And this is only writable if we are in secure boot mode.

这只有在我们处于安全启动才可写。

And it's not only that only this register is writable if that flag is set to one, but also the other one, this SPCSR0.

如果个标志被设个，不仅这个寄存器是可写的，而且另个，这个 SPCSR0 也是可写的。