Speakers
Mr
Fredrik Johansson
(Cobham Gaisler)Mr
Jan Andersson
(Aeroflex Gaisler)
Description
**Abstract**
This paper describes a mixed-signal LEON3FT microcontroller ASIC (Application Specific Integrated Circuit) targeting embedded control applications with hard real-time requirements. The prototype device is currently in development at Cobham Gaisler, Sweden, and IMEC, Belgium, in the activity Microcontroller for embedded space applications, initiated and funded by the European Space Agency (ESA).
The presentation and paper will describe the architecture and functionality of the device. This abstract describes an on-going development where the devices are in the specification stages before detailed implementation phase. The presentation and final paper will contain further details on the device and will describe progress made during the activity.
**Background**
Software based data acquisition/processing and simple control applications are widely used in many spacecraft subsystems. They allow implementing software based control architectures that provide a higher flexibility and autonomous capability versus hardware implementations. For this type of applications, where limited performances are requested to the processor, general purpose microprocessors are usually considered not compatible due to high power consumption, high pin count packages, need of external memories and peripherals. Low-end microcontrollers are considered more attractive in many applications such as:
- propulsion system control
- sensor bus control
- robotics applications control
- simple motor control
- mechanism control
- power control
- particle detector instrumentation
- radiation environment monitoring
- thermal control
- AOCS/GNC (Gyro, IMU, MTM)
- antenna pointing control
- RTU control
- Simple instrument control
- Wireless networking
In these kind of applications the microcontroller device should have a relatively low price, a low power consumption, a limited number of pins and must integrate small amount of RAM and most of the I/O peripherals for control and data acquisition (serial I/Fs, GPIO’s, PWM, ADC etc.). The requirements for memory and program length are usually minimal, with no or very simple operating system and low software complexity.
**Microcontroller Applications**
Spacecraft subsystem control and monitoring of parameters such as power supply voltages, currents, pressures and temperatures are ideal applications for the LEON3FT microcontroller. Bridges between different communication standards or interface of an equipment towards a higher level controller or the central On Board Computer (OBC) are also ideal applications for the LEON3FT microcontroller.
The LEON3FT microcontroller can perform advanced data handling to offload any higher level controller or the central On Board Computer (OBC). By hiding the data handling details the transmitting data volume can be reduced and simplified functionalities and timing requirements are requested to the higher level controller.
The LEON3FT microcontroller integrates several on-chip data bus standards, such as SpaceWire, CAN, MIL-STD-1553, I2C, SPI, UART and can easily provide data packetization for serial communication using standard protocols. The microcontroller can also efficiently replace FPGAs in accomplishing the above functionalities. Generally the FPGA implementation is faster but much more complexity and flexibility can be captured in the software of a microcontroller even with limited processing capability. The correct use of FPGAs in space applications can be complex to achieve and also cost, package size and availability of integrated analog functions can favour the use of a microcontroller with respect to FPGA.
Below are listed a number of possible microcontroller use cases and specific applications.
- Nanosatellite controller
- Instrument Control Unit
- Remote Terminal control
- Propulsion Unit control
- Electric Motor Control
**Processor Performance and Determinism**
For applications demanding determinism on nested interrupts, a special interrupt handling scheme will be implemented in software where nested interrupts are allowed to occupy one additional register window. The number of levels of nested interrupts that can be handled without additional timing penalty depends on the complexity of the software implementation.
In the architecture, deterministic interrupt latency will be achieved by:
- Running software (including interrupt handlers) from local RAM and accessing any data needed during the interrupt handling through port separate from AMBA ports.
- Adapting the register window usage (using a flat model) structure to avoid unexpected window over/underflow traps. This is done in the compiler and application code, and most OS code does not need modification.
- The alternate window pointer feature from the SPARC V8E extension to allow window over/underflow handlers to run with traps enabled.
- Register file partitioning to allow partitioning of the register file (the windows) to different “contexts”. Contexts can for example be threads to speed up context switching and/or interrupt contexts to dedicate windows to ISRs.
**SPARC Reduced instruction set**
LEON-REX is an extension to the SPARCv8 instruction set. Similar extensions exist for other architectures such as THUMB/THUMB2 for ARM and MIPS16 for MIPS. The reduced SPARC V8 instruction set variant has been developed by Cobham Gaisler and is integrated into the device.
The main design goal has been to reduce code size, thereby reducing memory storage needed for the code, and to reduce memory bandwidth needed for the instruction code fetching.
Another design goal is to allow retrofitting the extension in existing LEON3/LEON3FT pipelines and into the existing software/compiler stack, and to provide backward compatibility. User can develop C code as usual (with bare-C or a small RTOS) and the existing LEON environment (GRMON, compilers etc) can be used for development.
LEON REX is designed to allow gradual transition where existing SW environment can be used unmodified and converted piece by piece to use new instruction set.
The compressed instruction set is an optional extension of the SPARC V8 ISA, and existing code can be used without modification. Compressed and regular code can be mixed in the same application, thus the user can avoid changing critical code that has already been validated.
The first version of the instruction set extension has been specified and tested on prototype hardware and tests has shown that a compression ration of 30-50% compared to normal SPARC V8 code is achievable in a real world scenario.
Summary
This paper describes a mixed-signal LEON3FT microcontroller ASIC (Application Specific Integrated Circuit) targeting embedded control applications with hard real-time requirements. The prototype device is currently in development at Cobham Gaisler, Sweden, and IMEC, Belgium, in the activity Microcontroller for embedded space applications, initiated and funded by the European Space Agency (ESA).
The presentation and paper will describe the architecture and functionality of the device. This abstract describes an on-going development where the devices are in the specification stages before detailed implementation phase. The presentation and final paper will contain further details on the device and will describe progress made during the activity.
Primary author
Mr
Fredrik Johansson
(Cobham Gaisler)
Co-author
Mr
Jan Andersson
(Aeroflex Gaisler)
