UM10850
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© NXP B.V. 2016. All rights reserved.
User manual
Rev. 2.4 — 13 September 2016
74 of 464
NXP Semiconductors
UM10850
Chapter 4: LPC5410x System configuration (SYSCON)
4.6.4.5 PLL usage
As previously noted, the PLL divider settings used in the PLL registers are not simple
binary values, they are encoded as shown in the PLL register descriptions. The divider
values and their encoding can be found by calculation using the information in this
document. For simple PLL usage with no pre- or post-divide, the set_pll API can be used
(see
). Also, a PLL setting calculator can be found on the NXP website. The
latter two possibilities are recommended in order to avoid PLL setup issues.
4.6.4.5.1
Procedure for determining PLL settings
In general, PLL configuration values may be found as follows:
1. Identify a desired PLL output frequency. This may depend on a specific interface
frequency needed or be based on expected CPU performance requirements, and
may be limited by system power availability.
2. Determine which clock source to use as the PLL input. This can be influenced by
power or accuracy required, or by the potential to obtain the desired PLL output
frequency.
3. Identify PLL settings to obtain the desired output form the selected input. The Fcco
frequency must be either the actual desired output frequency, or the desired output
frequency times 2 x P, where P is from 2 to 32. The Fcco frequency must also be a
multiple of the PLL reference frequency, which is either the PLL input, or the PLL input
divided by N, where N is from 2 to 256.
4. There may be several ways to obtain the same PLL output frequency. PLL power
depends on Fcco (a lower frequency uses less power) and the divider used.
Bypassing the input and/or output divider saves power.
5. Check that the selected settings meet all of the PLL requirements:
–
Fin is in the range of 32 kHz to 24 MHz.
–
Fcco is in the range of 75 MHz to 150 MHz.
–
Fout is in the range of 1.2 MHz to 150 MHz.
–
The pre-divider is either bypassed, or N is in the range of 2 to 256.
–
The post-divider is either bypassed, or P is in the range of 2 to 32.
–
M is in the range of 3 to 32,768.
Also note that PLL startup time becomes longer as Fref drops below 500 kHz. At 500
kHz and above, startup time is up to 500 microseconds. Below 500 kHz, startup time
can be estimated as 200 / Fref, or up to 6.1 milliseconds for Fref = 32 kHz. PLL
accuracy and jitter is better with higher values of Fref.
4.6.4.5.2
PLL setup sequence
The following sequence should be followed to initialize and connect the PLL:
SYSPLLPDEC
PLL P divider
SYSPLLSSCTRL0
PLL spread spectrum control 0
SYSPLLSSCTRL1
PLL spread spectrum control 1
Table 102. System PLL status register (SYSPLLSTAT, address 0x4000 01B4) bit description
Register
Description
Reference