1. Slide 1P106/MAPLD 2004Woodroffe MAPLD 2004 Washington D.C. 8 th – 10 th September 2004 Application and Experience of CAN as a low cost OBDH bus system Adrian Woodroffe Engineer Surrey Satellite Technology Limited, Guildford, UK

2. Slide 2P106/MAPLD 2004Woodroffe Contents Introduction to Surrey Satellite Technology Ltd. (SSTL) SSTL Missions using CAN SSTL CAN bus topology SSTL CAN hardware solutions COTS CAN RadCAN SSTL CAN for Spacecraft Usage (CAN-SU) protocol Future development Conclusion

3. Slide 3P106/MAPLD 2004Woodroffe Surrey Satellite Technology Ltd Predominantly Low Earth Orbit (LEO) missions Extensive use of Commercial Off The Shelf (COTS) technology, plus Module level redundancy Passive fail safe system design Provides a low cost solution Rapid utilisation of new technology Now moving to small GEO platforms British National Space Centre funded GEMINI small GEO communications platform Deployable panels Harsher radiation environment compared with LEO Higher reliability requirements

4. Slide 4P106/MAPLD 2004Woodroffe SSTL CAN Flight History SatelliteLaunchOrbit UoSAT-11984560 km UoSAT-21984700 km UoSAT-31990900 km UoSAT-41990900 km UoSAT-51991900 km S80/T19921330 km KitSAT-119921330 km KitSAT-2*1993900 km PoSAT-11993900 km HealthSAT-21993900 km Cerise1995735 km FASAT-Alfa1995873 km FASAT-Bravo1998873 km Clementine1999735 km SatelliteLaunchOrbit UoSAT-121999650 km Tsinghua-12000750 km TiungSAT-120001020 km SNAP-12000650 km PicoSAT2001650 km ALSAT-12002686 km UKDMC2003686 Km NigeriaSAT-12003686 km BilSAT-12003686 km TOPSAT2005650 km ChinaSAT2005650 km CFESAT2005650 km GSTBv2200520000 km GEMINITBC38000 km No CAN BusPartial CAN BusCAN Bus for all TTC

5. Slide 5P106/MAPLD 2004Woodroffe CAN Bus System Topology DRIVER CAN μC TCTLM °C SEMI-SMART MODULE DRIVER CAN μC TCTLM °C SEMI-SMART MODULE DRIVER CAN μC TCTLM °C TYPICAL DATA PROCESSING MODULE (OBC/SSDR) DRIVER CAN PERIPHERAL μPROC/DSP DRIVER CAN μC TCTLM °C DRIVER CAN PERIPHERAL μPROC/DSP PRIMARY CAN BUS SECONDARY CAN BUS TYPICAL DATA PROCESSING MODULE (OBC/SSDR) Two sub-system categories are connected to the CAN bus Semi-smart modules that respond to tele-commands and provide telemetry For example a reaction wheel will have a tele-command to set it speed, and a telemetry value to read it’s speed. The microcontroller will provide the control loop to maintain the set speed. Data processing modules, onboard computers etc that control the satellite Generally based round a microprocessor, the OBC will control the attitude and payload functions etc by issuing a set of tele-commands on the CAN bus Redundant busses are selected via latching relay A module can be commanded to change to the secondary bus, or Will automatically switch buses if the module does not receive any messages for 5 minuets

6. Slide 6P106/MAPLD 2004Woodroffe The Environment Low Earth Orbit 500Km to 1000Km Relatively benign radiation environment 2mm Alu shielding from module boxes & structure) SEU rate of approximately 1 per MByte of SRAM per day SEL, only 4 confirmed cases in SSTL’s flight history A COTS solution is therefore acceptable if; The system design is passively fail safe for attitude, power generation and thermal control Module level cold redundancy Higher Earth Orbits MEO and GEO orbits >10Krad TID per year depending on orbits More complex systems require guaranteed active attitude control for deployable panels etc A radiation tolerant (for both TID and SEU / SELs) must be used

7. Slide 7P106/MAPLD 2004Woodroffe COTS Solution DRIVER CAN μC TCTLM °C TYPICAL DATA PROCESSING MODULE (OBC/SSDR) DRIVER CAN PERIPHERAL μPROC/DSP Semi-Smart Modules Based round a 8051 microcontroller On Chip CAN controller Makes maximum use of microcontroller peripherals 8 channel, 10 bit ADC for telemetry gathering PWMs and URATS Physical CAN transceiver Latching relay to select bus Data Processing Units Microprocessor base CAN controller memory mapped Physical CAN transceiver Latching relay to select bus

8. Slide 8P106/MAPLD 2004Woodroffe COTS Components Flown Component Flown on number of Mission Used on Approx number of modules per mission Approx. accumulated Orbit years of operation from last 4 mission only Philips CAN Transceiver: Physical CAN driver, current production 41 (example UKDMC)4.5 Phillips PCA82C250: Physical CAN driver, going obsolete 1025 (example UKDMC)>60 * Philips P87C592: CAN microcontroller, obsolete 10 None currently Up to 20 N/A Philips CAN 8-bit peripheral, current production 42 (example UKDMC)4.5 * Philips PCA82C200: CAN 8-bit peripheral, obsolete 6 None currently Use to be 2 N/A Infineon: 8-bit CAN microcontroller (A/D, PWM etc. 8051), current production 626 (example UKDMC)>60 * Microchip CAN SPI peripheral42 (example UKDMC)9 * assuming 50% are in cold redundant modules & thus off

