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I implement bit-banging I2C on Atmega328P and met the problem when the Master receives data from the slave. I'm in depression in debugging this code since I found no problem. Master Write work perfectly in many cases (Receive ACK, NAK if wrong slave address, sen multiple data,... ) But When Master want to read data from slave it only send 7/8 bit :( The receive algorithm seems correct just missing 1 bit! Please help me!

Master Arduino Using bit-banging i2c on AVR, Slave Arduino using Wire.h (Master writing work perfect)

As you can see, just missing one bit!

Master AVR code

#include <util/delay.h>
#include <stdint.h>

// CODE from STM32
#define SDA PC4
#define SCL PC5
//OUTPUT REGISTER: PORTC
// PC4: Pin A4 - SDA | PC5: Pin A5 - SCL | Arduino (Atmega328P)
#define SDA_ON (PORTC |= (1 << PC4))
#define SDA_OFF (PORTC &= ~(1 << PC4))
#define SCL_ON (PORTC |= (1 << PC5)) // PORTC = 0b0000 0010
#define SCL_OFF (PORTC &= ~(1 << PC5))

#define READ_SDA() (PINC & (1 << SDA))
#define READ_SCL() (PINC & (1 << SCL))

#define SET_SDA_OUT() DDRC |= (1 << SDA)
#define SET_SDA_IN() DDRC &= ~(1 << SDA)

#define I2C_READ 0x01
#define I2C_WRITE 0x00

#define ADDR 0x08 //Slave Address
#define ADDR_W (ADDR << 1)
#define ADDR_R (ADDR_W | 1)

void init()
{
    DDRC = (1 << DDC5) | (1 << DDC4);
    SCL_ON;
    SDA_ON;
}

void start()
{
    SDA_ON;
    _delay_us(4.5);
    SCL_ON;
    _delay_us(4.5);
    SDA_OFF;
    _delay_us(4.5);
    SCL_OFF;
    _delay_us(4.5);
}

/*  i2c stop sequence */
void stop()
{
    SDA_OFF;
    _delay_us(4.5);
    SCL_ON;
    _delay_us(4.5);
    SDA_ON;
    _delay_us(4.5);
}

/* Transmit 8 bit data to slave */
uint8_t Tx(uint8_t dat)
{

    for (uint8_t i = 0; i < 8; i++)
    {
        if (dat & 0x80)
        {
            SDA_ON;
        }
        else
        {
            SDA_OFF;
        }
        _delay_us(3);
        SCL_ON;
        _delay_us(5);
        SCL_OFF;
        _delay_us(2);
        dat <<= 1;
    }
    SET_SDA_IN();
    _delay_us(3);
    SCL_ON; // the 9th pulse is high
    _delay_us(3);
    if (READ_SDA())
    {
        _delay_us(2);
        SCL_OFF; // the 9th pulse is low
        PORTC = 0x00;
        SET_SDA_OUT();
        return 0;
    }
    else
    {
        _delay_us(2);
        SCL_OFF; // the 9th pulse is low
        PORTC = 0x00;
        SET_SDA_OUT();
        return 1;
    }
}

uint8_t Rx(void)
{
    uint8_t byte = 0;
    uint8_t bit = 0;

    _delay_us(10);
    SET_SDA_IN();

    for (bit = 0; bit < 8; bit++)
    {
        byte <<= 1;
        do
        {
            SCL_ON;
        } while (READ_SCL() == 0);
        _delay_us(3);
        if (READ_SDA())
        {
            byte |= 0x01;
        }
        _delay_us(2);
        SCL_OFF;
        _delay_us(5);
    }

    PORTC = 0b00010000;
    SET_SDA_OUT();
    SDA_ON;
    SCL_ON;
    _delay_us(3);
    SCL_OFF;
    _delay_us(3);

    return 1;
}

int main(void)
{
    init();
    _delay_ms(3000);
    unsigned char ack;
    while (1)
    {
        start();
        ack = Tx(ADDR_R);
        if (!ack)
        {
            stop();
            break;
        }
        Rx();
        stop();

        _delay_ms(1000);
    }
}```

    
**Slave code using Wire.h**

```#include <Wire.h>

void setup() {
  Wire.begin();        // join i2c bus (address optional for master)
  Serial.begin(9600);  // start serial for output
  pinMode(10,OUTPUT);
}

void loop() {
  Wire.requestFrom(0x08, 1);    // (a,b) request b bytes from slave device #a

  while (Wire.available()) { // slave may send less than requested
    char c = Wire.read(); // receive a byte as character
    Serial.print("Master received data: ");Serial.println(c);
    delay(100);
  }
}```
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3
  • \$\begingroup\$ Can you describe what exactly is the issue? Dumping code and flawless-looking logic capture is of little use. The largest issue is that you seem to think SDA and SCL are push-pull outputs. They are not, they are open-drain outputs, they must never be push-pull outputs, that would prevent any MCU slave from working properly with the master if it is slow enough to stretch the clock. \$\endgroup\$
    – Justme
    May 28, 2021 at 18:36
  • \$\begingroup\$ As justme says, i2c is open drain. You should set the ddr bit (ie make the pin an output) in order to output a ‘0’ and clear it (ie make the pin an input) for a ‘1’ and the pullup resistors do their job. \$\endgroup\$
    – Kartman
    May 29, 2021 at 0:21
  • \$\begingroup\$ Thank you for your response, I have solved this problem by stretching the SCL before go into Rx(void) by 12us and now the data frame is correct. \$\endgroup\$
    – MinhTem
    May 29, 2021 at 1:32

1 Answer 1

Reset to default
0
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I just using Master Wire.h then compare the waveform then stretching and it work :) :) Only 2us difference

enter image description here

{
    uint8_t byte = 0;
    uint8_t bit = 0;

    _delay_us(12); <---- From 10 to 12us
    SET_SDA_IN(); <--- Already release the SDA so Slave can pull down the SDA line 

    for (bit = 0; bit < 8; bit++)
    {
        byte <<= 1;
        do
        {```
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