STM Prescaler - what happens if it's negative or too big?
The ST timer prescalers are unsigned 16 bit numbers.
You cannot represent a negative nunmber in an unsigned 16 bit register.

Set your Windows calculator to Progammer made, select Word size, and enter -1.
The result is 0xFFFF, maximum prescaler. Due to the two's complement signing method.

You also can't fit a number larger than 65535. You can try to write 0xFFFF'FFFF,
but the high halfword (16 bit) will be discarded.