Variables That Survive Soft Reset

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Introduction

When working with microcontrollers, it's essential to preserve variables during a soft reset or crash. This is particularly crucial when dealing with critical data that needs to be retained even after a system failure. In this article, we'll explore the variables that survive soft reset and provide alternative solutions when the default methods fail.

Understanding Soft Reset

A soft reset is a type of system reset that occurs when the microcontroller's power supply is interrupted or when the system crashes. During a soft reset, the microcontroller's memory is cleared, and all variables are lost. This can be a significant issue when working with critical data that needs to be preserved.

Default Methods for Preserving Variables

In the ESP32 ecosystem, there are several methods for preserving variables during a soft reset. One of the most common methods is to use the Non-Volatile Storage (NVS) feature. NVS allows you to store data in a non-volatile memory that persists even after a power cycle or soft reset.

However, in some cases, you may need to write data to a file or a specific location in memory. In such cases, you can use the RTC_NOINIT_ATTR attribute to preserve variables during a soft reset.

Using RTC_NOINIT_ATTR

The RTC_NOINIT_ATTR attribute is a special attribute that tells the compiler to preserve variables during a soft reset. This attribute is typically used with the RTC_NOINIT macro, which is defined in the ESP32 SDK.

Here's an example of how to use RTC_NOINIT_ATTR to preserve variables during a soft reset:

RTC_NOINIT_ATTR int buff[20];

This code snippet preserves the buff array during a soft reset. However, as you mentioned, this method may not work as expected in all cases.

Alternative Solutions

If the RTC_NOINIT_ATTR method doesn't work for you, there are several alternative solutions you can try:

1. Using a File System

One of the most common alternative solutions is to use a file system to store data. The ESP32 SDK provides a file system API that allows you to create, read, and write files to a non-volatile storage device.

Here's an example of how to use the file system API to store data:

#include <stdio.h>
#include <string.h>

// Create a file system object
FS *fs = esp_vfs_fat_sdcard_mount("/sdcard");

// Create a file object
FILE *file = fopen("/sdcard/data.txt", "w");

// Write data to the file
fwrite(buff, sizeof(buff), 1, file);

// Close the file
fclose(file);

This code snippet creates a file system object and a file object, writes data to the file, and closes the file.

2. Using a Memory-Mapped File

Another alternative solution is to use a memory-mapped file to store data. A memory-mapped file is a file that is mapped into memory, allowing you to access its contents as if it were a regular array.

Here's an example of how to use a memory-mapped file to store data:

#include <stdio.h>
#include <sys/mman.h>

// Create a memory-mapped file object
void *mmap = mmap(NULL, sizeof(buff), PROT_READ | PROT_WRITE, MAP_SHARED, 0, "/sdcard/data.txt");

// Write data to the memory-mapped file
memcpy(mmap, buff, sizeof(buff));

// Unmap the memory-mapped file
munmap(mmap, sizeof(buff));

This code snippet creates a memory-mapped file object, writes data to the memory-mapped file, and unmaps the memory-mapped file.

3. Using a Custom Storage Mechanism

If the above solutions don't work for you, you can create a custom storage mechanism to store data. This can be a simple array or a more complex data structure that is designed to store data in a non-volatile manner.

Here's an example of how to create a custom storage mechanism to store data:

#include <stdio.h>

// Define a custom storage structure
typedef struct {
    int data[20];
} storage_t;

// Create a custom storage object
storage_t storage;

// Write data to the custom storage object
memcpy(storage.data, buff, sizeof(buff));

// Read data from the custom storage object
memcpy(buff, storage.data, sizeof(buff));

This code snippet defines a custom storage structure, creates a custom storage object, writes data to the custom storage object, and reads data from the custom storage object.

Conclusion

In conclusion, preserving variables during a soft reset is a critical aspect of microcontroller programming. While the RTC_NOINIT_ATTR method is a convenient way to preserve variables, it may not work as expected in all cases. In such cases, you can use alternative solutions such as a file system, a memory-mapped file, or a custom storage mechanism to store data in a non-volatile manner.

