The XBeeBoot bootloader provides XBee Series 2 Over-The-Air firmware update capability to Atmel AVR devices, as well as supporting direct firmware update via the standard Optiboot protocol.

It provides the following features:

• Bootloader fits within 1kB: This is larger than [Optiboot's] (https://github.com/Optiboot/optiboot) smallest build, but is still half the size of a standard Arduino bootloader.

• Reasonably fast Over-The-Air firmware updates: Less than three minutes to write a 20kB firmware image.

• Natively supports XBee API-mode protocol: No need to use AT-mode XBee firmware.

• Over-The-Air updates are secure: XBee security models apply - so firmware updates are encrypted and authenticated with AES encryption if the ZigBee network is configured with encryption.

• Over-The-Air updates are robust: The protocol uses direct addressing, so the other devices on the ZigBee network are unaffected. You could even Over-The-Air update firmware on multiple devices concurrently.

• Full access to Optiboot-supported bootloader facilities with a direct connection via standard [avrdude] (http:https://www.nongnu.org/avrdude/) software: This bootloader detects an attempt to program via the standard Arduino/avrdude firmware update protocol and automatically switches to the standard Optiboot protocol.

• Full access to Optiboot-supported bootloader facilities Over-The-Air via [avrdude] (http:https://www.nongnu.org/avrdude/) version 6.4 and later.

The Atmega device needs to be able to communicate serially with the XBee. This requires connecting the XBee DOUT (pin 2) to the Atmega RXD (Atmega328P pin 2), and connecting the XBee DIN (pin 3) to the Atmega TXD (Atmega328P pin 3). You've probably already done that.

Apart from the serial link, there is only one additional connection required: We need a mechanism to hard-reset the Atmega to enter the bootloader. This is supported by connecting the XBee DIO3 pin (pin 17) to the Atmega RESET pin (Atmega328P pin 1). Optionally, a 0.1uF capacitor can be included in the reset pin connection - this isn't mandatory, but including the capacitor does eliminate the possibility of the XBee accidentally holding the Atmega in a permanent state of reset.

This is intentionally identical circuitry to [SparkFun's tutorial] (https://www.sparkfun.com/tutorials/122) for use with XBee Series 1 devices.

No. In particular, it doesn't matter if the coordinator node is a separate and unrelated node. Any XBee address can bootload to any other ZigBee-networked XBee-hosted AVR device, so long as they share encryption keys.

Yes it does, although it is of course a little slower getting going if the remote device is sleeping. Once it is going it appears to run something like 25% slower than a normal router mode device, E.g. a little over three minutes for a 20kB update, rather than a little under three minutes.

Not intentionally. Because the AT firmware isn't really usable in any environment with more than two XBee devices, I haven't looked at AT mode very carefully.

There are no known limits. I currently have eight active XBee devices on the same ZigBee network, meshed over some fairly complicated terrain, with a mix of router and endpoint devices, and can update the firmware successfully and reliably across the network.

Yes it can. In two ways, in fact:

1. You can still use it as if it were a normal Optiboot/Arduino bootloader, with the caveat that the default serial baud rate is 9600 baud, to match the XBee default.

2. You can also bootload, with the avrdude xbee programmer directly, by not giving a ZigBee address. This is useful for diagnosis and proof of concept testing. It may also be useful for bootloading over unreliable serial links (E.g. AT mode XBee links), as the XBee bootloader protocol includes checksums, is packet-based and can recover from lost or corrupted packets in the serial data stream.

• XBeeBoot uses the hardware watchdog to guarantee that the bootloader doesn't hang the chip, and does eventually exit. The hardware watchdog time-out can be set to a maximum of eight seconds on AVR devices. A time-out of eight seconds is a little on the short side for Over-The-Air updates, so if your mesh network is prone to packet loss it may be necessary to restart the firmware update because the eight second time-out was tripped.

• XBee traffic on an encrypted network is already digitally authenticated via AES encryption, which is "pretty good". Realistically, standard XBee security will normally be good enough. Perfection would be if XBeeBoot supported an extra level of digital signature on firmware updates as an additional security measure. But then XBeeBoot wouldn't fit into 1kB.

• By necessity, XBeeBoot needs to be talked to from programmer software that also knows how to talk to an XBee device in API mode. These features are available as standard in [avrdude] (http:https://www.nongnu.org/avrdude/) version 6.4 and later.

XBeeBoot is an amalgamation of an Over-The-Air protocol by David Sainty, and a core bootloader based directly on Optiboot.

Optiboot builds on the original work of Jason P. Kyle (stk500boot.c), Arduino group (bootloader), Spiff (1K bootloader), AVR-Libc group, Ladyada (Adaboot), Peter Knight (aka Cathedrow) and [Bill Westfield (aka WestfW)] (https://github.com/Optiboot/optiboot).