1. Article purpose[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 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";
};
The LPTIM connected to HSI and used for HSE over frequency monitoring has to be be assigned to the Arm® Cortex®-A secure context (OPTEE) and to be enabled in counter node in OP-TEE device tree with:
&lptimer3 {
status = "okay";
counter {
status ="okay";
};
};
&lptimer1 {
status = "okay";
counter {
status = "okay";
};
};