OBC Time Service

Time Service is component responsible for maintaining mission time.

Time Service's processes:

• TS1: Time update
• TS2: Time correction and storage
• TBD: TS3: Time restore on startup

TS1: Time update

• Updates in-memory mission time.
• Uses internal Backup Real Time Counter (BURTC) to increment mission time.
  • Use LFRCO oscilator (burtcInit.clkSel = burtcClkSelLFRCO;)
• Procedure:
  • Startup:
    • Initialize semaphore
    • Initialize BURTC
    • Enable Interrupt (NVIC_EnableIRQ(BURTC_IRQn))
    • Start task
  • ISR (void BURTC_IRQHandler(void)):
    • Give semaphore
  • Time monitoring task:
    • [In infinite loop]
    • Take semaphore
      • Success:
      • call time service with TIME_UPDATE_INTERVAL ms increment
      • Optionally: substract 1 ms every 50 intervals to reduce measuring error
      • Failure: log
• Values:
  • Prescaler set to 3 (244 μs) (burtcInit.clkDiv = 3)
  • Compare value set to 205 (BURTC_CompareSet(BURTC_CounterGet() + 205))
  • TIME_UPDATE_INTERVAL = 50 ms (~ 244 μs * 205)
  • Accuracy with above settings:
    • Interrupt interval every 50.02 ms
    • 0.04% error: (20 μs error every interrupt, 1 ms error every 50 interrupts, 360 ms error every 15 minutes)

TS2: Time correction and storage

• Corrects the mission time by taking into account value from external real time clock (EXRTC)
• Persists the mission time to external flash
• Runs every TIME_PERSIST_INTERVAL minutes
• Task procedure:
  • Read initial values on startup:
    • Read current mission time from time service -> MCU[t-1]
    • Read current time from EXRTC -> RTC[t-1]
  • [Infinite loop]:
  • Delay TIME_PERSIST_INTERVAL minutes
  • Read update values:
    • MCU[t] from time service
    • RTC[t] from EXRTC
  • Correct the MCU[t]
    • ΔMCU = (MCU[t] - MCU[t-1])
    • ΔRTC = (RTC[t] - RTC[t-1])
    • If either Δ is negative set it to 0
    • MCU[t] = MCU[t-1] + (ΔMCU + ΔRTC)/2
  • Persist the MCU[t] and RTC[t] on external flash
  • RTC[t-1] = RTC[t], MCU[t-1] = MCU[t]
• Values:
  • TIME_PERSIST_INTERVAL = 15 minutes