General

A three processor architecture has been implemented to manage operations onboard of TIsat-1.
Two microcontrollers of different make relay each other in interaction with sensors and communication devices. The inactive processor is completely powered off. Both processors provide the same capabilities acheived through two different but functionally equivalent firmware pieces.
The designers expect this scheme to provide information on dependability of the chosen processors for space applications along with fault tolerance of the system through redundancy.

Firmware

C and assembly were used in the project. The C code running on the two central processors was developped following a simple, confortable and powerful programming scheme of an home grown time-preemptive scheduler, codenamed Minosse.

On-Board Computer

Two very low power processors (from the MSP430 and PIC18 family), were used. Both, in turn, access sensors, actuators and communication subsystems.
A signal multiplexing/demultiplexing scheme which prevents powering the shut off parts through the signal lines was developped and applied throughout.
Onboard digital communication is based on the I2C standard and uses devices designed for dual-master, high-reliability I2C applications. If one master fails, the bus will still be accessible by the second (backup) master.

Two Ferroelectric RAM chips, set up as RAID1 (i.e. mirror), are used for data logging and system needs. The data is stored with EDAC (Error Detection And Correction) encoding to counter single event upset induced data corruption.

Timing Unit

The third processor stems from a space proven device family (PIC16F), successfully used in several previous CubeSat missions.
The Timing Unit (TU) is responsible for operation startup after space deployment and for managing timeslot assignment to the OBC processors, while periodically requiring a valid signature from them.

Software Modulation

Both processors access the radio subsystem and are able to audio-modulate/demodulate via software.
Digital-to-Analog and Analog-to-Digital converters along with DMA-channels are used to generate waveforms to be sent over the FM radio.

With PSK modulation using NRZ data, a bitrate of 1’400bps is acheived, while 700bps are obtained with Manchester coded data.
With FSK modulation (varying amplitude for different frequencies) the bit rate is about 200bps with NRZ and 100bps with Manchester encoding.

Operating Modes

TIsat-1 executes a periodic sequence of operations going through so called Operating Modes. Each Mode executes and terminates within a known timeframe, allowing the system to step over to the next Mode.

The diagram shows the Operating Modes of TIsat-1. Transitions and times have been tuned to the real needs.

Acknowledgements

The OBC structure and operation was inspired by the PicPoT project from Politecnico di Torino.

Documents

On Board Computer Survey (PDF, Italian)