Application of Enumeration in MCU Programming

When do we need to use enum? When the value of a variable is between several ranges, such as a week variable that can only take values ​​of Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday. It is better to use enum in this case. Of course, define can also be used. However, define is more troublesome to maintain and is suitable for when the value range of the variable is small.

1. Using the enum keyword to describe constants (i.e., describing enumeration constants) has the following advantages: 
(1) It makes the program easier to maintain and more intuitive. For example, the enumeration defined below describes the number of transmission bits supported by the UART. From the definition of the enumeration type, we can know the several modes of transmission bits provided by the UART module.

enum
{
          usartDatabits4 = USART_FRAME_DATABITS_FOUR, /* 4 databits (not available for UART). */
          usartDatabits5 = USART_FRAME_DATABITS_FIVE, /**< 5 databits (not available for UART). */
          usartDatabits6 = USART_FRAME_DATABITS_SIX, /**< 6 databits (not available for UART). */
          usartDatabits7 = USART_FRAME_DATABITS_SEVEN, /**< 7 databits (not available for UART). */
          usartDatabits8 = USART_FRAME_DATABITS_EIGHT, /**< 8 databits. */
          usartDatabits9 = USART_FRAME_DATABITS_NINE, /**< 9 databits. */
          usartDatabits10 = USART_FRAME_DATABITS_TEN, /**< 10 databits (not available for UART). */
} USART_Databits_Enum;

(2) Make the program safer 
. For example, when writing the underlying driver, a UART initialization function UartInit(uint32 baudrate, uint32 bit) is provided to the upper layer. When calling this function, the programmer may provide the wrong bit parameter because he does not understand the transmission bit mode supported by the module UART. To avoid this phenomenon, we can of course check the bit range in the UartInit() function. If the parameter values ​​are continuous, it is fine. If they are not continuous, multiple if-else or switch will be used. It will consume a certain amount of ROM and CPU time. 
We can define the function as UartInit(uint32 baudrate, USART_Databits_Enum bit). On the one hand, we can easily know the transmission bit mode supported by UART by checking the USART_Databits_Enum enumeration type. On the other hand, we can avoid passing wrong parameters during compilation.

2. The difference and connection between enumeration and macro definition:

The difference between macro and enumeration lies in the period of action and the form of storage. Macro is replaced in the preprocessing stage, it replaces the text of the code segment, and the macro no longer exists during the program running. Enumeration takes effect after the program is running, and the enumeration constant is stored in the static storage area of ​​the data segment. (In the IAR compilation environment, the enumeration type defaults to character type. In programming under the M3 core, in order to reduce unnecessary character extension instructions, you can use the ?enum_is_int instruction to force all enumeration types to be 4 bytes.)

But we cannot say that macros are better than enumerations. If you need to define a lot of constants, using one enum {…} is obviously clearer than a bunch of defines. Enumerations can also be used to define some special types, such as Bool, such as: type enum {FALSE,TRUE} Bool;