5 ms·

Quite likely a radiation hardened variant, e.g.: https://en.wikipedia.org/wiki/RAD750 https://en.wikipedia.org/wiki/RAD750 The CPUs used in space applications

by ch_123 6y ago

Quite likely a radiation hardened variant, e.g.: https://en.wikipedia.org/wiki/RAD750 https://en.wikipedia.org/wiki/RAD750 The CPUs used in space applications typically lag ~20 years behind the consumer market due to the need to be very sure that the kinks have been ironed out, and the need to produce variants that can withstand the environmental conditions outside earth's atmosphere. EDIT: Also worth pointing out - the price of one of these specialized chips is in the six digits, due to the higher tolerances and low production volumes.

plutonorm 6y ago
Crazy. Just put 10 complete systems in some radiation shielding.
- iancmceachern 6y ago
  Too heavy
- myrandomcomment 6y ago
  Which is more weight. The single highest factor in all space design is cost per kg lifted. The only thing that is more costly - lifting it successfully and then having it break.
  - wlesieutre 6y ago
    For a rover it's not just lifting it, you have to put it back down once it gets to Mars. That's a 3-stage process with the aeroshell, parachute, and powered descent. Heavier rover? You'll need more fuel for the skycrane's engines. And that fuel adds weight, so don't forget that you'll need more fuel to lift the extra fuel. Now your rover and skycrane are bigger and heavier. Do you need a bigger parachute? Does everything still fit in the aeroshell? Is the fuel capacity for orienting and stabilizing the aeroshell still sufficient with the increased mass?
  - plutonorm 6y ago
    The board in my phone is less than 100g. That's 1kg for ten of them.
    - colejohnson66 6y ago
      You’re assuming these computers are the size of a mobile phone
- scottLobster 6y ago
  We're talking launching stuff into space, every kg of extra weight is critical, particularly for scientific missions that are trying to do as much as possible. Also no amount of radiation shielding is going to make you perfectly shielded from cosmic rays. It's actually a chronic problem with Voyagers 1/2. The teams that maintain them regularly experience weird, unplanned behavior because a cosmic ray bit-flipped some register or other, and then they have to manually figure out what erroneous command got executed and how to recover from it, and those are extremely simple systems compared to what's running on Perseverance. So yeah, you need radiation hardened parts in space or you're just begging for a dead probe.
  - pklausler 6y ago
    There are three computer systems on each Voyager. They are dedicated to specific tasks (Flight Control System, Attitude and Articulation Control System, and Computer Command Subsystem), each with two instances for redundancy.
    - scottLobster 6y ago
      Right, but that redundancy to my understanding is in case of component/system failure. It's a backup, not a judge-actor or error-correcting architecture. Case in point: https://www.jpl.nasa.gov/news/engineers-diagnosing-voyager-2-data-system-update https://www.jpl.nasa.gov/news/engineers-diagnosing-voyager-2... "Engineers successfully reset a computer onboard Voyager 2 that caused an unexpected data pattern shift, and the spacecraft resumed sending properly formatted science data back to Earth on Sunday, May 23. Mission managers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., had been operating the spacecraft in engineering mode since May 6. They took this action as they traced the source of the pattern shift to the flip of a single bit in the flight data system computer that packages data to transmit back to Earth. In the next week, engineers will be checking the science data with Voyager team scientists to make sure instruments onboard the spacecraft are processing data correctly. "
- retSava 6y ago
  It's not just the weight, there's also the complexity of handling 10 (or whatever) systems. It's not always easy to know when a system fails. How can you be sure that the output is invalid? Detecting complete blackouts is fairly easy, after a while when a watchdog tickle is absent for too long, but bad output due to flipped memory? Consensus-style systems can be used, but they are also a complexity in itself, and the point here was to replace a hardened single point of failure with redundancy.
- moftz 6y ago
