Talking SMBus between RaspberryPi and ATMEGA 324PA - AVR not clock stretching

I'm trying to get an ATMEGA 324PA to run as an SMBus slave.

I'm using the following code on the Pi:

import smbus as smbus
i2c = smbus.SMBus(1)
i2c_addr = 0x30
result = i2c.read_block_data( i2c_addr, reg )

On the AVR, I'm using:

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/sleep.h>
#include "smb_slave.h"

#define SMB_COMMAND_RETURN_VENDOR_STRING    0x10

int main(void)
{
    // Set data direction of PORTB as output and turn off LEDs.
    DDRA = 0xff;
    PORTA = 0xff;

    // Initialize SMBus
    SMBusInit();
    SMBEnable();

    // Enable interrupts globally

    sei();
    for (;;)
    {
    }

    return 0;
}


void ProcessReceiveByte(SMBData *smb)
{
    smb->txBuffer[0] = ~PIND;
    smb->txLength = 1;
}

static void ReturnVendorString(SMBData *smb)
{
    unsigned char *vendor = (unsigned char*) "Vendor\0";
    unsigned char i;
    unsigned char temp;


    i = 0;
    // Copy vendor ID string from EEPROM.
    while ((temp = vendor[i]) != '\0')
    {
        i++;
        smb->txBuffer[i] = temp;
    }
    smb->txBuffer[0] = i; // Byte count.
    smb->txLength = i + 1; // Number of bytes to be transmitted including byte count.

    smb->state = SMB_STATE_WRITE_READ_REQUESTED;
    PORTA ^= 0x40; // debug
}

static void UndefinedCommand(SMBData *smb)
{
    // Handle undefined requests here.
    smb->error = TRUE;
    smb->state = SMB_STATE_IDLE;
}


void ProcessMessage(SMBData *smb)
{
    if (smb->state == SMB_STATE_WRITE_REQUESTED)
    {
        switch (smb->rxBuffer[0]) // Command code.
        {
            case SMB_COMMAND_RETURN_VENDOR_STRING:  // Block read, vendor ID.
                ReturnVendorString(smb);
                break;
            default:
                UndefinedCommand(smb);
                break;
        }
    }
    else
    {
        smb->state = SMB_STATE_IDLE;
    }
}

With a (gcc-adapted) version of: http://www.atmel.com/images/AVR316.zip from http://www.atmel.com/devices/ATMEGA324A.aspx?tab=documents

Something's partially working, as my logic analyser shows:

But I assume I'm doing something wrong as the AVR is not ACK'ing the READ, nor clock-stretching nor sending a response.

Where should I look next?

Can I have confidence in the Python smbus module on the RasPi?

---

Could what I'm seeing be related to https://github.com/raspberrypi/linux/issues/254 ?

The issue you linked is the problem -- i2c clock stretching is simply broken on the Raspberry Pi. More info: http://www.advamation.com/knowhow/raspberrypi/rpi-i2c-bug.html

If a sensor has alternative output such as UART sometimes that's an option but for some projects I've had to use a micro or Beaglebone or some other thing.