Main Flight Computer

Main Flight Computer

The Main Flight Computer is a modularized computer design with very little in the way of function-specific design. With 8 connectors supplying 3 voltages, I2C data, and a GPIO line, this flight computer can very easily become a controller for just about any application requiring medium-scale hardware integration and modularity, at the slight expense of weight. The Main CPU is an Atemega 324P, although any Atmega Chips in the **4P line will work just as well. On board sensors include temperature, TXCO Real time clock, and Absolute barometric pressure. I2C connectors 1 through 8 provide 12v, 5v, 3.3v, I2C data, and a GPIO, while I2C connectors 9 and 10 provide 12V, and boosted I2C lines for extended runs of I2C data. These connectors are completely non-specific to any particular module, thus any module may be plugged into any connector (with respect to connectors 9 and 10, whose form-factor differs). The connectors are nominally Molex .1” Friction-fit headers, but their standard layout ensures that any .1” or 2.5mm connector may be used without issue.

Bill of Materials

Part	Package	Quantity	Notes
Atmega 324P	TQFP44	1
DS3231	SO16 Wide	1	Optional (If not included, do not connect battery backup or decoupling capacitors)
TMP100	SOT23	1	Optional (If not included, do not connect decoupling capacitor)
BMP085	Annoying and difficult leadless package	1	Optional. Only part which required hot-air to solder (Unless you are a soldering messiah).
P82B96	TSSOP	2	Required for I2C boost to sensor strings
P-Channel MOSFET	SOT23-3	2	Required to kill power to sensor strings. Use logic level MOSFETs.
N-Channel MOSFET	SOT23-3	2	Required to kill power to sensor strings. Use logic level MOSFETs.
LEDs	1206	3	For some debug IO.
Tactile Switch	Standard	1	Reset
.1” Breakaway Headers		6-42	Only solder in what you need.
12MHz Resonator	3.20mm x 1.30mm	1	Board runs at a max of 12MHz. Good luck finding a correctly sized resonator (Digikey 490-1197-1-ND seems to work).
Molex friction-fit headers (Molex PN: 22232081)		10	Arrow seems to have these in abundant stock, and cheap. Digikey stocks them, but at inflated prices.)
100nF Capacitors	0804	7	C1-C7, these decouple everything (4 for CPU, 1 for BMP085, 1 for TMP100, 1 for DS3231).
4.7k Resistor	0804	7	R1-3, used to pull reset, SDA and SCL high. R21-R24 used for I2C boost.
220 ohm Resistor	0804	3	Used for LEDs.
10k Ohm Resistor	0804	3	Used in power disconnect circuitry.
Anything > 1k Resistor	0804	2	R4 and R5 used to pull unused connections on DS3231 high. Use anything bigger than 1k.

Connector Details

If there's one thing we've learned on this project, it's that connectors are a real pain. After a lot of searching, the group settled on a derivative of the connectors used in the SNOX flights. Furthermore, any .1” or 2.5mm connector should fit in the space alloted, so there is some flexibility in choosing this time around. The specced connectors are as follows:

Board Connectors

Located on the Balloon CPU and any submodules. Molex PN: 22232081, available from Arrow and Digikey.

Wire Housings

Used to interconnect modules. Molex PN: 22013087, available from Arrow and Digikey.

Crimp Connectors

Used to attach wire to wire housings. Molex PN: 08520123, available from Arrow and Digikey. Success in criming these connector can be had by carefully using a D-SUB crimper (as available from Radioshack for $10), and soldering the wire for extra stability.