  There is also a time-domain problem with regards to total integrated dose. All the parts will degrade over time. Radiation hardened parts can have lives of 15 years or more in a GEO environment. A commercial part might be lucky to make it 1 year. Radiation shielding is not as easy as you may think. Heavy ions in space can pass through anything and will impart a charge in whatever they hit as well as creating secondary sources of radiation. Neutrons are not as common in space but can be found near reactors, they are also very penetrating.
- Rebelgecko 6y ago
  Not all radiation problems can be solved by rebooting/voting/watchdogging. Over time the hardware itself will die.
DrBazza 6y ago
> The CPUs used in space applications typically lag ~20 years This. Worked in physics department in the 90s that built satellites, and all the chips were late 70s or early 80s chips. Programming them was "fun".
jvanderbot 6y ago
Fun fact, a friend of mine bought and old mac, installed linux, and used it to do profiling for some code he wanted to fly. Great way to quickly find the problem areas! Those old macs run Ubuntu just fine :)
- timw4mail 6y ago
  An old version anyway. I think 2014.4 or around there was the last PPC version of ubuntu.
  - trasz 6y ago
    Up-to-date FreeBSD should run just fine, though.
    - deleted 6y ago
      [deleted]
    - boogies 6y ago
      Or more Ubuntu-like, Debian (in fact IIUC they just recently added support for ppc64, as it’s only in unstable while ppc64el is in stable).
quercusa 6y ago
Once you have a rad-hard CPU, what do you do for the RAM, some super ECC?
- kevin_thibedeau 6y ago
  There are memories available in shielded packages. SRAM is used wherever possible. Some of the rad hard micros have on board ECC, others don't and you'd have to spin your own with an ASIC or FPGA or have another mitigation strategy.
ooOOoo 6y ago
RAD750 usage is confirmed in https://mars.nasa.gov/mars2020/spacecraft/rover/brains/ https://mars.nasa.gov/mars2020/spacecraft/rover/brains/
avianlyric 6y ago
Also worth mentioning that chips from 20 years ago are built on old fabrication technology, with ginormous transistor sizes (compared to modern CPUs). The RAD750 has a 150nm to 250nm minimum feature sizes, a Ryzen Zen 3 chip has a feature size of 7nm, two orders of magnitude smaller! Feature size is super important in radiation exposed environments because resistance to bit flips from a stray proton (or other charged particles flying around in space) is inversely proportional to the size of your transistors. So small transistors mean they’re more likely to be activated by stray charged particles causing all kinds of interesting and exciting problems (which you don’t want to be debugging from one end of a 12 light minute comms channel).
- mlyle 6y ago
  > The RAD750 has a 150nm to 250nm minimum feature sizes, a Ryzen Zen 3 chip has a feature size of 7nm, Yup. And while the transistor sizes haven't really shrunk by the 35x that this suggests, they've still shrunk by a lot. And, of course, the area has shrunk by this amount squared. > causing all kinds of interesting and exciting problems Sometimes super exciting. Latchup can turn the whole package into a white-hot parasitic transistor connecting Vcore to ground at low impedance. Rad-hard variants can largely eliminate this possibility.
  - wtallis 6y ago
    If we compare the RAD750 against the 5nm Apple A1, the newer chip has about 1660 times as many transistors per mm^2. Minimum feature size (ie. line widths) may not have kept pace with the nanometer-based fab node names, but transistor sizes definitely have.
    - mlyle 6y ago
      Eh. Don't compare the RAD 750. It's better to compare something that's not domain specific and rad hard. Compare the PPC 740, which was ~300,000 transistors/mm^2 at 260nm. Apple M1 is ~130,000,000 transistors/mm^2 at 5nm. (260/5)^2 =~ 2700x, but the actual difference is ~430x. So it's not as bad as line widths, but it still isn't quite keeping pace.
wmf 6y ago
If RAD750 came out in 2001 that means it was only ~5 years behind. Which raises the question of why isn't there a RAD-A72 now.
Demonsult 6y ago
Many of my clients in aerospace are still designing in PPC parts in 2021.
foobarian 6y ago
Aren't these Motorola-lineage architectures specifically good at doing lockstep computation? I.e. there are a bunch processors executing identical instructions and comparing the outputs?
- mech422 6y ago
  The moto 68K was used in this fashion in the Stratus Super Mini's.

