topic Documentation of oscillator cap bits on LPC55sxx in LPC Microcontrollers

Documentation of oscillator cap bits on LPC55sxx

apines — Sun, 15 Dec 2024 00:34:32 GMT

Bits 21:8 of "32 KHz Crystal oscillator (XTAL) control register" and "High-speed crystal oscillator control register (XO32M_CTRL)" are used to set the internal crystal oscillator capacitors. The data sheet doesn't describe what these bits mean and assumes that the user will just call the SDK function. Not everyone uses the SDK; we need to be able to write these directly. We can try to divine it from the SDK's functions (POWER_Xtal32khzCapabankTrim and POWER_Xtal16mhzCapabankTrim) but it would be better if it were actually documented somewhere. Is it, and we're just missing it? Otherwise, could someone explain how these bits are actually used?

Also, the app note (AN13057) in table 1 in section 5.1 calls out these functions as POWER_Xtal16mhzCapbankTrim and POWER_Xtal32mhzCapbankTrim, both of which are wrong.

Re: Documentation of oscillator cap bits on LPC55sxx

Alice_Yang — Tue, 17 Dec 2024 02:07:59 GMT

Hello @apines

Please refer to User Manual of the chip you are using.

For example LPC55s69, we can find the definitions for the two registers:

Alice

Re: Documentation of oscillator cap bits on LPC55sxx

apines — Tue, 17 Dec 2024 16:42:48 GMT

This doesn't answer the question and highlights my complaint. "Capabank setting input" and "Capabank setting output" doesn't give any insight into what's supposed to be written here.

You even included the quote which is my principal objection: "The values written to this register should not be changed. These registers are handled by the SDK power library."

Not everyone uses the SDK. What I'm looking for is a detailed description of exactly what these bits mean. The documentation on this part appears to be lacking.

Re: Documentation of oscillator cap bits on LPC55sxx

Alice_Yang — Wed, 18 Dec 2024 10:06:00 GMT

Hello @apines

Thanks for your clarify.

I have confirmed with our internal, as UM mentioned, these registers only can be handled by SDK power library. Maybe break the chip if you directly change register. So I highly recommend you use the SDK power library to config. Sorry for the inconvenient to you. Thanks.

Alice