UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 48 of 841
NXP Semiconductors
UM10360
Chapter 4: LPC176x/5x Clocking and power control
4.6 PLL1 (Phase Locked Loop 1)
PLL1 receives its clock input from the main oscillator only and can be used to provide a
fixed 48 MHz clock only to the USB subsystem. This is an option in addition to the
possibility of generating the USB clock from PLL0.
PLL1 is disabled and powered off on reset. If PLL1 is left disabled, the USB clock can be
supplied by PLL0 if everything is set up to provide 48 MHz through that route. If PLL1 is
enabled and connected via the PLL1CON register (see Section 4.6.2
), it is automatically
selected to drive the USB subsystem (see Figure 7
).
PLL1 activation is controlled via the PLL1CON register. PLL1 multiplier and divider values
are controlled by the PLL1CFG register. These two registers are protected in order to
prevent accidental alteration of PLL1 parameters or deactivation of PLL1. The protection
is accomplished by a feed sequence similar to that of the Watchdog Timer. Details are
provided in the description of the PLL1FEED register.
PLL1 accepts an input clock frequency in the range of 10 MHz to 25 MHz only. The input
frequency is multiplied up to the range of 48 MHz for the USB clock using a Current
Controlled Oscillator (CCO). The multiplier can be an integer value from 1 to 32 (for USB,
the multiplier value cannot be higher than 4. The CCO operates in the range of 156 MHz
to 320 MHz, so there is an additional divider in the loop to keep the CCO within its
frequency range while PLL1 is providing the desired output frequency. The output divider
may be set to divide by 2, 4, 8, or 16 to produce the output clock. Since the minimum
output divider value is 2, it is insured that the output of PLL1 has a 50% duty cycle. A
block diagram of PLL1 is shown in Figure 10
.
4.6.1 PLL1 register description
PLL1 is controlled by the registers shown in Table 29. More detailed descriptions follow.
Writes to any unused bits are ignored. A read of any unused bits will return a logic zero.
Warning: Improper setting of PLL1 values may result in incorrect operation of the
USB subsystem!
Table 29. PLL1 registers
Name Description Access Reset
value
[1]
Address
PLL1CON PLL1 Control Register. Holding register for
updating PLL1 control bits. Values written to this
register do not take effect until a valid PLL1 feed
sequence has taken place.
R/W 0 0x400F C0A0
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