by ch_123 6y ago

plutonorm 6y ago

Crazy. Just put 10 complete systems in some radiation shielding.

iancmceachern 6y ago
Too heavy
myrandomcomment 6y ago
Which is more weight. The single highest factor in all space design is cost per kg lifted. The only thing that is more costly - lifting it successfully and then having it break.
- wlesieutre 6y ago
  For a rover it's not just lifting it, you have to put it back down once it gets to Mars. That's a 3-stage process with the aeroshell, parachute, and powered descent. Heavier rover? You'll need more fuel for the skycrane's engines. And that fuel adds weight, so don't forget that you'll need more fuel to lift the extra fuel. Now your rover and skycrane are bigger and heavier. Do you need a bigger parachute? Does everything still fit in the aeroshell? Is the fuel capacity for orienting and stabilizing the aeroshell still sufficient with the increased mass?
- plutonorm 6y ago
  The board in my phone is less than 100g. That's 1kg for ten of them.
  - colejohnson66 6y ago
    You’re assuming these computers are the size of a mobile phone
scottLobster 6y ago
We're talking launching stuff into space, every kg of extra weight is critical, particularly for scientific missions that are trying to do as much as possible. Also no amount of radiation shielding is going to make you perfectly shielded from cosmic rays. It's actually a chronic problem with Voyagers 1/2. The teams that maintain them regularly experience weird, unplanned behavior because a cosmic ray bit-flipped some register or other, and then they have to manually figure out what erroneous command got executed and how to recover from it, and those are extremely simple systems compared to what's running on Perseverance. So yeah, you need radiation hardened parts in space or you're just begging for a dead probe.
- pklausler 6y ago
  There are three computer systems on each Voyager. They are dedicated to specific tasks (Flight Control System, Attitude and Articulation Control System, and Computer Command Subsystem), each with two instances for redundancy.
  - scottLobster 6y ago
    Right, but that redundancy to my understanding is in case of component/system failure. It's a backup, not a judge-actor or error-correcting architecture. Case in point: https://www.jpl.nasa.gov/news/engineers-diagnosing-voyager-2-data-system-update https://www.jpl.nasa.gov/news/engineers-diagnosing-voyager-2... "Engineers successfully reset a computer onboard Voyager 2 that caused an unexpected data pattern shift, and the spacecraft resumed sending properly formatted science data back to Earth on Sunday, May 23. Mission managers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., had been operating the spacecraft in engineering mode since May 6. They took this action as they traced the source of the pattern shift to the flip of a single bit in the flight data system computer that packages data to transmit back to Earth. In the next week, engineers will be checking the science data with Voyager team scientists to make sure instruments onboard the spacecraft are processing data correctly. "
retSava 6y ago
It's not just the weight, there's also the complexity of handling 10 (or whatever) systems. It's not always easy to know when a system fails. How can you be sure that the output is invalid? Detecting complete blackouts is fairly easy, after a while when a watchdog tickle is absent for too long, but bad output due to flipped memory? Consensus-style systems can be used, but they are also a complexity in itself, and the point here was to replace a hardened single point of failure with redundancy.
moftz 6y ago
There is also a time-domain problem with regards to total integrated dose. All the parts will degrade over time. Radiation hardened parts can have lives of 15 years or more in a GEO environment. A commercial part might be lucky to make it 1 year. Radiation shielding is not as easy as you may think. Heavy ions in space can pass through anything and will impart a charge in whatever they hit as well as creating secondary sources of radiation. Neutrons are not as common in space but can be found near reactors, they are also very penetrating.
Rebelgecko 6y ago
Not all radiation problems can be solved by rebooting/voting/watchdogging. Over time the hardware itself will die.

