uint16_t VS __IO uint16_t

Typically in one or more header files you'll find something like,

#ifdef __cplusplus
#define __I volatile /*!< defines 'read only' permissions */
#else
#define __I volatile const /*!< defines 'read only' permissions */
#endif
#define __O volatile /*!< defines 'write only' permissions */
#define __IO volatile /*!< defines 'read / write' permissions */

volatile is important for peripheral registers, they don't act like memory, and operate independently of the processor, require strict sequencing and reading every time they are used.

Hi SamTheMan,

thank you