AVR Freaks

Yah. So, the Attiny85 core Wire implementation basically uses AVR312 implementation. I tossed the Wire library, but took their low-level stuff (which is very close, or identical to AVR312). I added a new routine to set/clear a busy flag, then hacked up USI_Counter_Overflow_ISR(void) when it does USE_SLAVE_CHECK_ADDRESS; instead of sending SET_USI_TO_SEND_ACK() blindly I check my busy flag and send SET_USI_TO_SEND_NACK() instead if we are busy. A test app that allows you to put it into busy when you press a button (using our modified lib):

extern "C" {
#include "USI_TWI_Slave.h"
	};

void yay_data(int amount)
{
	while (USI_TWI_Data_In_Receive_Buffer())
	{
		byte val = USI_TWI_Receive_Byte();
		if (val == 0 || val == '0')
			digitalWrite(4, HIGH);
		else
			digitalWrite(4, LOW);
	}
	return;
}

void yay_write()
{
}

void initial_i2c()
{
	USI_TWI_Slave_Initialise(0x20);
	USI_TWI_On_Slave_Receive = yay_data;
	USI_TWI_On_Slave_Transmit = yay_write;
}

void setup()
{

	pinMode(3, OUTPUT);
	pinMode(4, OUTPUT);
	digitalWrite(3, HIGH);
	digitalWrite(4, HIGH);
	initial_i2c();
}
bool busy = false;

void loop()
{
	bool old_busy = busy;
	busy = digitalRead(1)==0 ? true : false;
	digitalWrite(3, busy ? LOW : HIGH);
	if (old_busy != busy)
	{
		if (busy)
			USI_TWI_Busy(1);
		else
			USI_TWI_Busy(0);
	}
	delay(50);
}

Yes, no switch debounce or anything :) (note the LED's are reversed, LOW lights them) Ran a master against that which sends 0/1/0/1 and backs off and ack-polls if a write fails, and it all works very nicely; press the switch, the master backs off and polls, let go the switch and it all works nicely. Setting busy when I have a pending EEPROM write will work out perfectly.

Thanks for the ideas and pointer, it got me going in the write (*) direction!

PK

(*) Sorry, I couldn't help it.

For posterity, if you happen to use circuit python, here is a test master and some quick and dirty proof of concept library files are attached.:

import busio
import board
import time

i2c = busio.I2C(board.SCL, board.SDA, frequency=100000)

def ack_poll(i2c):
    i2c.writeto(32, b'')

val = 0
def toggle(i2c):
    global val
    i2c.writeto(32, bytes([val]))
    val = 1 if val == 0 else 0

print("Hi there")
while not i2c.try_lock():
    pass
while True:
    try:
        toggle(i2c)
    except OSError:
        print("Boom, will poll for a bit")
        while True:
            # TODO: Add timeout
            try:
                ack_poll(i2c)
                print("We're back, baby")
                break
            except OSError:
                # TODO: Shorter...
                time.sleep(0.1)
                pass

    time.sleep(0.5)
i2c.unlock()

None of this is production, of course, but might help someone get on the write foot.