What's Inside the Mars Rovers

Captain Zion writes "Space.com has a story about the hardware and software of Mars Rovers Spirit and Opportunity. Basically, they're radiation-shielded, 20MHz PowerPC machines wirh 128Mb RAM and 256Mb of flash memory, running VxWorks. I wonder if I could make a nice firewall with one of these for my home network..."

Comments

Re:Radiation Shielding

[the real darkskye]

The CPU is fabricated to withstand the radiation, a brief summary can be found here or by googling

Flying VxWorks to Mars

[Hiroto. S]

I googled across following presentation with a little more details.

Flying VxWorks to Mars

Re:Redundency Check?

[vofka]

If I recall correctly, the Shuttle has 5 GPC's (General Purpose Computers), three of which are "online" at any one time.

The online GPC's each carry out the same set of calculations (potentially each uses code designed to do the same thing, but written by different programmers), and they compare each others results. If any single GPC is considered to be too far wrong, the offline GPC's submit their answers. The three GPC's that are in closest agreement then become the new online GPC's, and the remaining two go offline. The GPC's can reboot themselves if they are too far out of whack, if they fail in one of the "results elections", and of course when they are told to do so by the crew.

Also, whenever a GPC is sent offline by one of the others, a specific caution indicator (and potentially the master caution indicator and klaxon) is activated, and the relevant error codes are shown on one of the forward CRT's. The error codes, along with other information such as the currently running program and the current mission phase, determine the crew's actions. Actions can be as simple as disabling the master caution klaxon for the current alert, all the way to hand-checking certain results and manual GPC restarts.

This is all from memory (from about 5 years back), so some of this may have changed recently, particularly on Atlantis with the "glass cockpit" upgrade that happened 18 months or so ago, but the general gist should be about right (and I'm sure I'll soon know if it isn't!!)

Re:Wait a second...

[Rootbear]

Actually they are running at 20MHz. I've seen several write ups which clearly state that. The RAD6000 can apparently run at up to 33MHz, with a claimed 35MIPS. The rovers are "underclocked", probably due to power budget concerns.

Go to
http://www.iews.na.baesystems.com/space/rad600 0/ra d6000.html
and click on the rover picture to get a PDF brochure, which gives the 33MHz/35MIPS figure.

Rootbear

Processor is *not* a PowerPC

[GileadGreene]

radiation-shielded, 20MHz PowerPC machines

No, they're not.

The processors in MER are RAD6000's, which are radiation-hardened versions of the RS/6000, the predecessor to the PowerPC (see this for details). The RAD6000's younger brother, the RAD750, is indeed a rad-hardened PowerPC.

As an aside, there is a big difference between a radiation-shielded processor and a radiation-hardened processor. Shielding implies just sticking some kind of rad-absorbent material between the processor and the environment. A rad-hardened processor is actually manufactured in a different way - different gate layout, different design rules, often different materials (Silicon-on-Insulator is popular). These things are done to minimize or prevent the effects of single-event upsets (when a bit is flipped by high-energy particles) and single-event latchups (which basically turn a couple of gates into a glorified short-to-ground). The materials changes may also improve the overall total dose tolerance of the processor. The work required for redesign is one of the reasons that space-qualified rad-hard processors lag the commercial market. The NASA Office of Logic Design has some good papers on space processors available online if you're interested in learning more.

RAD6K

[Anonymous Coward]

I am an engineer that works with the RAD6K processor boards. A couple of observations here.

1. The RAD6K really does run at 20 Mhz. They're creakingly slow. They're spec'd to run up to 33 Mhz, but the customer can get them to clock at lower speeds (I've seen them run at 12.5 Mhz). The only drawback is the PCI bus is also clocked as the same speed as the CPU. This is a mixed bag - but a slower PCI bus helps improves signal integrity and decreases power consumption.
2. The board is PCI, but NOT compact PCI. There is a proprietary PCI connector and a proprietary PCI backplane. You cannot plug commercial PCI products unless you have an adapter to interface to the proprietary PCI connectors.
3. For those who are not aware, there are three types of memory being used on the rovers. There is the SRAM (the RAD6K boards use SRAMs, not DRAMs), the EEPROM, and apparently, FLASH RAM. The EEPROM and the SRAM are on the processor board itself - there is probably more EEPROM memory in the system on another board. The EEPROM usually holds the flight code, and there are usually two copies. An original version that was launched with the spacecraft, and one patched version made possible via uplinks.
   
4. I am amazed at the presence of FLASH RAM's. I am not aware of any rad-hardened FLASH RAM devices for spaceflight use. In addition to radiation hardness, the device must be made reliable with an approved QML-Q or V manufacturing flow. Radiation hardness is just icing on the cake, but the key is that the device must be reliable to withstand temperature extremes, shock and vibration. So, I have yet to see a FLASH RAM device that can be used. I am aware of the Chalcogenide based RAM's which are essientially uses the same substrates on CD-ROMs as memory cells. These products are hard to come by right now and are a high risk because we don't have sufficient data and flight heritage. A catch-22 in flight design is if it hasn't flown before, we don't want to fly it. But at some point, someone has to fly the first generation (someone who is willing to take a huge risk). Anyway, the FLASH RAM's on the rovers are in all likelihood upscreened commercial products. In other words, a mass buy of an entire lot of dies of commercial FLASH RAM's may have been bought, packaged in-house or through a vendor, and then screened for reliablity at extended specifications. This is not the same as the manufacturer who can guarantee the specs by design by designing it from the outset with increased reliability in mind.
5. Radiation shielding? Minimal at best! The RAD6K shields its SRAMs by placing it on the underside of the processor board and orienting it such that the particles hit the processor side of the board instead of the RAM side of the board. There is some degree of radiation shielding for particles of sufficiently low energy. The truly high speed particles are going to get through and the only thing that will truly stop them is shielding whose thickness is measured in feet. That amount of shielding is too heavy for launch. The best we can do is mitigate the effects of radiation by guaranteeing devices can withstand a certain amount of radation dosage (measured in kRads) and design for latchup protection (latchup is a parasitic condition in which an ionizing particle impacts a transistor structure in a way that causes a SCR to be formed and a runaway current condition is initated leading to the device being burned out by high currents). Radiation effects in the form of SEE's (single event effects) such as bit flips can be mitigated by redundancy and voting circuits, memory scrubbing, and error checking using checksums/CRC's.