Do not reply with NAK on last byte

At least the bcm2835 (Raspberry Pi 3) I2C/SMBus master has problems
when replying with a NAK to the last byte written to a slave device.

Fix this by replying with NAK one byte later.

.gitignore

@@ -3,4 +3,5 @@
 *.bin
 *.hex
 *.lst
+*.lss
 *.map

LICENSE

@@ -0,0 +1,262 @@
+GNU GENERAL PUBLIC LICENSE
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+Version 2, June 1991
+
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Makefile

@@ -1,47 +1,103 @@
-PRG            = twiboot
-OBJ            = main.o
-MCU_TARGET     = atmega88
-OPTIMIZE       = -Os
+CC	:= avr-gcc
+LD	:= avr-ld
+OBJCOPY	:= avr-objcopy
+OBJDUMP	:= avr-objdump
+SIZE	:= avr-size
 
-DEFS           =
-LIBS           =
+TARGET = twiboot
+SOURCE = $(wildcard *.c)
 
-# Override is only needed by avr-lib build system.
-override CFLAGS        = -g -Wall $(OPTIMIZE) -mmcu=$(MCU_TARGET) $(DEFS)
-override LDFLAGS       = -Wl,-Map,$(PRG).map,--section-start=.text=0x1C00
+# select MCU
+MCU = attiny85
 
-CC             = avr-gcc
-OBJCOPY        = avr-objcopy
-OBJDUMP        = avr-objdump
-SIZE           = avr-size
+AVRDUDE_PROG := -c avr910 -b 115200 -P /dev/ttyUSB0
+#AVRDUDE_PROG := -c dragon_isp -P usb
 
-all: $(PRG).elf lst text
-	$(SIZE) -x -A $(PRG).elf
+# ---------------------------------------------------------------------------
 
-$(PRG).elf: $(OBJ)
-	$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ $(LIBS)
+ifeq ($(MCU), atmega8)
+# atmega8:
+# Fuse L: 0x84 (8Mhz internal RC-Osz., 2.7V BOD)
+# Fuse H: 0xda (512 words bootloader)
+AVRDUDE_MCU=m8
+AVRDUDE_FUSES=lfuse:w:0x84:m hfuse:w:0xda:m
+
+BOOTLOADER_START=0x1C00
+endif
+
+ifeq ($(MCU), atmega88)
+# atmega88:
+# Fuse L: 0xc2 (8Mhz internal RC-Osz.)
+# Fuse H: 0xdd (2.7V BOD)
+# Fuse E: 0xfa (512 words bootloader)
+AVRDUDE_MCU=m88
+AVRDUDE_FUSES=lfuse:w:0xc2:m hfuse:w:0xdd:m efuse:w:0xfa:m
+
+BOOTLOADER_START=0x1C00
+endif
+
+ifeq ($(MCU), atmega168)
+# atmega168:
+# Fuse L: 0xc2 (8Mhz internal RC-Osz.)
+# Fuse H: 0xdd (2.7V BOD)
+# Fuse E: 0xfa (512 words bootloader)
+AVRDUDE_MCU=m168 -F
+AVRDUDE_FUSES=lfuse:w:0xc2:m hfuse:w:0xdd:m efuse:w:0xfa:m
+
+BOOTLOADER_START=0x3C00
+endif
+
+ifeq ($(MCU), atmega328p)
+# atmega328p:
+# Fuse L: 0xc2 (8Mhz internal RC-Osz.)
+# Fuse H: 0xdc (512 words bootloader)
+# Fuse E: 0xfd (2.7V BOD)
+AVRDUDE_MCU=m328p -F
+AVRDUDE_FUSES=lfuse:w:0xc2:m hfuse:w:0xdc:m efuse:w:0xfd:m
+
+BOOTLOADER_START=0x7C00
+endif
+
+ifeq ($(MCU), attiny85)
+# attiny85:
+# Fuse L: 0xe2 (8Mhz internal RC-Osz.)
+# Fuse H: 0xdd (2.7V BOD)
+# Fuse E: 0xfe (self programming enable)
+AVRDUDE_MCU=t85
+AVRDUDE_FUSES=lfuse:w:0xe2:m hfuse:w:0xdd:m efuse:w:0xfe:m
+
+BOOTLOADER_START=0x1C00
+CFLAGS_TARGET=-DUSE_CLOCKSTRETCH=1 -DVIRTUAL_BOOT_SECTION=1
+endif
+
+# ---------------------------------------------------------------------------
+
+CFLAGS = -pipe -g -Os -mmcu=$(MCU) -Wall -fdata-sections -ffunction-sections
+CFLAGS += -Wa,-adhlns=$(*F).lst -DBOOTLOADER_START=$(BOOTLOADER_START) $(CFLAGS_TARGET)
+LDFLAGS = -Wl,-Map,$(@:.elf=.map),--cref,--relax,--gc-sections,--section-start=.text=$(BOOTLOADER_START)
+LDFLAGS += -nostartfiles
+
+# ---------------------------------------------------------------------------
+
+$(TARGET): $(TARGET).elf
+	@$(SIZE) -B -x --mcu=$(MCU) $<
+
+$(TARGET).elf: $(SOURCE:.c=.o)
+	@echo " Linking file:  $@"
+	@$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^
+	@$(OBJDUMP) -h -S $@ > $(@:.elf=.lss)
+	@$(OBJCOPY) -j .text -j .data -O ihex $@ $(@:.elf=.hex)
+	@$(OBJCOPY) -j .text -j .data -O binary $@ $(@:.elf=.bin)
+
+%.o: %.c $(MAKEFILE_LIST)
+	@echo " Building file: $<"
+	@$(CC) $(CFLAGS) -o $@ -c $<
 
