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UM10850
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User manual
Rev. 2.4 — 13 September 2016
50 of 464
NXP Semiconductors
UM10850
Chapter 4: LPC5410x System configuration (SYSCON)
4.5.37.4 System PLL P-divider register
Remark:
The PLL P-divider register does not use the direct binary representation of P
divide value directly. Instead, it uses an encoded version PDEC.
Remark:
While the PLL0 output is in use, do not change the PDEC value. Changing the
PDEC value changes the PLL output frequency and can cause the system to fail.
•
The valid range for P is from 1 to 2^5. This value is encoded into a 7-bit PDEC value.
The relationship can be expressed through the following pseudo-code:
P_max=0x20, x=0x10;
switch (P) {
case 0: x = 0xFFFFFFFF;
case 1: x = 0x00000062;
case 2: x = 0x00000042;
default: for (i = P; i <= P_max; i++)
x = (((x ^ (x>>2)) & 1) << 4) | ((x>>1) & 0xF); }
PDEC[6:0] = x;
4.5.37.5 Spread spectrum control with PLL0
The spread spectrum functionality can be used to provide a spread spectrum clock, which
can decrease electromagnetic interference (EMI).
The Spread Spectrum Clock Generator can be used in several ways:
•
It can encode M-divider values between 1 and 255 to produce the MDEC value used
directly by the PLL, saving the need for executing encoding algorithm code, or
hard-coding predetermined values into an application.
•
It can provide a fractional rate feature to the PLL.
•
It can be set up to automatically alter the PLL CCO frequency on an ongoing basis to
decrease electromagnetic interference (EMI).
If the spread spectrum mode is enabled, choose N to ensure 2 MHz < Fin/N < 4 MHz.
Spread spectrum mode cannot be used when Fin = 32 kHz.
When the modulation (MR) is set to zero, the PLL becomes a fractional PLL.
Table 69.
System PLL P-divider register (SYSPLLPDEC, address 0x4000 01BC) bit description
Bit
Symbol
Description
Reset value
6:0
PDEC
Decoded P-divider coefficient value
0
7
PREQ
PDEC reload request. When a 1 is written to this bit, the PDEC value is loaded into the PLL.
Must be cleared by software for any subsequent load, or the PLL can be powered down and
back up via the PDEN_SYS_PLL bit in the PDRUNCFG register if the PDEC value is
changed.
0
31:8
-
Reserved. Read value is undefined, only zero should be written.
-