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i2c start doesn't do anything on the bus

Question asked by KernelSanders on Oct 28, 2012
Latest reply on Oct 28, 2012 by KernelSanders
I am trying to write an i2c driver for my stm32f100 discovery board. To verify that it is setup right, I want to send a start condition and check to see that the SDA line goes low while SCL stays high. I have the following code (borrowed from the web)...

void I2C_Setup(void)
{
    GPIO_InitTypeDef  GPIO_InitStructure;
    I2C_InitTypeDef  I2C_InitStructure;
      
    /* I2C1 clock enable */
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_I2C1, ENABLE);
  
    /*enable I2C*/
    I2C_Cmd(I2C1,ENABLE);
  
    /* I2C1 SDA and SCL configuration */
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_6 | GPIO_Pin_7;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_OD; // Alternate function Open Drain
    GPIO_Init(GPIOB, &GPIO_InitStructure);
    /*SCL is pin06 and SDA is pin 07 for I2C1*/
  
    /* I2C1 configuration */
    I2C_InitStructure.I2C_Mode = I2C_Mode_I2C;
    I2C_InitStructure.I2C_DutyCycle = I2C_DutyCycle_2;
    I2C_InitStructure.I2C_OwnAddress1 = 0x04;
    I2C_InitStructure.I2C_Ack = I2C_Ack_Enable;
    I2C_InitStructure.I2C_AcknowledgedAddress = I2C_AcknowledgedAddress_7bit;
    I2C_InitStructure.I2C_ClockSpeed = I2C_SPEED ;
    I2C_Init(I2C1, &I2C_InitStructure);
}

I call this function first and then try to send the start condition...
/* initiate start sequence */
    I2C_GenerateSTART(I2C1, ENABLE);
    /* check start bit flag */
    while(!I2C_GetFlagStatus(I2C1, I2C_FLAG_SB));

I have an o-scope connected to both SDA and SCL but I don't see SDA go low as I would expect. I just put in the own address as 0x4 for now. There is no slave on the bus, I just have the two gpio lines going to a breadboard fed by 3.3 from the discovery board and those pins are pulled high with 4.7K pullups. I was expecting to see SDA go low.
Is my understanding of the protocol incorrect or is the web sample missing something?

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