megas2 is an ATMEGA32 full system emulator designed primarily with simplicity and hackability in mind. It is intended to help me with my various ATMEGA projects whenever they:

• use sophisticated or ad-hoc peripherals that are not available in mainstream simulation packages (e.g. Ethernet chips, or sensors connected to a complex process)
• have such complex code that it requires numerous testing cycles that would be time-consuming and wear out the Flash memory of the chip

It cannot be stressed enough that megas2 is intentionally designed to favor simplicity over (often unnecessary) accuracy. As such, it is not cycle-exact, communication over SPI/I2C is instantaneous, etc. (a more complete list of limitations can be found later in this document). The emulation is only as accurate as is stricly needed for moderately complex projects such as my own.

Performance is not a big priority either. Even though simulating a machine that is literally 100 times slower than the host is not an excuse to be careless, I would rather avoid premature optimization in this case. Should the emulation turn out to be too slow (which I doubt), only then will I consider a rethinking and re-engineering process to be in order.

Last but not least, megas2 is also a nice exercise for stretching those programming muscles in a new direction. It is not my first emulator writing experience, and I know it can be quite fun and ultimately educational.

megas2 is a personal project, and, though I would certainly be glad for it to be of use to anyone else, I would certainly not recommend it before trying these two alternatives first:

• simavr is the newest kid on the block and it seems to be developing into a highly professional and reliable solution for your ATMEGA/AVR simulation needs. Unfortunately, it lack support for the very ATMEGA variants I am interested in.
• simulavr seems fairly complete, accurate, and hackable. That said, I wasn't able to get it to work properly on my system, and I wasn't satisfied with the interfaces it provides for you to attach your own simulated devices.

Both of these solutions are also far, far more complex than is generally needed for simple projects, and this can be an impediment to the hackability that is inevitably required in these situations.

• Optional but highly recommended: the uml-utils package for the tunctl command, useful in setting up a virtual network.

The following is an inventory of the emulation limitations in megas2.

• Emulation is not cycle-exact (all instructions take 1 cycle)
• FMUL* instructions are not supported
• SPM instructions are not supported
• SLEEP, BREAK, WDR instructions are not supported (they are treated as NOP)

• IVSEL, BOOTRST options not supported

• Timers 0 and 2 not supported
• Partial support for Timer 1:
  • Only 0, 4 and 12 WGMs supported
  • COM* and FOC* not supported
  • Input Capture not supported
  • External clock sources not supported

• Auto Trigger Modes not supported
• All conversions take 15 ADC clock cycles

• Only master transmitter/receiver behavior supported
• Operations complete instantaneously and always succeed
• SPI bandwidth settings are ignored
• Data order, clock polarity and clock phase options not supported
• Interrupts not supported

• Only master behavior supported
• Operations complete instantaneously and always succeed
• TWI bandwidth settings are ignored
• Write collision detection not supported
• Interrupts not supported

• Not supported

• Only these commands are supported:
  • GO_IDLE_STATE
  • SET_BLOCKLEN
  • READ_SINGLE_BLOCK
  • WRITE_SINGLE_BLOCK
• Operations complete instantaneously
• Only 512-byte blocks are supported

• Stub available, development in progress

• Basic send/receive is supported
• Transmission is instantaneous and network errors never occur
• DMA is not suppoted
• Interrupts are not supported
• Hash Table, Magic Packet, Pattern Match receive filters are not supported
• Complex reset logic is not supported

• Simple support for tri-state and directionality
• Virtual network emulation