
Typical f
f
MHz to 2 MHz
f
MHz to 2 MHz
ext
ext
ext
ext
/ R must be in the range of
/ R must be in the range of
/ R must be in the range of 1
/ R must be in the range of 1
MCGOUT
int
= 32 kHz
Note
Freescale Semiconductor
= 16 MHz