DrBazza 6y ago

> The CPUs used in space applications typically lag ~20 years This. Worked in physics department in the 90s that built satellites, and all the chips were late 70s or early 80s chips. Programming them was "fun".

jvanderbot 6y ago

Fun fact, a friend of mine bought and old mac, installed linux, and used it to do profiling for some code he wanted to fly. Great way to quickly find the problem areas! Those old macs run Ubuntu just fine :)

timw4mail 6y ago
An old version anyway. I think 2014.4 or around there was the last PPC version of ubuntu.
- trasz 6y ago
  Up-to-date FreeBSD should run just fine, though.
  - deleted 6y ago
    [deleted]
  - boogies 6y ago
    Or more Ubuntu-like, Debian (in fact IIUC they just recently added support for ppc64, as it’s only in unstable while ppc64el is in stable).

quercusa 6y ago

Once you have a rad-hard CPU, what do you do for the RAM, some super ECC?

kevin_thibedeau 6y ago
There are memories available in shielded packages. SRAM is used wherever possible. Some of the rad hard micros have on board ECC, others don't and you'd have to spin your own with an ASIC or FPGA or have another mitigation strategy.

ooOOoo 6y ago

RAD750 usage is confirmed in https://mars.nasa.gov/mars2020/spacecraft/rover/brains/ https://mars.nasa.gov/mars2020/spacecraft/rover/brains/

avianlyric 6y ago

Also worth mentioning that chips from 20 years ago are built on old fabrication technology, with ginormous transistor sizes (compared to modern CPUs). The RAD750 has a 150nm to 250nm minimum feature sizes, a Ryzen Zen 3 chip has a feature size of 7nm, two orders of magnitude smaller! Feature size is super important in radiation exposed environments because resistance to bit flips from a stray proton (or other charged particles flying around in space) is inversely proportional to the size of your transistors. So small transistors mean they’re more likely to be activated by stray charged particles causing all kinds of interesting and exciting problems (which you don’t want to be debugging from one end of a 12 light minute comms channel).

mlyle 6y ago
> The RAD750 has a 150nm to 250nm minimum feature sizes, a Ryzen Zen 3 chip has a feature size of 7nm, Yup. And while the transistor sizes haven't really shrunk by the 35x that this suggests, they've still shrunk by a lot. And, of course, the area has shrunk by this amount squared. > causing all kinds of interesting and exciting problems Sometimes super exciting. Latchup can turn the whole package into a white-hot parasitic transistor connecting Vcore to ground at low impedance. Rad-hard variants can largely eliminate this possibility.
- wtallis 6y ago
  If we compare the RAD750 against the 5nm Apple A1, the newer chip has about 1660 times as many transistors per mm^2. Minimum feature size (ie. line widths) may not have kept pace with the nanometer-based fab node names, but transistor sizes definitely have.
  - mlyle 6y ago
    Eh. Don't compare the RAD 750. It's better to compare something that's not domain specific and rad hard. Compare the PPC 740, which was ~300,000 transistors/mm^2 at 260nm. Apple M1 is ~130,000,000 transistors/mm^2 at 5nm. (260/5)^2 =~ 2700x, but the actual difference is ~430x. So it's not as bad as line widths, but it still isn't quite keeping pace.

wmf 6y ago

If RAD750 came out in 2001 that means it was only ~5 years behind. Which raises the question of why isn't there a RAD-A72 now.

Demonsult 6y ago

Many of my clients in aerospace are still designing in PPC parts in 2021.

foobarian 6y ago

Aren't these Motorola-lineage architectures specifically good at doing lockstep computation? I.e. there are a bunch processors executing identical instructions and comparing the outputs?

mech422 6y ago
The moto 68K was used in this fashion in the Stratus Super Mini's.