mcf51jf128 Freescale Semiconductor, Inc, mcf51jf128 Datasheet - Page 452

• Manufacturer Part Number: mcf51jf128
• Description: Mcf51jf128 Reference Manual
• Manufacturer: Freescale Semiconductor, Inc

Functional Description
20.4.4 External Reference Clock
The MCG module can support an external reference clock in all modes. Refer to the
device datasheet for external reference frequency range. When C1[IREFS] is set, the
external reference clock will not be used by the FLL or PLL. In these modes, the
frequency can be equal to the maximum frequency the chip-level timing specifications
will support.
If the CME bit is asserted the slow internal reference clock gets enabledincluding the
clock monitor. If the external reference falls below a certain frequency for the case when
C6[CME]=1 (f
or f
depending on C2[RANGE]), the MCU will reset and
loc_high
loc_low
S[LOCK] will be cleared.
20.4.5 MCG Fixed Frequency Clock
The MCG Fixed Frequency Clock (MCGFFCLK) provides a fixed frequency clock
source for other on-chip peripherals. This clock is driven by either the slow clock from
the internal reference clock generator or the external reference clock from the Crystal
Oscillator, divided by the FLL reference clock divider. The source of MCGFFCLK is
selected by C1[IREFS]. Additionally, this clock is divided by two.
Finally, this clock is synchronized to the peripheral bus clock and altered to have a duty
cycle width equal to one peripheral bus clock period. This clock is only valid when it’s
frequency is not more than 1/8 of the MCGOUT clock frequency. When it is not valid, it
is disabled and held high. This clock is also disabled in Stop mode.
20.4.6 MCG Background Debug Controller Clock
The MCG Background Debug Controller Clock (MCGBDCCLK) provides a clock
source to the Background Debug Controller (BDC). This clock is driven by either the
CORECLK or the fast clock from the 4 Mhz internal reference clock generator as
selected by the CLKSW bit. The CLKSW bit is located in the BDC Status and Control
Register.
MCF51JF128 Reference Manual, Rev. 2, 03/2011
Preliminary
452
Freescale Semiconductor, Inc.