best common practice I2C register map

Question

Just wondering what the best practice regarding I²C register maps in C or rather what other people use often/prefer. Up to this point, I have usually done lots of defines, one for every register and one for all the bits, masks, shifts etc. However, lately I've seen some drivers use (possibly packed) structs instead of defined. I think these were Linux kernel modules. Anyway, they would ``` struct i2c_sensor_fuu_registers { uint8_t id; uint16_t big_register; uint8_t another_register; ... } __attribute__((packed)); ``` Then they'd use offsetof (or a macro) to get the i2c register and use sizeof for the number of bytes to read. I find that both approaches have their merit: struct approach: - (+) Register offsets are all logically contained inside a struct instead of having to spell each register out in a define. - (+) Entry sizes are explicitly stated using a data type of appropriate size. - (-) This doesn't account for bit fields which are widely used - (-) This doesn't account for regi…

Accepted Answer

WARNING: The method described here uses bitfields, whose arrangement in memory is implementation specific. If you do this, make sure you know how your compiler works in this regard.

As you point out, there are advantages and disadvantages to each method. I like a hybrid approach. You can define register offsets, but then use a struct for the contents and a union to specify the bits or the entire register. Inside the union, use the correct size variable for the size of the register (as you mentioned sometimes they're not byte addressable). You don't need quite as many defines, and you're less likely to mess up bit shifts and don't need masks. For example:

```
#define unsigned char u8;
#define unsigned short u16;

#define CTL_REG_ADDR  0x1234
typedef union {
  struct { 
    u16 not_used:10; //top 10 bits ununsed
    u16 foo_bits:3;  //a multibit register
    u16 bar_bit:1;   //just one bit
    u16 baz_bits:2;  //2 more bits
  } fields;
  u16 raw;
} CTL_REG_DATA;

#define STATUS_REG_ADDR 0x58
typedef union {
  struct { 
    u8 bar_bits:4;  //upper nibble
    u8 baz_bits:4;  //lower nibble
  } fields;
  u8 raw;
} STATUS_REG_DATA;

//use them like the following
u16 readregister(u16);
void writeregister(u16,u16);

CTL_REG_DATA reg;
STATUS_REG_DATA rd;
rd = readregister(STATUS_REG_ADDR);
if (rd.fields.bar_bit) {
   reg.raw = 0xffff;        //set every bit
   reg.fields.bar_bit = 0;  //but clear this one bit
   writeregister(CTL_REG_ADDR, reg);
}
```

best common practice I2C register map

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