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TCA6424A: issues using python smbus2 read to read from TCA6424A input register

Mike Ryb
Prodigy 30 points
Part Number: TCA6424A
Other Parts Discussed in Thread: TCA6424

Hello,

I am trying to communicate with two TCA6424A devices via i2c using python smbus2 with Raspberry Pi 4 .

So, smbus2  read and  write i2c functions through no exceptions. 

But, it looks like read function doesn't read from the input register configured like this

bus = SMBus(1)

    TCA_Slave_Address = 0x22
    TCA_Reg_Address = 0x8C
    TCA_Data4B = [0x00,0x00,0x00]  # All ports output    
     bus.write_i2c_block_data(TCA_Slave_Address, TCA_Reg_Address , TCA_Data4B)
    time.sleep(0.3)  
    TCA_Slave_Address = 0x23
    TCA_Reg_Address = 0x8C
    TCA_Data4B = [0x00,0x00,0xFF] # 16 ports output, 8 ports input
    bus.write_i2c_block_data(TCA_Slave_Address, TCA_Reg_Address , TCA_Data4B)
    time.sleep(0.3)

  Next, I write data to  device address   0x23 output port-0

    TCA_Slave_Address = 0x23
    TCA_Reg_Address = 0x84
    TCA_Data4B = [0x02,0x02,0x02]  
    
    I2C.BMP_Write(TCA_Slave_Address, TCA_Reg_Address, TCA_Data4B)
    time.sleep(0.3)
  When I read from device 0x23 input register  I get data which has just been written to output port-0  in the previous step  
   
    write = i2c_msg.write(0x23, 0x02)
    read = i2c_msg.read(0x23, 3)               
    try:
       print( bus.i2c_rdwr(write, read))    >> [2,2,2]
    except Exception as e:
          
The i2c_rdwr(write, read)  function produces that below logic-analyzer diagram.  Any help or advice would be very much appreciated .   Thanks. 

  • 0
    Guru 303540 points

    The logic analyer shows that the "i2c_msg.write(0x23, 0x02)" message writes two zero bytes, which makes the following read access the output port.

    When calling i2c_msg.write, always give it a list of bytes, or a string containing the bytes: i2c_msg.write(0x23, [0x02]).

    (And why are reading the same register three times?)

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Thank you for your quick response ,  I will try   i2c_msg.write(0x23, [0x02]) as you are suggesting. 
    The goal is to read the input Port-2 so, the i2c_msg.write(0x23, 0x02) should write 0x2 to TCA6424 command byte to allow reading from input Port-2. , if I am not mistaken. 

    I am not sure about reading the same register three times. I just mimic with Python the original MCU STM32 C code Read function which works fine . The only difference is :  C code is using 8 bit address and smbus2 python can take only 7bit address. However, using i2c sniffer with 8bit addressing I can see my python read function  read = i2c_msg.read(0x23, 3) is reading from address 0x47 (as it should be, correct ?)

    void TCA_Read(void)
    {
    read_state = 0;
    I2C2->CR1 |= 0x00000040; //Transfer Complete Interrupt enabled TCIE
    I2C2->CR2 &= 0x00000000;
    I2C2->CR2 |= 0x00010046; //AUTOEND=0, NBYTES=1,Write Request,SADD[7:1]=Slave Address=0x46
    }
    switch (read_state)
    {
    case 0:
    I2C2->CR2 |= 0x00002000;  //Start condition
    read_state = 1;
    break;

    case 1:
    read_state = 2;
    I2C2->TXDR = 0x02;
    break;

    case 2:
    I2C2->CR1 &= 0xFFFFFFBF; //Transfer Complete Interrupt disabled TCIE
    I2C2->CR2 &= 0x00000000;
    I2C2->CR2 |= 0x00010000;
    I2C2->CR2 |= 0x02000446;  //AUTOEND=1, NBYTES=2 (3bytes),Read Request,SADD[7:0]=Slave write Address=0x46
    I2C2->CR2 |= 0x00002000;  //Re-start condition

    read_state = 3;
    break;

    case 3:
    TCA_Temp_Buff[TCA_Temp_Sample_Count] = I2C2->RXDR;

    read_state = 0;
    break;

    default:
    read_error_set();
    break;
    }

  • 0 in reply to Mike Ryb
    Guru 303540 points

    "i2c_msg.read(0x23, 3)" tells it to read three bytes. Because the previously written command byte is 0x02 (and not 0x82), the register index is not autoincremented, so you get the same value three times.

    An I²C address always has seven bits. The first byte of an I²C transaction contains the address and the R/W bit. This byte is sometimes called the "address byte", but this does not make the adress eight bits. (And some microcontrollers have a register that also combines the address and the R/W bit, such as in your example.)

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Thanks again for your explanations.
    I will try to use correct terms in the future but , my question is still standing , why it is not reading from the input port-2 ?

  • 0 in reply to Mike Ryb
    Guru 303540 points

    Because the command byte is zero (as shown by the logic analyzer).

  • 0 in reply to Mike Ryb
    TI__Guru 65020 points

    Hi Mike,

    I agree with Clemen's comment. Your logic analyzer image is showing that you are writing to the command byte 0x00h prior to the read which sets the read transaction to read from the INPUT PORT 0. You would need to set the write command to send the command byte 0x02h if you want to read from INPUT PORT 2. 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 30 points

    The question was why the logic analyzer was showing 0x00 instead of 0x02. 
    It appears Clemens suggestion to use brackets for a single byte  0x02 in  i2c_msg.write(0x23, [0x02]) is correct. 

    Thank you Clemens  one more time.