S12 Clock, Reset and Power Management Unit (S12CPMU)
Several examples of PLL divider settings are shown in Table 7-23. The following rules help to achieve
optimum stability and shortest lock time:
•
•
Use lowest possible f
/ f
ratio (SYNDIV value).
VCO REF
Use highest possible REFCLK frequency f
.
REF
Table 7-23. Examples of PLL Divider Settings
fosc REFDIV[3:0] fREF REFFRQ[1:0] SYNDIV[5:0]
fVCO
VCOFRQ[1:0] POSTDIV[4:0]
fPLL
fbus
off
off
$00
$00
$00
$00
1MHz
1MHz
1MHz
4MHz
00
00
00
01
$1F
$1F
$0F
$03
64MHz
64MHz
32MHz
32MHz
01
01
00
01
$03
$00
$00
$00
16MHz
8MHz
64MHz 32MHz
32MHz 16MHz
32MHz 16MHz
off
4MHz
The phase detector inside the PLL compares the feedback clock (FBCLK = VCOCLK/(SYNDIV+1)) with
the reference clock (REFCLK = IRC1M or OSCCLK/REFDIV+1)). Correction pulses are generated based
on the phase difference between the two signals. The loop filter alters the DC voltage on the internal filter
capacitor, based on the width and direction of the correction pulse, which leads to a higher or lower VCO
frequency.
The user must select the range of the REFCLK frequency (REFFRQ[1:0] bits) and the range of the
VCOCLK frequency (VCOFRQ[1:0] bits) to ensure that the correct PLL loop bandwidth is set.
The lock detector compares the frequencies of the FBCLK and the REFCLK. Therefore the speed of the
lock detector is directly proportional to the reference clock frequency. The circuit determines the lock
condition based on this comparison.
If PLL LOCK interrupt requests are enabled, the software can wait for an interrupt request and for instance
check the LOCK bit. If interrupt requests are disabled, software can poll the LOCK bit continuously
(during PLL start-up) or at periodic intervals. In either case, only when the LOCK bit is set, the VCOCLK
will have stabilized to the programmed frequency.
•
•
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 the tolerance ∆
and is cleared when
Lock
the VCO frequency is out of the tolerance ∆ .
unl
•
Interrupt requests can occur if enabled (LOCKIE = 1) when the lock condition changes, toggling
the LOCK bit.
S12P-Family Reference Manual, Rev. 1.13
238
Freescale Semiconductor
FREESCALE [ Freescale ]
