RAD750Mohsin Farooq

Nicolas Schrading

Overview• Single board computer and processor• Radiation hardened up to 10,000 gray• 2.5V, 250nm, 6-layer CMOS technology• Exact same Superscalar RISC as PowerPC 750• 110 - 200MHz• Split 32KB L1 Cache, 256KB - 1MB L2 Cache

Companies InvolvedDesigned by: Manufactured by:

Used by:

Cost

~$200,000 per CPU!

Why?•Radiation: huge problem!

o Testing takes much longer o It has to be 100% reliable

Space Problems

• Extremely high reliability.• Need for long product lifetime.• Thermal Management.• Electromagnetic compatibility.• Large amount of radiation!

All must be done while minimizing cost!

The Problem of Radiation

Van Allen Radiation Belt Solar Flares and Galactic Cosmic Radiation

Electrons up to 10 MeV, protons up to 100s MeV

Several GeVs

Most energetic observed: 300EeV!

- Anything > 100MeV harmful to microlectronics

The Problem of Radiation• Single Event Upsets (SEUs) caused by high-energy particles

striking sensitive nodes in microelectronic device.o Cause unwanted state changes in memory/registers.o Cause pulses in logic or support circuitry.

• Destructive strikes known as Single Event Latchup (SEL), Single Event Gate Rupture (SEGR), or Single Event Burnout (SEB)

The Problem of RadiationSEL caused by high energy particle passing through a p-n transistor junction, creating a Thyristor effect: low-impedance path between power supply rails of MOSFET circuit.

Without a powercycle, high current permanently damages the circuit.

p-n junction

Equivalent circuit of latchup

The Problem of Radiation• SEGR - Heavy ion hits the gate while a high voltage is

applied to it. Local breakdown occurs in the insulating layer, causing overheating and destruction of the gate.

• SEB - If substrate under source region is forward-biased by a particle, and the drain-source voltage is higher than breakdown voltage, get high current and overheating.

Illustration of SEGR and SEB

Solutions

• Lead Room?o 1 square foot of lead weighs 59 lbs.o cost? $10,000 per pound.o Not feasible.

• Radiation Hardening?

Radiation Hardening• Add an insulating layer (silicon dioxide or sapphire) around both NMOS and

PMOS transistors.o Makes chip immune to SEUs.o Removes most SELs.o Chip can still be high speed and low power.o Drastic increase in substrate cost.

Radiation Hardening• Bipolar integrated circuits (TTL) are generally better than CMOS

• Power MOSFETS are the most susceptible to SEBo SEB can occur at less than 30 V biaso BJTs are less susceptible beginning at 400 V

• TTL logic generally takes more power and produces more waste heat

Radiation Hardening• MRAM much greater tolerance to SEUs than SRAM and DRAM.• Tie points throughout the processor force high-energy particles to discharge to

ground.• Parity bits to fix SEUs• Redundant components: Independently calculate answers, compare. Any system

that produces a minority result recalculates. If repeated errors occur from the same system, that board is reset.

• Turn off cache during travel through particularly radioactive areas.• Watchdog timer causes hard reset after system unresponsive.

A Challenger Appears!

Commercial PowerPC 750 vs RAD750

Rad750 much larger due to radiation hardening redesigns

BUT:5 - 6 orders of magnitude better in terms of SEU performance (radiation upsets)

PowerPC Instruction Set

• RISC• Instruction Set: PowerPC 32-bit• Integer Size: 32 bit• Physical Address space: 32 bits• Virtual Address space: 52 bits

o Divided into 4-Kbyte pages, each of which can be mapped to a physical page.

Execution units• Floating-point unit (FPU)• Branch processing unit (BPU)• System register unit (SRU)• Load/store unit (LSU)• Two integer units (IUs) • IU1: execute all integer instructions.• IU2: execute all integer instructions except multiply and

divide.

Block Diagram6 Execution Units:

Branch

System

Floating Point

2 Integer

Load / Store

Register Set● UISA Registers are user-level● General-purpose registers

(GPRs)● Floating-point registers(FPRs)● Condition register (CR)● Floating-point status and control

register (FPSCR)● All can be accessed by software

with either user or supervisor privileges.

Cache• Split 32-Kbyte, eight-way set associative instruction and data cache. (Harvard

architecture)• Pseudo least-recently-used (PLRU) replacement algorithm.• 32-byte cache block.• Two coherency state bits for each data cache block.

o Modified (M)o Exclusive (E)o Invalid (I)

• Single coherency state bit for instruction cache block.o Invalid (INV)o Valid (VAL)

Dynamic Branch Prediction

Uses a 512 entry x 2-bit Branch History Table and maintains two speculations

Branch Unit connected to instruction fetch for Branch Target Buffer (BTB)

Power Management• Dynamic power management powers down unused functional units by shutting

down their clocks.• 3 levels of decreased power:

o Doze: Keeps cache active with “snooping”-based cache coherence.o Nap: Disables snooping, keeps Phase Locked Loop (PLL) to maintain higher

clock rate.o Sleep: Shuts down PLL, requires 100us delay to return to higher

performance.

• Also has low-speed continuous operation mode

Power Management

• Thermal Assist Unito Monitor junction temperature through the incorporation of

an on-chip thermal sensor.o TAU provides thermal control by comparing the 750’s

junction temperature against user-programmed thresholds.

• Instruction Cache Throttlingo Reduce the instruction execution rate without overhead of

dynamic clock control.

Why not Core i7?

• Long process for radiation hardening • Parity logic needed for memory such as

“scrubber”.• Voting logic needed for redundant check bit

check. (increases chip area by a factor of 5)• Thorough testing of all features.

Future Improvements

• Magnetic Shield coupling:o Previously dismissed as unrealistically expensive.o But new experiments show promising results to make

compact (implying cheaper) protection.o Separate out electrons and protons of the solar wind

creating a separation of charge which would deflect particles away from the shield.

• Cost of payload per pound to decrease significantly in the upcoming years.

Conclusion

RAD750 is successful, widely used5-6 orders of magnitude better in radiation than commercial counterpart

Reliable

Sourceshttp://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.48.1291

http://www.klabs.org/DEI/Processor/PowerPC/rad750/papers/spacewire_con_2007.pdf

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=00931184&tag=1

http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/11883/1/02-0576.pdf

http://parts.jpl.nasa.gov/docs/Radcrs_Final.pdf

http://www.smpstech.com/power-mosfet-single-event-burnout.htm

https://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/2F33B5691BBB8769872571D10065F7D5/$file/750cldd2x_ds_v2.6_16Oct2009dft.pdf

http://www.wired.com/wiredenterprise/2012/08/martian-computing-is-light-on-ram-heavy-on-radiation-shielding/