If you need to free up additional memory, resizing the buffer can be a useful option. If your Arduino communicates with an SD or MicroSD card, it will need to allocate a large 512 byte memory buffer to make it easier to read and write data. If you are using a microcontroller with very limited RAM (like ATmega328 on Arduino UNO) then you shouldn’t use heap at all.
Arduino has a heap. The purpose of a heap is to use memory for dynamic resource allocation. Blocks of memory can be reserved for the heap by using the malloc() call. The heap is removed in sections by using the free() function. The delete() function removes the heap altogether.
Use the stack instead of the heap whenever possible: Stack memory is preferred because the memory is completely released when the function returns, and the stack memory is not fragmented. Usually, this means using local variables instead of dynamic memory allocation (for example, calls to malloc, calloc, and realloc). Although it is a good solution to use dynamic memory allocation when programming a typical PC with hundreds of megabytes, gigabytes, or even terabytes of RAM, for embedded devices (such as the Arduino series), This is usually a bad idea.
A Look at the types of Arduino Memory
In many cases, due to the use of global variables (such as strings or numbers), a large amount of RAM is taken out of static memory (RAM area variable.data). In a microcontroller-based system, RAM is used to store temporary data. The RAM in Arduino, like any other RAM, is used to store temporary data and is also volatile. Flash and EEPROM are two types of ROM, used to store application code and small data.
Let’s take a look at the situation of these three kinds of memory on Arduino, and compare their sizes on different Arduino boards. Like any computer system, Arduino has two main memories: RAM and ROM. But what is interesting is how they are organized and their size.
A von Neumann processor has a memory area, all program codes, data, EEPROM and I / O registers share the same memory space, all mapped into unique address ranges. So freeMemory () gives us the amount of free memory available in between. The heap can grow (with a new allocation) or “possibly” decrease in size (when free memory, for example, when using free memory) as needed.
How the Heap Works
Hidden heap space cannot be used by the stack, but it can be used for subsequent heap allocations. In this case, the memory available to free the second allocation is called heap hidden space, and heap fragmentation has occurred. When this happens, the allocation of memory on the heap becomes chaotic, and it is often necessary to allocate a new section of memory for the final string object.
The idea is that you can prevent fragmentation of the String heap by using reservations (num) to pre-allocate memory for the growing String. Since the memory has already been allocated, if the string becomes longer, the String does not need to call realloc(). The idea is that you can prevent fragmentation of the String heap by using reservations (num) to pre-allocate memory for the growing String. The Arduino string library allows you to reserve buffer space for strings using the Reserve () function. Usually all you have to do is use fallback() on any long-lived String object, you know that as the text is processed, its length will increase.
In most cases, many other small String objects are temporarily used in these operations, forcing newline characters to be allocated to a new area on the heap, and leaving a large hole (memory fragmentation) in the previous area. In fact, when passing by value, function parameters are copied to the stack, so you can save some stack memory by reducing the size of the parameters. If you reduce the size of the parameter, you can save a lot of stack memory.
Declaring Byte Values for Arduino
Be responsible and try to use a type that meets both requirements for your data, but also has the smallest number of bytes used for storage in memory. When creating enums for Arduino / AVR it is unlikely that more than 256 values are required in most cases, declare them as byte type as follows, this will ensure they only take up 1 byte of SRAM.
For each type of Arduino memory, there are some basic changes we can make to our code to reduce the memory usage. On this page, we will see how we can write an Arduino program that uses less memory. Obviously, the focus is on JSON serialization and deserialization, so we’ll see how we can use ArduinoJson with less RAM.
ArduinoJson & the stack
ArduinoJson uses a stack with StaticJsonBuffer and a heap for DynamicJsonBuffer. But on the second line, since the strings are in Flash, the ArduinoJson has to make a copy of them in RAM. Unfortunately, flash uses a different address space, so software that manipulates both RAM and flash lines must temporarily copy flash lines into RAM. Another type of RAM is called DRAM, or dynamic random access memory, which uses a capacitor to store data.
In Arduino, SRAM stores temporary or runtime data (variables created by functions and interrupts). RAM in Arduino is actually called SRAM or static random access memory, a type of RAM that uses flip-flops to store 1-bit data. Above we saw the internal memory model of the Arduino, and on the left we saw the program memory storage. The code itself and everything we call PROGMEM are stored here. However, storing elements in program memory is not free, because special functions must be used to read the values.
As you can imagine, getLargestAvailableBlock () returns the size of the largest allocated block, and getTotalAvailableMemory () returns the total amount of free memory. As we have seen, repeated allocations of the same size do not cause fragmentation; so we could store our objects on the heap, but always use the same size. Allocating and freeing the heap causes overhead and fragmentation, so the code block uses much less RAM than it actually has on the device.
The heap & malloc ()
Every time you call malloc (), you are allocating a block of memory on the heap. Let’s say you just freed a block of memory and then created a hole in the heap.
Thus, there is nothing in this sketch that can be used by heap memory (which can be fragmented). Typically, our code dynamically loads static variables into SRAM, which results in a waste of valuable heap memory since we never have to change them. As the memory area of the stack grows towards the heap, the memory address of this new variable is the last memory address used by the stack when this method is called.
Conclusion
The heap is an area of RAM where heap allocation takes place. More often than not, your program will allocate memory on the heap without explicitly calling malloc (). Whether part of a standard library or a linked library, C ++ programs running on a computer include a heap allocator that is much more efficient than what our Arduinos have. As you can see, only a program that allocates and frees blocks of different sizes increases heap fragmentation.