LPC1114 bypassing the system oscillator

[heaton_1991]

I have a question regarding bypassing the system oscillator in LPC1114. If I enable the system oscillator bypass enable (set bit 0 of SYSOSCCTRL to be 1), I could connect an external clock source to XTALIN pin as the external clock source. However, from the following figure, I don't really know how this XTALIN pin is connected to the clock generation unit. Does anyone have any idea about this? 

To be more specific, if the system oscillator is bypass enabled and I connect a new clock source referred as CLK to the XTALIN pin, where is CLK connected to the above figure? Does it just replace the system oscillator?

[Alice_Yang]

Hello Haohao Liao,

About "I don't really know how this XTALIN pin is connected to the clock generation unit. ",

-> This is internal connect of chip, PLL input (sys_osc_clok) is fed directly from XTALIN pin, you can just regard it as the external clock source instead of  system oscillator . Then configure "System PLL clock source select registe "r to 0x1 can select it as PLL input.

Have a great day,
TIC

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[heaton_1991]

Hi, Alice,

Thanks a lot for your help.

So, if we choose bypass the system oscillator, and connect the external

clock source(maybe from a function generator with frequency 12 Mhz) to the

XTALIN, then this clock source will be fed directly to the PLL input. Thus,

if we choose the system PLL clock source to be the system oscillator and

configure the main clock select register to select system PLL clock out,

the main clock will be our external clock source * 4 (assume MSEL = 3 and

PSEL = 2), which is 48 Mhz. Am I right?

Alice_Yang <admin@community.nxp.com> 于2018年12月11日周二 上午12:59写道：

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Re: LPC1114 bypassing the system oscillator

reply from Alice_Yang

<https://community.nxp.com/people/Alice_Yang?et=watches.email.thread> in *LPC

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<https://community.nxp.com/message/1090105?commentID=1090105&et=watches.email.thread#comment-1090105>

[Alice_Yang]

Hello Haohao,

Yes, you are right.

Have a great day,
TIC

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