Repeat Start Condition

[asad-urrahman]

Thank so much for this work, its really helpful. I want to know how can i add Repeat start condition function; that master first send command to slave and start reading instantly without freeing i2c bus.

[NeuralSpaz]

If you know the number of bytes that you are expecting to read from the slave.
ReadFromReg(addr, reg byte, value []byte) should do what you want. (but I have not validated with a DSO)

Or
check out golang.org/x/exp/io/i2c. depending on the specifics it might also do what you need.

If you need a hand with any of that or an example I'll see if I can help you out.

[asad-urrahman]

Thank you so much, Yes i have checked golang.org/x/exp/io/i2c, there read() and write() routines are used while Repeat start condition in examples are using ioct().
( for example see here )

In my case, command is sent in one single packet, and response is received automatically.

TxBuffer actually contains TxBufferLength number of chars/bytes which are sent first to slave device like a command, I2C_M_RECV_LEN keep I2C bus busy for Repeat Start.
In case of success, a response from slave device is received. Received data can be found in packets.msgs[0].buf

// unsigned char * TxBuffer
// unsigned short TxBufferLength

    // These data types defined in linux/i2c.h
    struct i2c_rdwr_ioctl_data packets;
    struct i2c_msg messages[1];
    
    // .......

    messages[0].addr  = I2cDeviceAddress;   // 0x50
    messages[0].len   = TxBufferLength;   // 8
    messages[0].buf   = TxBuffer;   // [FF,FF,FF,FF,FF,FF,FF,FF]
    messages[0].flags = I2C_M_RECV_LEN; 
    // as defined in linux/i2c.h, this flag  can be used for repeated start condition

    /* Send the request to the kernel and get the result back */
    packets.msgs      = messages;
    packets.nmsgs     = 1;

    err = ioctl(i2cDevice, I2C_RDWR, &packets);

    if(err < 0) {
        return I2C_ERROR;
    }


    // Data recieved packets.msgs[0].buf
    // Recieved Number of Byets/chars are packets.msgs[0].buf[0]+1;

[NeuralSpaz]

So atm I'm thinking interface modification like this:
ReadFromReg(addr, reg byte, value []byte, mode ...func() error) error
with the defaults as the current behavior if mode function is passed. (backwards compatible)
and then a RepeatStart() error function.
Can you see any need for RepeatedStart on any other of the interface methods?

[NeuralSpaz]

have a look at https://github.com/NeuralSpaz/i2c/tree/experimental, I have not had a chance to test, but should be easy to mod/extend to suit your needs without too much fuss.

Let me know.

[asad-urrahman]

I am sorry i ddint get completely, but i think to adding new function would be better which do support Repeat start condition. I am working on smart cards and its working principal is like this; Master send an command (apdu) as a result Slave send back response, command and response may composed of many bytes.
I would say signature would be like this
WriteRead(addr, cmd []byte, value []byte) error
The length of response is first byte of response.

[NeuralSpaz]

The variadic is just a way to have per call options that are also backward compatible.

The function call for a function:
WriteRead(addr, cmd []byte, value []byte, mode ...func() error) error

would be like:
i2cConn.WriteRead(0x50, []byte{0x01,0x10}, rxbuf, i2c.RepeatedStart(i2cConn))
or other options like
i2cConn.WriteRead(0x50, []byte{0x01,0x10}, rxbuf, i2c.IgnoreNak(i2cConn))
or
i2cConn.WriteRead(0x50, []byte{0x01,0x10}, rxbuf) for default behavior.

[asad-urrahman]

I am sorry, still dint get because from code
ReadFromReg(addr, reg byte, value []byte, mode ...func() error) error
while in WriteRead() reg is not single byte but an array/slice .