PIC24f: How to Generate Start condition using SPI for I2C device?

Question

I am trying to communicate with the device having internal communication as I2C while externally it has two different pin for Input & Output along with a separate pin for Clock. Here is the link.

I am using SPI to communicate with this device. But I am unable to generate START/STOP condition to communicate with I2C device.

Can I get some suggestion to overcome this issue ???

I am using C Coding.

---

I Tried Bit Banging as follow

Dont know if any logical error.

Here is the Start Condition I m trying to implement :

bool generateStartSeq()
{
    SPI1STATbits.SPIEN      = 0;    // disable SPI port
//    SPI1CON1bits.DISSCK       = 1;    // Internal SPIx clock is enabled
//    SPI1STATbits.SPIEN        = 1;    // enable SPI port, clear status

    AD1PCFG = 0xFFFF;               // Convert IO pin to Digital
    TRISFbits.TRISF6 = 0;           // Configure RF6 pin as output;
    TRISFbits.TRISF8 = 0;           // Configure RF8 pin as output;

    LATFbits.LATF6 = 1;             // Make RF6 pin high

    LATFbits.LATF8 = 1;

    Nop;
    Nop;
    Nop;
    Nop;
    Nop;

    LATFbits.LATF8 = 0;

    //    writeSPI1(0xFF);                // SDO send data High
    //    
    //    Nop;
    //    Nop;
    //    Nop;
    //    Nop;
    //    Nop;
    //    
    //    writeSPI1(0x00);

    SPI1Init();

    return true;

}

Any suggestion will really help me to correct the logic.

Answer 1

DISCLAIMER: The voltages involved can easily damage your PIC, your programmer and USB hardware. If you use this design, you do so at your own risk and I am not responsible for any damage that may arise.

Instead of trying to find a software solution, I'd recommend a very simple hardware solution that would allow you to use the stock I2C controller in the PIC. Note that, as with your software solution, you probably need a level shifter to raise your pic's output to the module's 0-24V logic levels but with this solution you dont need a level shifter to drop the module's output to you pic's range (because of the blocking diode).

^{simulate this circuit – Schematic created using CircuitLab}

The diode can be any blocking diode with sufficiently low Vfwd (that is why I recommend a shotkey diode) and a high blocking voltage ( > 30V ). The BAT41 series seems to be a good fit according to it's specifications (disclaimer, I have never used it in this application).

This works because I2C is an open collector digital communication. That is, any IC on the I2C bus will only ever pull it low, the bus comes back up high because of the resistor pulling it to Vdd. Thus, if the HM DATA OUT goes low, it pulls the SDAx low with it but when it is high, the PIC is free to let it go high or pull it low itself.

If you need to implement your own level shifting hardware, I'd recommend a simple FET based level shifter. Although beware that the FET must be mounted in the right direction. If you reverse the direction, your pic will be exposed to 24V through it's body diode and it (your PIC) likely wont survive.

^{simulate this circuit}