1. Article purpose[edit | edit source]
The purpose of this article is to explain how to monitor the HSE oscillator.
If the HSE oscillator is used in the clock tree, it is possible to enable a monitoring function to detect and avoid over frequency. The LPTIM mechanism can detect any over frequency and triggers a tamper event which runs the appropriate custom actions.
2. How to activate the monitoring[edit | edit source]
By default, the HSE monitor is disabled in the dtsi "SoC" files.
To activate the HSE monitor, add this node in your board OP-TEE device tree
&hse_monitor {
status = "okay";
};
Moreover the LPTIM connected to HSI and used for HSE over frequency monitoring (LPTIM3 on STM32MP13x lines 
and LPTIM1 on STM32MP25x lines ) has to be:
- assigned to the Arm® Cortex®-A secure context (OPTEE)
- enabled in counter node in your board OP-TEE device tree with:
- on STM32MP13x lines :
- on STM32MP13x lines
&lptimer3 {
status = "okay";
counter {
status ="okay";
};
};
- on STM32MP25x lines :
- on STM32MP25x lines
&lptimer1 {
status = "okay";
counter {
status = "okay";
};
};