By following the examples and code snippets provided in this article, you should be able to create a robust and reliable storage mechanism that preserves variables during a soft reset.

Additional Resources

For more information on preserving variables during a soft reset, you can refer to the following resources:

Introduction

In our previous article, we explored the variables that survive soft reset and provided alternative solutions when the default methods fail. In this article, we'll answer some of the most frequently asked questions about preserving variables during a soft reset.

Q: What is a soft reset?

A: A soft reset is a type of system reset that occurs when the microcontroller's power supply is interrupted or when the system crashes. During a soft reset, the microcontroller's memory is cleared, and all variables are lost.

Q: Why do I need to preserve variables during a soft reset?

A: You need to preserve variables during a soft reset to ensure that critical data is retained even after a system failure. This is particularly crucial when working with data that needs to be preserved for a long time, such as sensor readings or user settings.

Q: What are the default methods for preserving variables during a soft reset?

A: The default methods for preserving variables during a soft reset include using the Non-Volatile Storage (NVS) feature and the RTC_NOINIT_ATTR attribute.

Q: What is the RTC_NOINIT_ATTR attribute?

A: The RTC_NOINIT_ATTR attribute is a special attribute that tells the compiler to preserve variables during a soft reset. This attribute is typically used with the RTC_NOINIT macro, which is defined in the ESP32 SDK.

Q: Why doesn't the RTC_NOINIT_ATTR method work for me?

A: The RTC_NOINIT_ATTR method may not work for you if you're using a newer version of the ESP32 SDK or if you're working with a specific type of data that requires a more complex storage mechanism.

Q: What are some alternative solutions for preserving variables during a soft reset?

A: Some alternative solutions for preserving variables during a soft reset include using a file system, a memory-mapped file, or a custom storage mechanism.

Q: How do I use a file system to preserve variables during a soft reset?

A: To use a file system to preserve variables during a soft reset, you can create a file system object and a file object, write data to the file, and close the file. Here's an example of how to do this:

#include <stdio.h>
#include <string.h>

// Create a file system object
FS *fs = esp_vfs_fat_sdcard_mount("/sdcard");

// Create a file object
FILE *file = fopen("/sdcard/data.txt", "w");

// Write data to the file
fwrite(buff, sizeof(buff), 1, file);

// Close the file
fclose(file);

Q: How do I use a memory-mapped file to preserve variables during a soft reset?

A: To use a memory-mapped file to preserve variables during a soft reset, you can create a memory-mapped file object, write data to the memory-mapped file, and unmap the memory-mapped file. Here's an example of how to do this:

#include <stdio.h>
#include <sys/mman.h>

// Create a memory-mapped file object
void *mmap = mmap(NULL, sizeof(buff), PROT_READ | PROT_WRITE, MAP_SHARED, 0, "/sdcard/data.txt");

// Write data to the memory-mapped file
memcpy(mmap, buff, sizeof(buff));

// Unmap the memory-mapped file
munmap(mmap, sizeof(buff));

Q: How do I create a custom storage mechanism to preserve variables during a soft reset?

A: To create a custom storage mechanism to preserve variables during a soft reset, you can define a custom storage structure, create a custom storage object, write data to the custom storage object, and read data from the custom storage object. Here's an example of how to do this:

#include <stdio.h>

// Define a custom storage structure
typedef struct {
    int data[20];
} storage_t;

// Create a custom storage object
storage_t storage;

// Write data to the custom storage object
memcpy(storage.data, buff, sizeof(buff));

// Read data from the custom storage object
memcpy(buff, storage.data, sizeof(buff));

Conclusion

In conclusion, preserving variables during a soft reset is a critical aspect of microcontroller programming. While the RTC_NOINIT_ATTR method is a convenient way to preserve variables, it may not work as expected in all cases. In such cases, you can use alternative solutions such as a file system, a memory-mapped file, or a custom storage mechanism to store data in a non-volatile manner.

By following the examples and code snippets provided in this article, you should be able to create a robust and reliable storage mechanism that preserves variables during a soft reset.

Additional Resources

For more information on preserving variables during a soft reset, you can refer to the following resources: