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DISCO-II Cortex-M7 scheduler

Source code and build setup for the Cortex-M7 auxiliary core payload scheduler aboard the DISCO-II student cubesat.

The Cortex-M7 is a secondary core on the NXP i.MX8MP applications processor inside the PicoCore™MX8MP system-on-module (SoM) used in the scientific payload of the DISCO-II student cubesat. In the context of the DISCO-II mission, the Cortex-M7 is used for low power operations while the main Cortex-A53 cores are put into a low power state. This mainly includes GNSS monitoring to time wake-up events for the main cores and orchestration of the satellite's components via programmable procedures in-flight.

TLDR

Got docker? Great, clone the repository (including submodules), open a shell and run:

docker compose build && docker compose run build-service

This will compile and install a binary into the bin/ directory, ready to be loaded onto a PicoCore™MX8MP.

Environment

The build process is handled by a docker container that contains the necessary GNU Arm Embedded toolchain and libraries to compile the application - see Dockerfile for reference. The actual build process is invoked via the src/armgcc/build-service-start.sh shell script, and CMake is used to manage the build process (src/armgcc/CMakeLists.txt). src/armgcc/MIMX8ML8xxxxx_cm7_ram.ld contains the linker script which specifies the memory layout of this application for the Cortex-M7.

Application

The application relies on FreeRTOS - as provided with the SDK from F&S Elektronik Systeme GmbH (fsimx8mp-m7-sdk/ subdirectory) - and the following libraries:

  • libcsp: CSP, CubeSat Space Protocol, a communication protocol used for network communication between the satellite's components as well as with the ground station. (lib/csp subdirectory)

  • libparam: A library that adds a parameter system to a CSP network. This provides a mechanism for nodes in the network to store and expose parameters over the network. (lib/param subdirectory)

  • csp_proc: A custom-made library that provides a framework for creating and running procedures on the nodes in a CSP network via a register-based virtual machine, utilizing the libparam parameter system for its register set. This is used to orchestrate the satellite's components in-flight via programmable procedures sent from the ground station. (lib/csp_proc subdirectory)

The custom application code contains the following components:

  • src/main.c: The main entry point for the application. Initializes the FreeRTOS scheduler and calls the initialization functions for e.g. network, clock and csp_proc configuration.
  • src/board_config.c: Contains configuration of memory regions and RDC.
  • src/clock_config.c: Contains the clock configuration for the Cortex-M7.
  • src/FreeRTOSConfig.h: Contains the FreeRTOS configuration, including e.g. heap size and supported features.
  • src/gnss.c: Contains the GNSS configuration and task for parsing NMEA strings.
  • src/hooks.c: Contains implementation of periodic tasks such as updating libparam parameters with temperature readings.
  • src/lpm.c: Contains low power mode configuration functionallity.
  • src/network_config.c: Contains the network configuration for the CSP network, including VMEM and parameter server task initialization for libparam to function properly.
    • src/can_iface.c: Contains a custom implementation of a CAN driver for the CSP network.
    • src/si_lock.c: Platform-specific implementation of semaphore for use with the CSP network.
  • src/param_config.c: Specifies the parameters that are exposed over the network via libparam.
  • src/pin_mux.c: Contains the pin multiplexing configuration (CAN and UART)
  • src/sys_comm_service.c: Contains an interface for using the MU to wake up the A53 cores.
  • src/tinygps.c: Contains functions for parsing NMEA strings from a GNSS module.
  • src/tmu.c: Contains an interface to the Thermal Management Unit (TMU) on the i.MX8MP for temperature monitoring.