Datasheet

Clocks and Reset Generator (CRG)
Functional Description
MC9S12DP256 — Revision 1.1
Clocks and Reset Generator (CRG)
Tracking mode — In tracking mode, the filter makes only small
corrections to the frequency of the VCO. PLL jitter is much lower in
tracking mode, but the response to noise is also slower. The PLL enters
tracking mode when the VCO frequency is nearly correct and the
TRACK bit is set in the CRGFLG register.
The PLL can change the bandwidth or operational mode of the loop filter
manually or automatically.
In automatic bandwidth control mode (AUTO = 1), the lock detector
automatically switches between acquisition and tracking modes.
Automatic bandwidth control mode also is used to determine when the
PLL clock, PLLCLK, is safe to use as the source for the system clock,
SYSCLK. If PLL LOCK interrupt requests are enabled, the software can
wait for an interrupt request and then check the LOCK bit. If CPU
interrupts are disabled, software can poll the LOCK bit continuously
(during PLL start-up, usually) or at periodic intervals. In either case, only
when the LOCK bit is set, is the PLLCLK clock safe to use as the source
for the system clock. If the PLL is selected as the source for the system
clock and the LOCK bit is clear, the PLL has suffered a severe noise hit
and the software must take appropriate action, depending on the
application.
The following conditions apply when the PLL is in automatic bandwidth
control mode (AUTO=1):
• The TRACK bit is a read-only indicator of the mode of the filter.
• The TRACK bit is set when the VCO frequency is within a certain
tolerance, ∆
trk
, and is clear when the VCO frequency is out of a
certain tolerance, ∆
unt
.
• The LOCK bit is a read-only indicator of the locked state of the
PLL.
• The LOCK bit is set when the VCO frequency is within a certain
tolerance, ∆
Lock
, and is cleared when the VCO frequency is out of
a certain tolerance, ∆
unl
.
• CPU interrupts can occur if enabled (LOCKIE = 1) when the lock
condition changes, toggling the LOCK bit.
Freescale Semiconductor, I
Freescale Semiconductor, Inc.
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