Reading I2C sensor value (bit-banging)

Question

I have a very simple I2C bit-banging library for ATTINY85.

#define PORT_SDA PB0
#define PORT_SCL PB2

#define SIGNAL_HIGH(PORT) PORTB |=  ( 1 << PORT )
#define SIGNAL_LOW(PORT)  PORTB &= ~( 1 << PORT )

void LED_ON(void);
void LED_OFF(void);

void i2c_init(void);
void i2c_start(void);
void i2c_read(void);
void i2c_stop(void);
void i2c_write(uint8_t byte);

void i2c_init()
{
    DDRB |= ( 1 << PORT_SDA );
    DDRB |= ( 1 << PORT_SCL );
}


void LED_ON( void )
{
    PORTB |= 0b00000010;
}


void LED_OFF( void )
{
    PORTB &= 0b111111101;
}


void i2c_start( void )
{
    SIGNAL_HIGH( PORT_SCL );
    SIGNAL_HIGH( PORT_SDA );
    SIGNAL_LOW(  PORT_SDA );
    SIGNAL_LOW(  PORT_SCL );
}


void i2c_stop( void )
{
    SIGNAL_LOW(  PORT_SCL );
    SIGNAL_LOW(  PORT_SDA );
    SIGNAL_HIGH( PORT_SCL );
    SIGNAL_HIGH( PORT_SDA );
}

void i2c_write(uint8_t byte)
{
    uint8_t bit;

    for ( bit = 0; bit < 0x08; bit++ )
    {
        if( ( byte << bit ) & 0x80 )
            SIGNAL_HIGH( PORT_SDA );
        else
            SIGNAL_LOW( PORT_SDA );

        SIGNAL_HIGH( PORT_SCL );
        SIGNAL_LOW( PORT_SCL );
    }

    SIGNAL_HIGH( PORT_SDA );
    SIGNAL_HIGH( PORT_SCL );
    SIGNAL_LOW( PORT_SCL );
}

I'm able to successfully write to I2C without any problems. I have tested this code with SSD1306 and LC2404B and everything works great even the timing if VCC is set to 4.2V.

i2c_init();

i2c_start();
    i2c_write( 0xA0 );
    i2c_write( 0x01 );
    i2c_write( 0x13 );
i2c_stop();

While writting works perfectly, I can't seem to be able to trigger any of my I2C modules with ATTINY85 to return me a value that I can read later.

I connected raspberry and GY-521 sensor (since it returns value even if internal address is not set). I can detect the sensor and read a value from it with raspberry the following way:

i2cdetect -y 1

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- 68 -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --  


i2cget -y 1 0x68

0x02

This is the output on the oscilloscope:

I can see the sensor data changing. The problem is I can't seem to replicate the same request from ATTINY85 to the sensor. The sensor simply doesn't respond with the value. I can't seem to understand if the last bit in the first byte is an ACK or 'READ' indicator bit, so I tried different addresses.

i2c_start();
i2c_write(  0b11010001 ); // address: 0xD1

i2c_start();
i2c_write(  0b11010000 ); // address: 0xD0

i2c_start();
i2c_write(  0b10100001 ); // address: 0xA1

i2c_start();
i2c_write(  0b10100000 ); // address: 0xA0

but regardless on what address I use, the sensor simply doesn't respond (in the oscioloscope) and the SDA line stays high after I send the address byte. I also tried to append another start() condition after I sent the address, but still no luck. Any hints on where I'm going wrong? I simply want to make the sensor respond to the ATTINY85 read request so I can later read the value. Thanks!

iheanyi iheanyi · Accepted Answer · 2017-06-21T20:39:08

I'd suggest reading a couple tutorials on I2C to get a general understanding of the protocol. See https://learn.sparkfun.com/tutorials/i2c for example.

In short, I2C is a two-wire multi-master bus with a clock line and a data line. In any communication, whether it is a read or a write, the master supplies the clock signal. Your i2c_write() implements this with the SCL transitions.

In order to read a value back, you need to also supply the clock for that the slave uses to output data. No clock, no data. So, you need to implement an i2c_read() similar to your i2c write, that generates the clock transitions, and shifts in each bit one at a time.

Reading I2C sensor value (bit-banging)

2 Answers