UM10503
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© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1.3 — 6 July 2012
127 of 1269
NXP Semiconductors
UM10503
Chapter 11: LPC43xx Clock Generation Unit (CGU)
The PLL contains three programmable dividers: pre-divider (N), feedback-divider (M) and
post-divider (P). The PLL contains a lock detector which measures the phase difference
between the rising edges of the input and feedback clocks. Only when this difference is
smaller than the so called “lock criterion” for more than seven consecutive input clock
periods, the lock output switches from low to high. A single too large phase difference
immediately resets the counter and causes the lock signal to drop (if it was high).
Requiring seven phase measurements in a row to be below a certain figure ensures that
the lock detector will not indicate lock until both the phase and frequency of the input and
feedback clocks are very well aligned. This effectively prevents false lock indications, and
thus ensures a glitch free lock signal.
To avoid frequency hang-up the PLL contains a frequency limiter. This feature is built in to
prevent the CCO from running too fast, this can occur if e.g. a wrong feedback-divider (M)
ratio is applied to the PLL.
Remark:
The PLL0 does not use the direct binary representations of M, N, and P directly.
Instead, encoded versions MDEC, NDEC, and PDEC of M, N, and P respectively.
and
for how to obtain the encoded values for M, N,
and P.
11.7.4.3 Use of PLL0 operating modes
11.7.4.3.1
Normal Mode
Mode 1 is the normal operating mode.
The pre- and post-divider can be selected to give:
Fig 27. PLL0 block diagram
Bypass
CTRL[1]
CLKOUT
CLKIN
32kHz
IRC
ENET_RX_CLK
ENET_TX_CLK
GP_CLKIN
CRYSTAL
PLL1
IDIVA
IDIVB
IDIVC
IDIVD
IDIVE
CTRL[27:24]
“1”
N-DIVIDER
NP_DIV[21:12]
Direct Input
CTRL[2]
PFD
Filter
CCO
Q
D
CLKEN
CTRL[4]
/2
NP_DIV[6:0]
P-DIVIDER
/2
M-DIVIDER
MDIV[16:0]
Direct Output
CTRL[3]
Bandwidth Select P,I,R
MDIV[31:17]
Table 92.
PLL0 operating modes
PLL0_Mode bit settings:
Mode
PD
CLKEN
BYPASS
DIRECTI
DIRECTO
FRM
1: Normal
0
1
0
1/0
1/0
0
3: Power Down
1
x
x
x
x
x