Delfi-n3Xt Command & Data Handling Subsystem
The Command and Data Handling Subsystem (CDHS) of Delfi-n3Xt comprise a redundant onboard computer, local microcontrollers, software and the onboard data bus. Furthermore, the databus and power bus connect all subsystems as the 'Delfi Standard System Bus'.
The microcontroller for the Onboard Computer (OBC) chosen for Delfi‑n3Xt is the MSP-430F1611 of Texas Instruments. This microcontroller has flight heritage on Delfi-C³ and many other nanosatellites. In a radiation test carried out by a TU Delft student in cooperation with EPFL and PSI in Switzerland, this microcontroller performed well up to a total ionization dose of 37 kRad without major errors or catastrophic failure. This will yield more than five years of lifetime in Low Earth Orbit. The heritage, results of the radiation test and the very low power consumption has led to the selection of this microcontroller.
The OBC is fully redundant to increase the overall reliability of the satellite. In case the primary OBC is silent on the databus for a while, the secondary OBC will take over. Both OBCs are placed on the same PCB, but with a fully separated circuit.
To provide unique frame identification to telemetry packets and correlate the measurements to time, the OBC is equipped with an Ellapsed Time Counter.
Delfi Standard System Bus
The Delfi Standard System Bus (DSSB) has been developed which connects all subsystems electrically. The electrical power interface is a single supply of 12 Volts. The data bus chosen for Delfi-n3Xt is Inter-Integrated-Circuits (I²C), which is a two wire serial interface also used in Delfi-C³. In the Delfi-C³ project, it has been discovered that microcontrollers can have failure modes in which they pull one or both of the I²C lines down indefinitely while powered. The DSSB therefore comprises of a small circuitry to monitor I²C activity and isolate potential malfunctioning nodes from the main bus. The OBC can then decide to retry the subsystem or to switch over to the redundant subsystem.
Most subsystems need to be able to be switched on or off according to the operational mode and available power. Next to this, as a short circuit can take down the main power bus, a form of over current protection should be placed on each subsystems. The DSSB therefore also comprises a piece of circuitry for over-current protection and power switching on OBC command.
The circuitry and the 20-pins Harwin connector are standardized and consume a bit less than 20% of the total PCB area, which is comparable to the CubeSat standard but adding failure tollerance to the whole satellite.