Introduction
Our current flight computer is a PowerPC-based single board computer, specifically chosen because of a "Linux on POWER" grant from IBM that Dr. Massey received a few years back.
We chose the TQ Components TQM5200 as our SBC because of it's neato feature list:
- 400 MHz Freescale MPC5200 PowerPC (that's a 603e core)
- 64 MB SDRAM / 32 MB FLASH
- Lots of peripherals: USB 1.1, CAN, UARTS, PCI, ATA/IDE, SPI, etc.
The amazing part is that the board is tiny: it's 80 x 60 mm. That's also it's down side: the way it's so small is that it has an array of high density surfacemount connectors on the bottom of the board to mount to a "carrier board" which adapts it to whatever system it's in.
TQ Components makes the STK5200, a giant, multi-purpose carrier board that breaks out the TQM5200 into a bzillion different connectors... in fact, it basically makes it into a PC motherboard, with VGA, USB, keyboard and mouse, etc.
That's good for prototyping, but it's too big and fragile for LV2.
Requirements
This is an initial requirements list, and should be reviewed before any serious design work is done.
- General requirements
- SHOULD cost (,100,200) $
- SHOULD be a (,2,4) layer PCB
- Size
- MUST be fit in the rocket (ID of ~ 133.3 mm)
- SHOULD be as small as possible, (48,100,150) cm^2
- MAY fit horizontally: 60 x 80 mm has a hypotenuse of 100 mm; however carrier board couldn't be much bigger and would be difficult to add components.
- MAY fit vertically: Probably better, with the 80 mm side going up/down. Carrier board could then be taller, say 150mm, and have components above and below the FC, as well as on the back side.
- Connectors
- MUST use standard "host" Rocketbus USB connector (See Capstone 2009 connector design: provides PWR, USB, SERIAL, CAN)
- MAY have other connectors as necessary:
- VGA out to ATV?
- Another host USB connector to go to the 802.11a card? Probably not, that should be its own rocketbus connector.
- Standard USB host connector for debug/? Probably not, that'll be a rocketbus to standard cable adapter.
- Data storage
- SHOULD have all onboard data storage: CF card or other SSD using ATA/IDE interface
- Power Supply
- MUST be low power at (,1.5,3) W
- MUST have its own switching power supply (Vbus to whatever supply voltage it needs: 3.3V? 5.0V?)
- SHOULD use the Capstone 2009-designed power supply without the battery-backed feature.
- TODO: more requirements!
Block Diagram(s)
Here's a block diagram to consider:
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- Advantages
- Simplest possible carrier board: it can be built quickly.
- Single connector to the rest of the rocket.
- 802.11a adapter now part of a "communication node", which has a regular RocketBus interface.
- Disadvantages
- Need to power up a different node to get wifi, and how do you switch the main power amp on and off?
- 802.11a on the same USB host controller as the rest of the nodes. Not terrible, but not great either.
Here's a slightly different one version:
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In this version, we put the 802.11a adapter on the carrier board. The disadvantage is you now need a 5V supply, an RF connector, and now you have to somehow mount the USB to 802.11a adapter on the board. But now at least you'll have a weak wifi signal when the FC is powered up, and the power amplifier becomes a separate node you can turn on or off.
- Advantages
- 802.11a link now always available to FC when the FC is powered on.
- Uses second USB host controller on the TQM5200, which saves bandwidth on the node hub.
- Disadvantages
- More complicated carrier board.
- Now there's a big coax snaking between the FC and the communication node, and an added connector.