topic Re: Using a external Clock vs. an Oscillator in LPC Microcontrollers

Using a external Clock vs. an Oscillator

lpcware — Wed, 15 Jun 2016 19:26:36 GMT

Content originally posted in LPCWare by liaochengyi on Wed Nov 26 22:56:21 MST 2014
Hi,

I have a external clock =10Mhz or 20Mhz, can I use the clock used as the clock source for the 11u67 CPU?
My application as below: USB, uart, SPI, I2C, and RTC.

If YES!!

How to connect the clock to 11u67 CPU? Connect to XTALIN and XTALOUT?
But the max. voltage of crystal input voltage = 1.95V.

Is there has example?

Thank you.
Chengyi

Re: Using a external Clock vs. an Oscillator

lpcware — Wed, 15 Jun 2016 19:26:37 GMT

Content originally posted in LPCWare by NXP_Paul on Thu Nov 27 11:42:55 MST 2014
Yes, you can use an external clock source connected to the XTALIN pin. The input voltage must be limited to 1.95V, so if you have a clock source with a higher voltage, you will need to use a capacitive or resistive divider to bring it down to an acceptable voltage range.
The problem you're going to have using a 10MHz or 20MHz clock is that you will not be able to generate the 48MHz frequency needed for full-speed USB. You will need a clock source that can be multiplied up to 48MHz, such as a 12MHz clock.

-Paul

Re: Using a external Clock vs. an Oscillator

lpcware — Wed, 15 Jun 2016 19:26:38 GMT

Content originally posted in LPCWare by liaochengyi on Tue Dec 16 02:22:24 MST 2014
Dear Paul,

Thank you for the fast reply! :)

I checked the user manual and found the 4.4.5 USB PLL control register

The PLL accepts an input frequency from 10 MHz to 25 MHz from various
clock sources. The input frequency is multiplied up to a high frequency, then divided down
to provide the actual clock 48 MHz clock used by the USB subsystem.

and PSEL just can select: 1, 2, 4, 8

But my external clock option is: 4, 5, 10, 20Mhz

If I need the 48Mhz USB clcok -->

1. Select 20Mhz input, MSEL = 5, --> need PSEL = 12. :((

2. Select 4Mhz input, MSEL = 12, --> PSEL = 0

But I can't use the 4Mhz clock input. Right?

Re: Using a external Clock vs. an Oscillator

lpcware — Wed, 15 Jun 2016 19:26:39 GMT

Content originally posted in LPCWare by R2D2 on Tue Dec 16 06:46:36 MST 2014

Quote: liaochengyi
Right?

Wrong :)

Read UM and just calculate P / PSEL and M / MSEL:

Quote:
FCLKOUT = M x FCLKIN = FCCO / (2xP)

UM:

Quote:
The input frequency range is 10 MHz to 25 MHz.

So there are just 2 options:

- 12 MHz

- 24 MHz

Re: Using a external Clock vs. an Oscillator

lpcware — Wed, 15 Jun 2016 19:26:39 GMT

Content originally posted in LPCWare by liaochengyi on Wed Dec 17 02:47:55 MST 2014
Dear R2D2,

Thank you for answer. 0:)

Because our new project care about the PCB size.

How about the main clock using the internal clock to drive USB, uart, etc....

Is it OK for those peripherals application?

Thank you very much. :)
BR
chengyi

Re: Using a external Clock vs. an Oscillator

lpcware — Wed, 15 Jun 2016 19:26:40 GMT

Content originally posted in LPCWare by R2D2 on Wed Dec 17 06:12:24 MST 2014
See: http://www.lpcware.com/content/forum/irc-oscillator-vs-system-oscillator