#include <M5Atom.h>
#include "CBACK_DRIVER.h"

bool flag = true;

CBACK_DRIVER Driver;
int C_BLACK = 0x000000;
int C_WHITE = 0xffffff;
int C_RED = 0xff0000;
int C_GREEN = 0x00ff00;
int C_BLUE = 0x0000ff;


void setup() {
        M5.begin(true, false, true);
        // let's check the atom is ok
        delay(50);
        M5.dis.drawpix(0, C_GREEN); // green
        delay(1000);
        M5.dis.drawpix(0, C_BLACK); // black
        Serial.begin(115200);
                Driver.Init();
                delay(1000);
                Serial.println("C Back Driver Test...");

}
void loop() {
        if(M5.Btn.wasPressed()){
                flag = !flag;
                Serial.println("toggle flag value");
        }
        if(flag){
                for(int i=1; i<=4;i++){
                        Driver.SetServoAngle(i,90);
                        Serial.println("servo 90");
                        // Driver.ReadServoPulse(i,1500);
                }
                Driver.SetOutput(1);
        }else{
                for(int i=1; i<=4;i++){
                        Driver.SetServoAngle(i,120);
                        Serial.println("servo 120");
                        // Driver.ReadServoPulse(i,2000);
                }
                Driver.SetOutput(0);
        }
        Serial.print("PortB ADC: ");
        Serial.println(Driver.ReadADC());
        M5.update();
}

I get this error:

Building in release mode
Linking .pio/build/m5stack-atom/firmware.elf
.pio/build/m5stack-atom/src/main.cpp.o:(.literal._Z5setupv+0x2c): undefined reference to `CBACK_DRIVER::Init()'
.pio/build/m5stack-atom/src/main.cpp.o:(.literal._Z4loopv+0x18): undefined reference to `CBACK_DRIVER::SetServoAngle(unsigned char, unsigned char)'
.pio/build/m5stack-atom/src/main.cpp.o:(.literal._Z4loopv+0x1c): undefined reference to `CBACK_DRIVER::SetOutput(unsigned char)'
.pio/build/m5stack-atom/src/main.cpp.o:(.literal._Z4loopv+0x20): undefined reference to `CBACK_DRIVER::ReadADC()'
.pio/build/m5stack-atom/src/main.cpp.o: In function `setup()':
/home/benalb/Projects/m5atom/cback/src/main.cpp:22: undefined reference to `CBACK_DRIVER::Init()'
.pio/build/m5stack-atom/src/main.cpp.o: In function `loop()':
/home/benalb/Projects/m5atom/cback/src/main.cpp:36: undefined reference to `CBACK_DRIVER::SetServoAngle(unsigned char, unsigned char)'
/home/benalb/Projects/m5atom/cback/src/main.cpp:40: undefined reference to `CBACK_DRIVER::SetServoAngle(unsigned char, unsigned char)'
/home/benalb/Projects/m5atom/cback/src/main.cpp:44: undefined reference to `CBACK_DRIVER::SetOutput(unsigned char)'
/home/benalb/Projects/m5atom/cback/src/main.cpp:47: undefined reference to `CBACK_DRIVER::ReadADC()'
collect2: error: ld returned 1 exit status
*** [.pio/build/m5stack-atom/firmware.elf] Error 1
========================================================= [FAILED] Took 13.90 seconds =========================================================

any ideas?