PLL0 input frequency range

lpcware — Wed, 15 Jun 2016 17:54:36 GMT

Content originally posted in LPCWare by Dragonheart77 on Sat Oct 05 00:16:57 MST 2013
Hi there,
I use the lpc1778 and the input frequency range for the PLL0 of the LPC17xx family is not clear to me.
The data sheet just tells me about a range of 10 to 25 MHz for PLL1. There is no such statement for PLL0. It just says "are functionally
identical but have somewhat different input possibilities and output connections". Is it the same input frequency range for PLL0 like PLL1?
For PPL1, mainly used for the USB clock, it sounds reasonable to allow 10 to 25 MHz and save something in the PLL block and silicon. Otherwise 2 different PLL blocks in silicon?
Also the source code of the "system_LPC177x_8x.c" says "F_in is in the range of 1 MHz to 25 MHz" as comment.
Currently I use an external 4 MHz oscillator. M = 30, P = 1 -> PLL Clk Out = 120 MHz, Fcco = 240 MHz OK. It works on 4 boards so far.
But does it always work in the temperature range?
I need the 4 MHz for other devices on my board and so I want to save parts and money. I'm not sure if the 4 MHz input is valid or not.
Thanks for your help.

Thomas

Re: PLL0 input frequency range

lpcware — Wed, 15 Jun 2016 17:54:37 GMT

Content originally posted in LPCWare by Rob65 on Tue Oct 08 22:54:10 MST 2013
Thomas,

I started out by stating that you should re-read the datasheet and user manual but it seems NXP messed up on this one.
The description on chapter 7.33.2 of the datasheet (LPC178X_7X.pdf) states that the main oscillator needs a crystal in the range of 10-25 MHz.
But then, in 14.2 there is something else ... ranges run from 1 up to 25 MHz.

The same is also written in 11.3 (Dynamic characteristics).

You are not using the oscillator in slave mode (according to fig. 36 in chapter 14.2) with a 4 MHz oscillator which is perfectly within spec.
So nothing to worry about.

Rob - NXP definitely made this a tricky one to answer ;)

topic Re: PLL0 input frequency range in LPC Microcontrollers

PLL0 input frequency range

Re: PLL0 input frequency range