topic Re: Max ext clock frequency LPC550x in LPC Microcontrollers

Max ext clock frequency LPC550x

ThomasO — Thu, 11 Nov 2021 06:41:27 GMT

Hi all!

I'm wondering what the max. frequency of external clock in bypass mode on the LPC550x is.

From LPC55S0x/LPC550x product datasheet Rev. 1.2 - 6. August 2021:

Page 4 reads: "Crystal oscillator with an operating frequency of 12 MHz to 32 MHz. Option for
external clock input (bypass mode) for clock frequencies of up to 25 MHz."

Page 98 reads: "In bypass mode, an external clock (maximum frequency of up to 24 MHz) can also be
connected to XTAL32M_P if XTAL32M_N is left open."

So that does not seem to be consistent. Was the spec. extended from 24 MHz to 25 MHz, and not all references updated?

Is it allowed to run LPC550x with 25 MHz external clock?

Many thanks in advance,

Thomas

Re: Max ext clock frequency LPC550x

xiangjun_rong — Thu, 11 Nov 2021 09:03:44 GMT

Hi,

I see that there is discrepancy for the required crystal clock frequency spec and external clock frequency spec in data sheet Rev1.2 of LPC550x, I have submitted a ticket to AE team so that they can confirm.

I will give you a feedback after I get response.

sorry to the inconvenience.

XiangJun Rong

Re: Max ext clock frequency LPC550x

xiangjun_rong — Wed, 17 Nov 2021 03:26:58 GMT

Hi, Thomas,

For crystal oscillator spec range, it is 12 MHz to 32 MHz. 1 MHz to 25 MHz is not correct.

For usage of bypass mode, maximum is indeed 24 Mhz since that is it what tested in bypass mode.

The bypass mode is not true bypass anything, the clock signal will be processed by Crystal oscillator.

so you should follow up the spec in datasheet as following for the external clock voltage:

Hope it can help you

XiangJun Rong