topic 12MHz FRO frequency change in LPC Microcontrollers

12MHz FRO frequency change

F_Vaquero — Thu, 29 May 2025 09:03:34 GMT

Hello,

We are using the LPC54608J512BD208 microcontroller in our product with 2 UART (FLEXCOMM2 and FLEXCOMM4) sourced from the 12MHz internal FRO.

We saw that, from time to time, the baud rate of the UART changed to a lower value (it could take several days) and we confirmed (using a pin to output the 12MHz internal FRO) that the reason is frequency of the 12MHz FRO changed more or less 5% to a lower value.

I saw in this forum a similar issue:

https://community.nxp.com/t5/LPC-Microcontrollers/LPC54605-fro-hf-drift-lower-than-48MHz-after-IEC-61000-4-3-EMC/td-p/1178745

Do you know why frequency is changing more than 1% indicated at datasheet?

Thank you.

Re: 12MHz FRO frequency change

Harry_Zhang — Fri, 30 May 2025 08:27:27 GMT

Hi @F_Vaquero

According the LPC546xx Data Sheet.

The 12 MHz FRO (Free Running Oscillator) on the LPC54608J512 should typically stay within ±1% frequency accuracy across operating conditions, as stated in the datasheet.

A drift of 5% is unusual and exceeds the expected spec. Since you're observing the FRO output changing after long durations (several days) and your UART baud rate is affected, this may indicate a hardware or environmental sensitivity issue.

Your FRO drift issue likely results from EMC-induced trim corruption.

So i think the most robust solution is to switch to a crystal oscillator for clocking, especially for timing-critical subsystems like UART.

Harry