Hi, guys 
Recently I wanna design a new autopilot for my roBoat(most of all for fun)
Half a year ago I used Arduino Mini to set up a simple one, with two pieces of 74LS157 too
but these days when I look schematic of arduilot I found a ATtiny12 on board, and I'm not quite sure what's the use of that. If anyone knows that please drop a comment below 
Thanks in advance 
Which version of ardupilot?
At one time I know they included an ATTiny as a watchdog. It would reset the main chip if a signal pulse from the primary processor stopped. So if you see a line from the ATTiny leading to the reset pin of the main chip, well, there you go.
Hi, it's a basic version of ArduPilot, here goes on SparkFun: https://www.sparkfun.com/products/8785
Thank you I notice the XRST connected to reset pin on ATmega 328 through a diode, and TMISO connected to D4
At first I thought it was used to lock the waypoints or something, lol
Thanks again for your nice answer
Plus I also wanna know which bootloader suits arduPilot? Will the the bootloader for Arduino Mini work?