 clean:
-	rm -rf *.o *.lst *.map $(PRG).elf *.hex *.bin
+	rm -rf $(SOURCE:.c=.o) $(SOURCE:.c=.lst) $(addprefix $(TARGET), .elf .map .lss .hex .bin)
 
-lst:  $(PRG).lst
-
-%.lst: %.elf
-	$(OBJDUMP) -h -S $< > $@
-
-text: hex bin
-
-hex:  $(PRG).hex
-bin:  $(PRG).bin
-
-%.hex: %.elf
-	$(OBJCOPY) -j .text -j .data -O ihex $< $@
-
-%.bin: %.elf
-	$(OBJCOPY) -j .text -j .data -O binary $< $@
-
-install: text
-	avrdude -c dragon_isp -P usb -p m88 -U flash:w:$(PRG).hex
+install: $(TARGET).elf
+	avrdude $(AVRDUDE_PROG) -p $(AVRDUDE_MCU) -U flash:w:$(<:.elf=.hex)
 
 fuses:
-	avrdude -c dragon_isp -P usb -p m88 -U lfuse:w:0xc2:m -U hfuse:w:0xdd:m -U efuse:w:0xfa:m
+	avrdude $(AVRDUDE_PROG) -p $(AVRDUDE_MCU) $(patsubst %,-U %, $(AVRDUDE_FUSES))

README.md

@@ -0,0 +1,119 @@
+# twiboot - a TWI / I2C bootloader for AVR MCUs ##
+twiboot is a simple/small bootloader for AVR MCUs written in C. It uses the integrated TWI or USI peripheral of the controller to implement a I2C slave.
+It was originally created to update I2C controlled BLMCs (Brushless Motor Controller) without an AVR ISP adapter.
+
+twiboot acts as a slave device on a TWI/I2C bus and allows reading/writing of the internal flash memory.
+As a compile time option (EEPROM_SUPPORT) twiboot also allows reading/writing of the whole internal EEPROM memory.
+The bootloader is not able to update itself (only application flash memory region accessible).
+
+Currently the following AVR MCUs are supported:
+
+AVR MCU | Flash bytes used (.text + .data) | Bootloader region size
+--- | --- | ---
+attiny85 | 954 (0x3BA) | 512 words
+atmega8 | 786 (0x312) | 512 words
+atmega88 | 810 (0x32A) | 512 words
+atmega168 | 810 (0x32A) | 512 words
+atmega328p | 810 (0x32A) | 512 words
+
+(Compiled on Ubuntu 18.04 LTS (gcc 5.4.0 / avr-libc 2.0.0) with EEPROM and LED support)
+
+
+## Operation ##
+twiboot is installed in the bootloader section and executed directly after reset (BOOTRST fuse is programmed).
+For MCUs without bootloader section see [Virtual bootloader section](#virtual-bootloader-section) below.
+
+While running, twiboot configures the TWI/USI peripheral as slave device and waits for valid protocol messages
+directed to its address on the TWI/I2C bus. The slave address is configured during compile time of twiboot.
+When receiving no messages for 1000ms after reset, the bootloader exits and executes the main application at address 0x0000.
+
+A TWI/I2C master can use the protocol to
+- abort the boot timeout
+- query information about the device (bootloader version, AVR signature bytes, flash/eeprom size, flash page size)
+- read internal flash / eeprom memory (byte wise)
+- write the internal flash (page wise)
+- write the internal eeprom (byte wise)
+- exit the bootloader and start the application
+
+As a compile time option (LED_SUPPORT) twiboot can output its state with two LEDs.
+One LED will flash with a frequency of 20Hz while twiboot is active (including boot wait time).
+A second LED will flash when the bootloader is addressed on the TWI/I2C bus.
+
+
+### Virtual Bootloader Section ###
+For MCUs without bootloader section twiboot will patch the vector table on the fly during flash programming to stay active.
+The reset vector is patched to execute twiboot instead of the application code.
+
+Another vector entry will be patched to store the original entry point of the application.
+This vector entry is overridden and MUST NOT be used by the application.
+twiboot uses this vector to start the application after the initial timeout.
+
+This live patching changes the content of the vector table, which would result in a verification error after programming.
+To counter this kind of error, twiboot caches the original vector table entries in RAM and return those on a read command.
+The real content of the vector table is only returned after a reset.
+
+
+## Build and install twiboot ##
+twiboot uses gcc, avr-libc and GNU Make for building, avrdude is used for flashing the MCU.
+The build and install procedures are only tested under linux.
+
+The selection of the target MCU and the programming interface can be found in the Makefile,
+TWI/I2C slave address and optional components (EEPROM / LED support) are configured
+in the main.c source.
+
+To build twiboot for the selected target:
+``` shell
+$ make
+```
+
+To install (flash download) twiboot with avrdude on the target:
+``` shell
+$ make install
+```
+
+Set AVR fuses with avrdude on the target (internal RC-Osz, enable BOD, enable BOOTRST):
+``` shell
+$ make fuses
+```
+
+
+## TWI/I2C Protocol ##
+A TWI/I2C master can use the following protocol for accessing the bootloader.
+
+Function | TWI/I2C data | Comment
+--- | --- | ---
+Abort boot timeout | **SLA+W**, 0x00, **STO** |
+Show bootloader version | **SLA+W**, 0x01, **SLA+R**, {16 bytes}, **STO** | ASCII, not null terminated
+Start application | **SLA+W**, 0x01, 0x80, **STO** |
+Read chip info | **SLA+W**, 0x02, 0x00, 0x00, 0x00, **SLA+R**, {8 bytes}, **STO** | 3byte signature, 1byte page size, 2byte flash size, 2byte eeprom size
+Read 1+ flash bytes | **SLA+W**, 0x02, 0x01, addrh, addrl, **SLA+R**, {* bytes}, **STO** |
+Read 1+ eeprom bytes | **SLA+W**, 0x02, 0x02, addrh, addrl, **SLA+R**, {* bytes}, **STO** |
+Write one flash page | **SLA+W**, 0x02, 0x01, addrh, addrl, {* bytes}, **STO** | page size as indicated in chip info
+Write 1+ eeprom bytes | **SLA+W**, 0x02, 0x02, addrh, addrl, {* bytes}, **STO** | write 0 < n < page size bytes at once
+
+**SLA+R** means Start Condition, Slave Address, Read Access
+
+**SLA+W** means Start Condition, Slave Address, Write Access
+
+**STO** means Stop Condition
+
+A flash page / eeprom write is only triggered after the Stop Condition.
+During the write process twiboot will NOT acknowledge its slave address.
+
+The multiboot_tool repository contains a simple linux application that uses
+this protocol to access the bootloader over linux i2c device.
+
+The ispprog programming adapter can also be used as a avr910/butterfly to twiboot protocol bridge.
+
+
+## TWI/I2C Clockstretching ##
+While a write is in progress twiboot will not respond on the TWI/I2C bus and the
+TWI/I2C master needs to retry/poll the slave address until the write has completed.
+
+As a compile time option (USE_CLOCKSTRETCH) the previous behavior of twiboot can be restored:
+TWI/I2C Clockstretching is then used to inform the master of the duration of the write.
+Please note that there are some TWI/I2C masters that do not support clockstretching.
+
+
+## Development ##
+Issue reports, feature requests, patches or simply success stories are much appreciated.