LPC Microcontrollers中的主题 LPCOpen : Why we use '~' in the Reseved bits masks for registers ?

LPCOpen : Why we use '~' in the Reseved bits masks for registers ?

karthikvenkates — Thu, 25 May 2017 00:15:26 GMT

Hi,

Below is the snapshot from "syscon_8xx.h" file of lpcopen library. If you see, everywhere they have used '~'.

For example, take SYSAHBCLKDIV register in LPC812.

Here Bits <31:8> are reserved bits. We could have defined

#define SYSCTL_SYSAHBCLKDIV_RESERVED (0xffffff)

But why did LPCOpen use #define SYSCTL_SYSAHBCLKDIV_RESERVED (~0xff) instead ?

Re: LPCOpen : Why we use '~' in the Reseved bits masks for registers ?

BlackNight — Thu, 25 May 2017 06:11:26 GMT

The '~' operator is a 'binary-negate' operator in C. The reserved bitmask would be 0xFFFFFF00 (and not 0xFFFFFF as you wrote). And ~0xFF will be exactly that 0xFFFFFF00. You could use both notations. I would prefer 0xFFFFFF00 too, but ~0xFF is shorter :-).

I hope this helps,

Erich