Why is the ADDR flag cleared without reading the SR2 register?

Question

I am doing I2C project with MPU6050 using STM32F4DISCOVERY board. The ADDR bit in SR1 register is clearing in one instruction execution without reading SR2 register. Because of clearing ADDR bit, I can't do acknowledge disable is made during EV6 in single byte receiving case.

The datasheet states:

ADDR: Address sent (master mode)/matched (slave mode) This bit is cleared by software reading SR1 register followed reading SR2.

In case a single byte has to be received, the Acknowledge disable is made during EV6 (before ADDR flag is cleared) and the STOP condition generation is made after EV6.

If I write any instruction (or declaration) between I2C->DR=address|0 and while(!(I2C2->SR1 & (1<<1))) in below code, then ADDR flag is cleared without reading the SR2 register (I2C2->SR2).

if(direction==i2c_transmitter_mode)
{

    I2C2->DR = address | 0;//SEND ADDRESS WITH ZERO LAST BIT
    while(!(I2C2->SR1 & (1<<1)))
    {
        if(--i2c1_timeout==00)
        {
            return 1;
        }
    }
    I2C2->SR2;
    return 0;

}

How can I fix this problem?

Answer 1

You can fix it by using the correct bit. ADDR bit is not 1<<7 but 1<<1.

Answer 2

1. Do not use magic numbers like 1<<1 1<<7 on those complex uCs. Use human readable definitions.

2. Your sequence is wqrong

    I2C1 -> CR1 |= I2C_CR1_START;
    while(!(I2C1 -> SR1 & I2C_SR1_SB ));
    I2C1 -> DR = addr;
    while(!(I2C1 -> SR1 & I2C_SR1_ADDR));
    (void)I2C1 -> SR2;

it is a very simple version - you should add timeouts, error checking, address ACK checking etc. But it shows the general idea.

3. I2C2->DR = address | 0; - it does not clear the last bit. Actually it does nothing. My advice - read about bitwise operators

to clear the particular bit:

address &= ~(1 << bitnumber);
I2C2->DR = address;

or for the bit zero

address &= ~(1 << 0);
I2C2->DR = address;

why I did not write address &= 0xfffffffe; - because it less understandable for humans. If you see the code with ~ you know that someone is zeroing some bits.