Practical Experience: Variable Initialization in Keil, IAR, CubeIDE

Sometimes in our applications, we require variables to maintain continuity or to retain state, such as during a Bootloader jump. For various reasons, during a reset process, some critical variables must not be initialized. Different compilation environments have different settings. This article summarizes this operation and introduces the methods used in Keil, IAR, and CubeIDE. The microcontroller used in this article is STM32G431RBT6.

The implementation in IAR is relatively simple; you can directly use the keyword “__no_init” to modify the variable:

To verify successful execution, you can periodically reset the system and observe the variable changes. For example, the following sample program resets the system periodically, and you will find that each time the Test_NoInit data increases by 10 based on the previous data, rather than being initialized and then increasing by 10.

In Keil, there is no equivalent keyword like in IAR, and there may be version differences. Below are the introductions:

Figure 1. Different compilation versions in Keil

To prevent uninitialized variables from being initialized to 0, they must be placed in a special section that meets the ZI data section (.bss), which has the UNINIT attribute in its execution region.

3.1. Arm® Compiler 5 Operation

Modify the project’s linker file, *.sct file

Figure 2. Modify the default linker file

Here, RAM is divided into two regions, where RW_IRAM2 is the region for uninitialized variables, with the attribute set to UNINIT, defining a region name NO_INIT.

Define variables in this section; AC5 requires the use of the zero_init modifier.

3.2. Arm® Compiler 6 Operation

In AC6, you need to add the .bss ZI definition, as shown in the following sct file modification:

Define variables in the section; AC5 and AC6 also have differences, and the zero_init modifier is no longer supported, as shown in the definition below:

The specific differences between versions AC5 and AC6 can be referenced in the Keil help file:

Figure 3. Keil help file regarding zero initialization

The implementation in CubeIDE is similar to that in Keil; the linker file *.ld needs to be modified. First, divide the RAM to delineate the uninitialized RAM area:

Figure 4. Dividing RAM regions

Add region descriptions and include region names:

Figure 5. Region description and name definition

Define variables in this uninitialized region:

Additionally, it is important to note that some STM32 series have specific option configuration bits for whether certain RAM regions will be initialized after reset. For example, in the STM32L4 series, to prevent SRAM2 variables from being initialized, you need to configure the SRAM2_RST bit in the option byte as shown below: