I'm trying to set the QUADSPI FIFO threshold when reading from external flash using interrupts and I want an interrupt to occur when there are 32 valid bytes in the FIFO. I'm setting the QUADSPI_CR_FTHRES to 31 according to the datasheet but when I look at the registers I can see that only the 4 first bits are set. I've tried to manually flip the 5th bit with the debugger but it's as if the bit is write protected or something. Why can't I set the FTHRES value to 0x1F?
Below are pictures when I step with the debugger:
