97 lines
4.2 KiB
Markdown
97 lines
4.2 KiB
Markdown
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# twiboot - a TWI / I2C bootloader for AVR MCUs ##
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twiboot is a simple/small bootloader for AVR MCUs with integrated TWI peripheral written in C.
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It was originally created to update I2C controlled BLMCs (Brushless Motor Controller) without an AVR ISP adapter.
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twiboot acts as a slave device on a TWI/I2C bus and allows reading/writing of the internal flash memory.
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As a compile time option twiboot also allows reading/writing of the whole internal EEPROM memory.
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The bootloader is not able to update itself (only application flash memory region accessible).
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Currently the following AVR MCUs are supported:
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AVR MCU | Flash bytes used (.text + .data) | Bootloader region size
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--- | --- | ---
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atmega8 | 900 (0x384) | 512 words
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atmega88 | 950 (0x3B6) | 512 words
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atmega168 | 1002 (0x3EA) | 512 words
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atmega328p | 1002 (0x3EA) | 512 words
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(Compiled on Ubuntu 18.04 LTS (gcc 5.4.0 / avr-libc 2.0.0) with EEPROM and LED support)
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## Operation ##
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twiboot is installed in the bootloader memory region and executed directly after reset (BOOTRST fuse is programmed).
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While running, twiboot configures the TWI peripheral as slave device and waits for valid protocol messages
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directed to its address on the TWI/I2C bus. The slave address is configured during compile time of twiboot.
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When receiving no messages for 1000ms after reset, the bootloader exits and executes the main application at address 0x0000.
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A TWI/I2C master can use the protocol to
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- abort the boot timeout
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- query information about the device (bootloader version, AVR signature bytes, flash/eeprom size, flash page size)
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- read internal flash / eeprom memory (byte wise)
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- write the internal flash (page wise)
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- write the internal eeprom (byte wise)
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- exit the bootloader and start the application
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As a compile time option twiboot can output its state with two LEDs.
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One LED will flash with a frequency of 20Hz while twiboot is active (including boot wait time).
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A second LED will flash when the bootloader is addressed on the TWI/I2C bus.
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## Build and install twiboot ##
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twiboot uses gcc, avr-libc and GNU Make for building, avrdude is used for flashing the MCU.
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The build and install procedures are only tested under linux.
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The selection of the target MCU and the programming interface can be found in the Makefile,
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TWI/I2C slave address and optional components (EEPROM / LED support) are configured
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in the main.c source.
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To build twiboot for the selected target:
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``` shell
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$ make
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```
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To install (flash download) twiboot with avrdude on the target:
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``` shell
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$ make install
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```
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Set AVR fuses with avrdude on the target (internal RC-Osz, enable BOD, enable BOOTRST):
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``` shell
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$ make fuses
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```
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## TWI/I2C Protocol ##
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A TWI/I2C master can use the following protocol for accessing the bootloader.
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Function | TWI/I2C data | Comment
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--- | --- | ---
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Abort boot timeout | **SLA+W**, 0x00, **STO** |
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Show bootloader version | **SLA+W**, 0x01, **SLA+R**, {16 bytes}, **STO** | ASCII, not null terminated
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Start application | **SLA+W**, 0x01, 0x80, **STO** |
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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
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Read 1+ flash bytes | **SLA+W**, 0x02, 0x01, addrh, addrl, **SLA+R**, {* bytes}, **STO** |
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Read 1+ eeprom bytes | **SLA+W**, 0x02, 0x02, addrh, addrl, **SLA+R**, {* bytes}, **STO** |
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Write one flash page | **SLA+W**, 0x02, 0x01, addrh, addrl, {* bytes}, **STO** | page size as indicated in chip info
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Write 1+ eeprom bytes | **SLA+W**, 0x02, 0x02, addrh, addrl, {* bytes}, **STO** |
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**SLA+R** means Start Condition, Slave Address, Read Access
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**SLA+W** means Start Condition, Slave Address, Write Access
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**STO** means Stop Condition
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The multiboot_tool repository contains a simple linux application that uses
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this protocol to access the bootloader over linux i2c device.
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## Development ##
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Issue reports, feature requests, patches or simply success stories are much appreciated.
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## Roadmap ##
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Some ideas that I want to investigate / implement in twiboot:
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- [ ] do not use interrupts and remove vector table to save flash space
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- [ ] find a way to not rely on TWI clock stretching during write access
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- [ ] support AVR TINYs (USI peripheral, no bootloader fuse, no Read-While-Write flash)
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