On Board Computer & Payload Controller

A BeagleBone Black (BBB) microcontroller will serve as the foundation for the On-Board Computer. The BBB is an open-source, communitydriven, small form-factor embedded system with a 1 GHz ARM Cortex-A8 as its primary processing unit. The BBB interfaces with the PC/104 bus via a Flight Computer motherboard, which conforms to the PC/104 spec, connects to the BBB’s expansion header pins, and provides the BBB with all necessary power and data connections. The Flight Computer Motherboard also adds additional functionality, including a real-time clock with local battery backup, external MEMS oscillators, CAN transceiver, external memory IC, temperature sensor, and external watchdog processor (TI MSP430FR2433). When the BBB and the Flight Computer motherboard are coupled together, the complete unit is referred to as the OBC.

The Payload Controller will be heavily based on the main On-Board Computer, with slight modifications to accommodate the unique requirements of the scientific Payload. A BBB will serve as the compute element, with an additional FPGA used for controlling the Payload’s transmission lasers on the same board, an architecture based on the Massachusets Institute of Technology’s CubeSat Laser Infrared CrosslinK (CLICK) mission. The Payload box is disconnected from the PC/104 connector stack; therefore, power and data connections will be made to form a custom wiring harness passed into the Payload box from the Payload Controller. For more information on this architecture, refer to the paper on PULSE-A’s Payload design.

MEMS Oscillators - The decision was made to use MEMS oscillators (https://www.digikey.com/en/products/detail/sitime/SIT1533AI-H4-DCC-32-768E/4898150) with our real-time clock. MEMS devices are far more resillient to extreme pressures and temperatures, and would provide a far more robust time source for our RTC. We decided to go with the SiT1533, a 20 ppm crystal oscillator replacement - there are two on the flight computer board, one of which acts as a clock source for the RTC.

Battery Powered Real Time Clock - A real-time clock was added such that in the event the BeagleBone Black needs to be reset, accurate time could be reistablished without the need for radio communication with the satellite. The RTC (https://ww1.microchip.com/downloads/aemDocuments/documents/MPD/ProductDocuments/DataSheets/MCP7940N-Battery-Backed-I2C-RTCC-with-SRAM-20005010J.pdf) connects to the BeagleBone over I2C, uses one of the MEMS oscillators as its clock source, and is powered by a CR2032 battery.

Watchdog Processor - A watchdog processor was added to monitor a heartbeat signal from the BeagleBone Black and reset the computer if it were to experience a catastrophic software fault. We decided to go with the Texas Instruments MSP430, as it provided all of the necessary functionality while being able to withstand our desired temerature ranges. Link: https://www.ti.com/lit/ds/symlink/msp430fr2433.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1755672532866&ref_url=https%253A%252F%252Fwww.ti.com%252Fgeneral%252Fdocs%252Fsuppproductinfo.tsp%253FdistId%253D10%2526gotoUrl%253Dhttps%253A%252F%252Fwww.ti.com%252Flit%252Fgpn%252Fmsp430fr2433