Flash Memory Optimization for CH32V003

[thevien2507]

I'm working with the CH32V003 microcontroller and noticed that peripheral libraries (Wire for I2C, SPI, and UART) consume a significant amount of flash memory.

For example, just using the Wire library for I2C and Serial takes up about 73% of the available flash memory (12,018 bytes out of 16,384 bytes).

Here is a minimal example of my I2C code:

#include <Wire.h>

void setup() {
    Wire.begin(8);  // Initialize as I2C slave with address 8
    Wire.onReceive(receiveEvent);
    Serial.begin(9600);
}

void loop() {
    // Do nothing, waiting for data from master
}

void receiveEvent(int howMany) {
    Serial.print("Received: ");
    while (Wire.available()) {
        char c = Wire.read();
        Serial.print(c);
    }
    Serial.println();
}

Flash & RAM Usage After Compilation:

Sketch uses 12,108 bytes (73%) of program storage space. Maximum is 16,384 bytes.  
Global variables use 680 bytes (33%) of dynamic memory, leaving 1,368 bytes for local variables. Maximum is 2,048 bytes.  

I also noticed that SPI and UART libraries take up a lot of flash memory as well, making it difficult to fit multiple peripherals into the available 16 KB flash.

Comparison with MounRiver

When compiling a similar I2C example using MounRiver Studio, the flash usage is significantly lower:

   text    data    bss    dec    hex    filename  
   4196     64    260   4520   11A8    I2C_7bit_Mode.elf  

This means that MounRiver only uses ~4.2 KB flash, while the Arduino Core version consumes ~12 KB, nearly 3 times more.

Questions:

• Are there any compiler flags or configuration options to disable unnecessary features and optimize resource usage?
• Is there any way to optimize flash usage while keeping I2C, SPI, and UART functionality?

Any suggestions for reducing flash usage would be greatly appreciated! 🚀

[maxint-rd]

Hello @thevien2507 ; you've encountered the same issue that I have. Insuffiecient flash space is the largest challenge I've faces in my CH32V003 projects. Since your example uses I2C slave, I assume you are using a recent IDE with latest core (i.e master branch plus all relevant PR's).

I've not found a definite solution, but this is what I do to combat this:

1. Switch off all unused features; UART, SPI, I2C, hardware timer, etc. The official way at the moment is to change the defines that are somewhere in a header file under system (don't know exactly by heart; it's mentioned in other related issues). For I2C-slave there is a separate define in the library to switch that featue off.
   (I've modified my header file and board file to make changing options easier using the IDE menu, but don't have these changes working for all CH32 chips, so I won't post it as a PR.)
2. Don't use Serial if you can avoid it. Use digitalWrite to blink a led and a logical analyzer to monitor I2C traffic. If you use serial, use short strings and never use printf.
3. Use the realtime debugger to debug specific code sections while other parts are switched off using ifdefs.
4. Use the CH32X033 if you need Serial for debugging. Its TSSOP20 package pinout has the closest match to the V003.The X033 has 64kB flash allowing for dev code that can be stripped out later using ifdefs.
5. Use an old Arduino IDE (1.8.19?) that has functioning -LTO option to produce smaller flash usage. I've not found the cause yet why Link Time Optimization doesn't work on the 2.3.2 IDE with latest core, while it does work on the old IDE.