Configuring AVR USI_IRQ_DO for two wire mode data output onto SDA line

examples/board_usi_i2cslave/Makefile

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+#
+# 	Copyright 2008-2012 Michel Pollet <buserror@gmail.com>
+#
+#	This file is part of simavr.
+#
+#	simavr is free software: you can redistribute it and/or modify
+#	it under the terms of the GNU General Public License as published by
+#	the Free Software Foundation, either version 3 of the License, or
+#	(at your option) any later version.
+#
+#	simavr is distributed in the hope that it will be useful,
+#	but WITHOUT ANY WARRANTY; without even the implied warranty of
+#	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#	GNU General Public License for more details.
+#
+#	You should have received a copy of the GNU General Public License
+#	along with simavr.  If not, see <http://www.gnu.org/licenses/>.
+
+target=	i2ctest_usislave
+firm_src = ${wildcard at*${board}.c}
+firmware = ${firm_src:.c=.axf}
+simavr = ../..
+
+IPATH = .
+IPATH += ${simavr}/examples/shared
+IPATH += ${simavr}/examples/parts
+IPATH += ${simavr}/include
+IPATH += ${simavr}/simavr/sim
+
+VPATH = .
+VPATH += ${simavr}/examples/shared
+VPATH += ${simavr}/examples/parts
+
+
+all: obj ${firmware} ${target}
+
+include ${simavr}/Makefile.common
+
+atmega1280_${target}.axf: atmega1280_${target}.c twimaster.c
+atmega1280_${target}.axf: ${simavr}/examples/shared/avr_twi_master.c
+atmega1280_${target}.axf: ${simavr}/examples/shared/avr_twi_master.h
+
+board = ${OBJ}/${target}.elf
+
+${board} : ${OBJ}/${target}.o
+${board} : ${OBJ}/i2c_eeprom.o
+
+${target}: ${board}
+	@echo $@ done
+
+clean: clean-${OBJ}
+	rm -rf *.hex *.a *.axf ${target} *.vcd .*.swo .*.swp .*.swm .*.swn

examples/board_usi_i2cslave/README

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+
+This example is similar to board_i2cslave, treating the AVR (ATtiny)
+as a bus slave, rather than bus master. It hooks directly to the
+AVR usi (i2c mode) pins, and bit bangs the I2C protocol as a master,
+and then runs a firmware that behaves as a TWI slave to talk to it.