LPC8xx XTALIN voltage compliance

[sh9]

Hi!

I am finding the data sheet somewhat unclear regarding the XTALIN pin. I understand that when used with a crystal, and the internal crystal oscillator active, the permissible voltage is limited to 1.95 V.

However, the pin can also be used with the oscillator bypassed, or it can be used as a general purpose pin. My question is: Does the voltage restriction also apply in those modes, or do I only have to obey it when using the pin with the crystal oscillator active?

My specific usage would call for a 3.3V clock signal fed into the pin with the oscillator bypassed. Is it necessary in this mode to limit the voltage to 1.8V?

Thanks!

[ZhangJennie]

HI 

In slave mode, with an amplitude between 200 mV (RMS) and 1000 mV (RMS). This corresponds to a square wave signal with a signal swing of between 280 mV and 1.4 V.

For detail, see DS:

Have a nice day,

Jun Zhang

[sh9]

Thank you, Jun Zhang, for your reply!

Let me ask one more question to see if I understand you correctly:

Is it correct that when the data sheet refers to "slave mode", this is the same as "system oscillator bypass" mentioned in the user manual (for example UM11029 chapter 8.6.4)?

If this is the case, then is it correct to say that the voltage limit of 1.8V applies whenever the XTALIN function is enabled in the PINENABLE register, and is independent from the setting in the SYSOSCCTRL register?

Thanks for any clarification you can provide!

[ZhangJennie · Accepted solution]

>>Is it correct that when the data sheet refers to "slave mode", this is the same as "system oscillator bypass" mentioned in the user manual (for example UM11029 chapter 8.6.4)?

Yes, correct.

>>If this is the case, then is it correct to say that the voltage limit of 1.8V applies whenever the XTALIN function is enabled in the PINENABLE register, and is independent from the setting in the SYSOSCCTRL register?

Yes, but if you use bypass mode( or say, slave mode), you need follow the rule as I pointed out in my first reply.

Have a nice day,

Jun Zhang

[sh9]

This solves my problem, thank you!