9. Slide 9P106/MAPLD 2004Woodroffe Radiation Hard Solution No Radiation Hard 8051 microcontroller with on chip CAN is currently available The COTS 8051 does not meet radiation for orbits higher then LEO Aurelia CAST A (8051 with the CASA2 CAN peripheral on the same die is presently under development (see www.caen.it/micro) Solution for Semi-Smart Modules Separate 8051 CPU, with an external CASA2 CAN controller (Aurelia design), external PROM, SRAM and ADC had to be provided Shown as a block diagram on the following slide Solution for Data Processing Modules The Existing architecture can be copied using available radiation hard components Processor: Atmel SPARC 695 CAN Peripheral: Aurelia CASA2 (Based on ESA HurriCAN IP Core) CAN Physical No Radiation Hard device is currently Available The COTS Phillips device is manufactured Silicon on Insulator and has shown promising total dose performance Aurelia have EM samples of a Radiation Hard Device RS485 drives can be interface to the CAN bus with some additional pull-up / pull-down resistors. This option is currently under evaluation

10. Slide 10P106/MAPLD 2004Woodroffe Radiation Hard Solution

11. Slide 11P106/MAPLD 2004Woodroffe COTS / Radiation Hard comparison COTS Radiation Hard COTS µControllerRadCAN Size1”sq>6”sq Mass<5g>50g Power <0.75W (max) 0.20W nominal Estimated 1W Total Dose<10Krads100Krad SEE ProtectionNone Highly SEU tolerant, Latch-up immune Component Cost<$10 (US)>$15000 (US) Export IssuesNone USA Department of State ITAR (up to 18 weeks for license)

12. Slide 12P106/MAPLD 2004Woodroffe Protocol Introduction Higher layer protocol for CAN developed by SSTL: CAN for Spacecraft Usage (CAN-SU) Used for telemetry, tele-commands and data transfers Developed in 1995 before the extended 29 identifier was available Driven by Requirements: Wide range of required telemetry values from many modules Support multiple system configurations Keep it simple Limitations 11 bit CAN ID field dictates the need for extended addressing in the CAN data field

13. Slide 13P106/MAPLD 2004Woodroffe CAN-SU Protocol ID (8 bits) Seq (3 bits) Address (10bits) From ID (8 bits) Control (8 bit) Data (32bits) 11 Bit CAN IdentifierCAN Control8 bytes of data in each CAN message Address of module or task the message is being sent to i.e. reaction wheel A sequence number used in burst of 8 packets Not discussed further here Various control bits as defined in the CAN protocol Including data field Length Address of module or task Sending the message i.e. the OBC Defines message type; Tele-command, telemetry request etc Telemetry or Telecommand Address 4 Bytes of Data CAN-SU Bit definitions CAN Bit definitions

14. Slide 14P106/MAPLD 2004Woodroffe Protocol Telecommand Request SourceRequest Sink Tele-command request Tele-command acknowledge Tele-command request Tele-command ack ID (8 Bits) Seq = 0 Len = 7 From (8 Bits) Address (10 Bits) Data Response 32 Bits Control TC-Ack ID (8 Bits) Seq = 0 Len = 7 From (8 Bits) Address (10 Bits) Data 32 Bits Control TC-Req

15. Slide 15P106/MAPLD 2004Woodroffe Protocol Telemetry Data SinkData Source Telemetry request Telemetry response Telemetry request Telemetry response ID (8 Bits) Seq = 0 Len = 7 From (8 Bits) Address (10 Bits) Data Response 32 Bits Control TL-Res ID (8 Bits) Seq = 0 Len = 3 From (8 Bits) Address (10 Bits) Control TL-Req

16. Slide 16P106/MAPLD 2004Woodroffe Future Developments (1) COTS Microchip PIC 3.3V microcontroller with on chip CAN running at 3.5 MIPS (compared with 1 MIP 8051). Lower power FLASH based program code for in-circuit programming (last resort in orbit) RAD Hard Evaluate Aurelia CASTA RadCAN next generation will be a System on a Chip (SoC) solution. Currently targeted at an Actel RTAX1000 Combines an open source 8051 IP core with the ESA licensed HurriCAN core, uses FPGA SRAM with EDAC and hard code the 8051 program code, to produce a single chip solution (minus the ADC)

17. Slide 17P106/MAPLD 2004Woodroffe Future Development (2) RadCAN – Next Generation

18. Slide 18P106/MAPLD 2004Woodroffe Future Developments (3) Protocol The use of the 11 bit identifier is severely limiting to CAN-SU SSTL is investigating a re-work of the CAN-SU protocol to use the 29 bit identifier The 29 bit identifier could incorporate both CAN task ID and telemetry / tele- command channel number 29 bit identifier could also mean use of the Remote Frame is possible CANOpen SSTL is also investigation the option to migrate to the industry standard CANOpen http://www.can-cia.de/canopen/ Heavier protocol than CAN-SU but selected parts could be used to replace and improve on CAN-SU Provides more complex synchronisation factions if required

19. Slide 19P106/MAPLD 2004Woodroffe Conclusions COTS components 10’s of orbit years use on SSTL satellites in LEO Suitable for radiation benign LEO missions Sub-system redundancy and fault tolerant system design has resulted in no observed failures of the CAN bus or it’s constituent components. Radiation Hard CAN solutions Not many components are available at the present time but this is changing Hurricane (ESA IP core) Aurelia CASA 2 (CAN peripheral) Aurelia CAST A (8051 micro + CAN peripheral), in development Aurelia CAN Physical driver, in development