# Is it a good idea to omit HSE in favour of HSI?

I'm considering using stm32 L412 chip that comes with a 48Mhz HSI with +/- 0.25% accuracy. On the datasheet it says HSI can be used to drive usb fs peripheral. I will also using SPI peripheral.

https://www.mouser.in/datasheet/2/389/dm00481909-1799148.pdf

My question is, Is HSI reliable and can it be used in production examples?

The internal oscillators are reliable, and will meet the accuracy specified in the datasheet, if that's adequate for your application, then by all means use them.

If you're not sure, and you have the pins and PCB area available, add a crystal or oscillator to your circuit, and board, and don't populate them unless you find they're required.

In the part you specify, there's an HSI48 which can be used to clock the USB peripheral, this oscillator gets trimmed by the chips 'Clock Recovery System' which uses the incoming USB Start Of Frame packets to adjust its frequency.

• extremely helpful advice, also in cubemx RCC option, there is option for crs sync with gpio, LSE and USB. Going by your last sentence, should I turn crs sync with usb on? – Mridul May 6 at 10:27
• Yes, if you're using it with USB that would be a sensible option. – Colin May 6 at 10:28

HSI16 and HSI48 are both reliable.

HSI16 +/- 2% accuracy over the whole temperature interval is extremely good for an internal oscillator.

HSI48 is less accurate but still reliable.

SPI clock recovery is far less critical than USB because it has a dedicated clock pin.

For safety critical applications use an external hardware watchdog to control your CPU code execution.