UM10850
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User manual
Rev. 2.4 — 13 September 2016
51 of 464
NXP Semiconductors
UM10850
Chapter 4: LPC5410x System configuration (SYSCON)
4.5.37.5.1
System PLL spread spectrum control register 0
PLL0 M-divider register:
The PLL0 M-divider value (MDEC) can be set directly if the
PLL0 is not used with the spread spectrum clock generator (SSCG). If the SSCG is
enabled via the SEL_EXT bit, then the SSCG sets the MDEC value.
Remark:
MDEC does not use the direct binary representations of M directly. Instead, it
uses an encoded version of M. The valid range for M is 1 to 2^15. This value is encoded
into a 17-bit MDEC value.
The relationship between M and MDEC is expressed via the following pseudo-code.
M_max=0x00008000, x=0x00004000;
switch (M) {
case 0: x = 0xFFFFFFFF;
case 1: x = 0x00018003;
case 2: x = 0x00010003;
default: for (i = M; i <= M_max; i++)
x = (((x ^ (x>>1)) & 1) << 14) | ((x>>1) & 0x3FFF); }
MDEC[16:0] = x;
The values for SELP, SELI, and SELR depend on the value for M as expressed by the
following pseudo-code:
if (M < 60) then
SELP = (M>>1) + 1
else
SELP = 31;
if (M > 16384) then
SELI = 1
else if (M > 8192) then
SELI = 2
else if (M > 2048) then
SELI = 4
else if (M >= 501) then
SELI = 8
Table 70.
System PLL spread spectrum control register 0 (SYSPLLSSCTRL0, address 0x4000 01C0) bit description
Bit
Symbol
Value
Description
Reset
value
16:0
MDEC
Decoded M-divider coefficient value
0
17
MREQ
MDEC reload request. When a 1 is written to this bit, the MDEC 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 MDEC value is changed.
0
18
SEL_EXT
Select spread spectrum mode. Selects the source of the feedback divider value. For
normal mode, this must be the value from the MDEC field in this register.
For spread spectrum mode: SEL_EXT = 0, BANDSEL = 0, and UPLIMOFF = 1.
0
0
The PLL feedback divider value comes from the spread spectrum controller.
1
The PLL feedback divider value comes from the MDEC field in this register.
31:19
-
Reserved. Read value is undefined, only zero should be written.
-