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I'm trying to reduce my sketch size and managed to reduce both SRAM and Flash memory.

From:
1570 SRAM - 32144 Flash memory
To:
644  SRAM - 19458 Flash memory

This are what I've done so far:

Enabling LTO

Using PROGMEM

Dropping string and using char

Dropping bootloader

Using port manipulation

Using registers to declare pins output/input

Removing/Optimizing/Combining; unnecessary/repeated codes in the sketch

Using EEPROM is not an option for me.

What else i can do to take optimization one or two step further?

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    ArduinoIDE is designed to be easy and not fast. To really optimise your code switch to AtmelStudio and write logic on registry level. ArduinoIDE loads many libraries out of box. Some of them are probably unnecessary in your sketch. – Filip Franik Jul 21 at 9:24
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    Change the compiler optimization levels. I looks like you can do that within the code, though I've done it by changing the boards.txt file in the past. – Gerben Jul 21 at 9:32
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    Also use the right type for (global) variables. Use byte instead of int for variables whose values are always between 0 and 255. – Gerben Jul 21 at 9:33
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What you can do in addition to the items you mentioned:

SRAM reduction

  • Use the smallest amount of data type for integers, e.g. use an 8 bit data type instead of the default integer, this saves you one byte for every variable. See remark below about types. I made my own types (if not existing) for int32_t, uint32_t, int16_t, uint16_t, int8_t and uint8_t and always use the smallest possible.
  • Use unsigned instead of signed for types that cannot be negative. This might save you one byte if you can prevent using an unsigned byte instead of a signed int because a value can be in range [128..255]. The same for unsigned int instead of signed long.
  • Use bit fields if you use multiple booleans inside a struct or class. Make sure the bit fields are declared consecutively.
  • Pack data in smart structures ... I.e. if you have two variables, where one can be 0..7 and another can also be 0..7, store this together using bit fields (in combination with boolean to bit field above).
  • Prevents float/doubles (see KIIV's comment below), instead:
  • Instead of using a float, multiply the value by the needed accuracy and store it in an integer (if you don't need high accuracy and it fits in the integer type).
  • In case you need a lot of strings to be stored, and those strings do not have characters above 128, you can save 1 bit per byte, of course this means quite some programming work to handle 7 bits 'ASCII' characters.
  • The same, if you have to store a lot of values that can be e.g. 0 to 50, you can use 6 bits instead of 8 bits; you cannot use bit fields, but you can make a smart array and program your own get/set functions to access the 6 bit values.
  • If you use libraries which declare big buffers, try to minimize those buffers, this might cause to copy them and manually change them yourself.
  • Prevent your own buffers to be made global, instead use them as a local variable temporarily when you need them. If you have two of such buffers, you don't have both buffers allocated at the same time; note however, that during execution, that buffer still needs space (so you have to calculate or make a guess/estimation what the maximum stack size will be).

Flash Reduction

  • Don't use duplicated code blocks, but make functions with parameters to differ between the differences of the duplicated code.
  • Instead of lookup tables, create a function that calculates it (if possible). Of course the gain depends on the length of the lookup table and the length of the function.
  • Instead of debugging strings, use numbers.

Both SRAM/Flash Reduction

  • Split your application over multiple MCUs, of course this adds some kind of communication protocol (UART, I2C, SPI).
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    It's more like: Don't use floats/doubles on architectures without hardware FPU (pretty much just some Cortex M3/M4 boards have it). And for example on AVR based arduinos is double just an alias for float. – KIIV Jul 22 at 8:45
  • @KIIV Thanks for the comment, didn't know the aliases were the same.k – Michel Keijzers Jul 22 at 10:35
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    You can also change to a processor with more SRAM, such as 16kbytes in Atmega1284P. I use that chip in many Arduino-based projects. Dual hardware serial ports,128K flash, 16K sram, and 32 IO. – CrossRoads Jul 22 at 11:23
  • @CrossRoads I didn't took hardware solutions into account (because of the OP not wanting EEPROM, which is more or less equal than using another MCU). Too bad they don't make a (cheap) Arduino with that MCU, I think the 2 KB SRAM is really a limit for many projects. I'm now making mine with a Mega and using an external EEPROM for storage. I was thinking about an external SPI RAM but I try to make my project without that. – Michel Keijzers Jul 22 at 11:30
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    Yep, China is not offering '1284P based boards like they do for Uno, Nano, and Promini. I sell boards, but I get my parts from Digikey & Mouser, so there is some markup there, but it ensures I get parts from a known good supply. – CrossRoads Jul 22 at 11:36
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I'm having flashbacks to my early days as a C programmer in the 80's.

Some thoughts in addition to all of the excellent advice already given ...

  • Depending on how much data your application uses, you may want to read / write data to an SD card (or other external storage). While there is a certain amount of overhead for the libraries, the payoff is you then have virtually unlimited storage. This is the equivalent of how we used to store data on 5 1/4" diskettes back in the old days.

  • Avoid static variables whenever possible.

  • Keep the scope of a variable as small as possible.

  • As a philosophy you should consider unsigned char to be your default data type and only 'upgrade' to a bigger footprint when your code requires.

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    Re “use dynamic memory as much as possible”: You should probably be more specific. When hearing “dynamic memory”, most would think malloc(), which is something to be avoided if you can. I guess you mean “automatic allocation”, i.e. local variables not qualified as static. – Edgar Bonet Jul 21 at 19:02
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    Re “re-use variables if you don't need them at the same time”: −1, as this is terrible advice! Please, refrain from encouraging such horribly bad practice. It may have been a reasonable optimization 30 years ago, but since at least 20 years, it has become quite difficult to find a compiler dumb enough to not do this optimization itself. Trust your compiler, and write clear code. Or ditch the compiler if you managed to find one that is that bad. – Edgar Bonet Jul 21 at 19:02
  • Funny, keeping scope as small as possible is mutualy exclusive with reusing variables. Reusing variables is also error prone, but maybe little bit less error prone than variable declaration and initialization later (often later than it's used) – KIIV Jul 22 at 8:32
  • Agreed with feedback, updated my answer to reflect that. – Rob Sweet Jul 22 at 15:35

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