I2C found the wrong address (Actually 1 shifter version)

[SKAŞ.1]

Hello Folks,

I was doing my I2C example, and I realize that the STM board found an address with one left-shifted.

I set my Arduino address as 70 -> 0x46 for hexadecimal

void setup()
{
  Wire.begin(0x46); // Wire communication begin
}

I connect my STM32F429I_DISC1 and finally make it work with the IsDeviceReady function.

It returns as 0-1-130-131 (I don't know why but not relevant.) and then it finds 140 and 141

When I check my binaries,

70 -> 0100 1100

140 -> 1000 1100

141 -> 1000 1101

when I left shift 70 with 1 bit, I got 140, I have no idea why I am getting 141 (last bit not important maybe?)

So I'm just curious about why it's happening?

I supply my project in here, I didn't add any additional library or so,

I just use CubeMX and CubeIDE (Not even Keil uVision 5)

But since, I am capable of doing I2C communication happen (even with shifted addresses), I want to look forward to complex examples.

Now, what should I send to my slave Arduino, 70 or 140?

Also when I make a scan at Arduino, it gives some I2C addresses, I am not sure which one belongs to my STM32.

 

There is only STM32 and Arduino connection with SDA and SCL, they have additional 4.7k pull-up resistor connected to 5V output of STM32, and ground of STM and Arduino connected.

Thanks for your attention,

Kind Regards.

[Pavel A.]

> When I check my binaries,

> 70 -> 0100 1100

Nope. 0100 0110.

> I got 140, I have no idea why I am getting 141 (last bit not important maybe?)

This is actually an important reminder to refresh the basics of I2C protocol 🙂

The LSB of the address set means read request. If this bit is 0 it means a write request.

If the device supports both reads and writes it will respond to both addresses.

> Now, what should I send to my slave Arduino, 70 or 140?

140. Or 141.

-- pa

[Mike_ST · Accepted solution]

Hello,

>> So I'm just curious about why it's happening?

I2C Addresses are 7 bits + 1 bit for R/W => 8bits address

Write => bit 0 =0

Read => bit 0 =1

>> Now, what should I send to my slave Arduino, 70 or 140?

HAL layer will expect a 7 bits address shifted 1 bit left, so, 140 in your case.

Depending of what operation you are doing, read or write, the HAL functions will take care of the bit 0.

[SKAŞ.1]

Thanks for your response people,

Yes I got it. As you guys mention, it's essential part of I2C protocol. Respectively that is my first time I really use these things. Also thanks for your support. I totally understand the I2C examples. I saw your message too late, but at least I saw it. I complete my examples on this one now, I am going to use UART and then SPI. I am gonna open a new topic if I need. THANKS AGAIN!