CMSIS - Peripheral Definitions - Structs With Unions Syntax? - c

Question

I've moving over from 8 bit bare-metal registers, and having to learn some new C Kungfu to wrap my head around the CMSIS Core approach.
I have a snippet of code here from a Peripheral Access Layer from a ARM Cortex M vendor. They create this SN_WDT_TYPE structure, which you can use to set watch-dog timer registers using their notation.
Why do they use the unions? I haven't seen this kind of syntax before.
If you use unions to create structures like that, do you they go several layers deep with pointers? Memory management with the unions? Is there some C syntax thing I'm missing here?
This might be CMSIS specific, does anyone know what the ": 1" is doing in those struct declarations...? I know the __IO ties back to some CMSIS definition of read/write.
HELPFUL STUFF I FOUND AFTER COMMENTS & ANSWERS:
ARM'S CMSCIS PERIPHERAL NAMING CONVENTION -- This example code doesn't seem to confirm too gracefully...
ARM'S BITFIELD COMPILER NOTES ON STRUCTS & UNIONS
/**
* #brief Watchdog Timer (SN_WDT)
*/
typedef struct { /*!< SN_WDT Structure */
union {
__IO uint32_t CFG; /*!< Offset:0x00 WDT Configuration Register */
struct {
__IO uint32_t WDTEN : 1; /*!< WDT enable */
__IO uint32_t WDTIE : 1; /*!< WDT interrupt enable */
__IO uint32_t WDTINT : 1; /*!< WDT interrupt flag */
uint32_t : 13;
__O uint32_t WDKEY : 16; /*!< Watchdog register key */
} CFG_b; /*!< BitSize */
};
union {
__IO uint32_t CLKSOURCE; /*!< Offset:0x04 WDT Clock Source Register */
struct {
__IO uint32_t CLKSOURCE : 2; /*!< WDT clock source */
uint32_t : 14;
__O uint32_t WDKEY : 16; /*!< Watchdog register key */
} CLKSOURCE_b; /*!< BitSize */
};
union {
__IO uint32_t TC; /*!< Offset:0x08 WDT Timer Constant Register */
struct {
__IO uint32_t TC : 8; /*!< Watchdog timer constant reload value */
uint32_t : 8;
__O uint32_t WDKEY : 16; /*!< Watchdog register key */
} TC_b; /*!< BitSize */
};
union {
__O uint32_t FEED; /*!< Offset:0x0C WDT Feed Register */
struct {
__O uint32_t FV : 16; /*!< Watchdog feed value */
__O uint32_t WDKEY : 16; /*!< Watchdog register key */
} FEED_b; /*!< BitSize */
};
} SN_WDT_Type;

Answer 1

The union allows you to access the hardware register as either a 32 bit word or as the bit fields contained in the register. Both representations have their uses. Perhaps you are missing the bit field syntax as it is not used in most application level coding. Bit field layout is compiler specific, but CMSIS headers are built to work with the intended compilers.