Startup Shutdown Synchronization Protocol (SSSP v1.0)

SSSP defines signal handling during the startup phase until all AMiRo modules are fully initialized, and during the shutdown phase so that the system turns off in a controlled and safe manner or restarts, if requested.
The complexity of the protocol is quite low and designed in a way that modules which do not implement SSSP will not compromise system operation.
Hence, only two signals are required:

  • S - synchronize
  • PD - power down

Both must be designed in a way, that they realize a logical OR on activation (one or more nodes are active) and a logical AND on deactivation (all nodes are inactive) respectively.
Electrically this can be implemented using active-low open-drain signals with pull-up resistors.

Startup Phase

All modules must initialize the signals in a way, that S is active and PD is inactive.
Although only S is used for startup, PD must be inactive during the startup phase, or the shutdown phase will be initiated either immediately by the bootloader or by the operating system as soon as it is active.

Each module executes the following steps:

  1. basic initialization
    1. initialization of required signals, voltages, or other hardware
      When a module has finished this stage, it sets S to inactive.
      In order to prevent erroneous behavior due to incorrect signals during the initialization, this stage takes at least one millisecond.
    2. waiting for synchronization
      Each module waits for S to become inactive (all modules are initialized) as a first synchronization.
    3. synchronous start of stage 2
      As soon as S is inactive, the master node activates it again in order to start the next stage.
      To ensure that each module had enough time to detect the inactive state of S, the master node must delay the activation by at least one millisecond.
  2. operating system initialization
    1. complete system startup
      Each module activates S again and fully initializes (e.g. starts the operating system).
      As soon as it is ready, it deactivates S again.
      When a module indicates to be ready, at least the main communication channel (for AMiRo this is CAN) must be fully operational.
      Again, S must be active for at least one millisecond, so every module can detect the activation.
    2. waiting for synchronization
      Each module waits for S to become inactive (all modules are ready).
      Only now it is safe to use the main communication channel and all modules are able to receive messages correctly.

At the end of the startup phase both signals, S and PD, are inactive.
Note that a module, which does not implement the protocol, will not interfere and cause no errors as long as it does not activate S.
However, such a module might cause errors after the startup phase, if it does not receive crucial information because communication is not set up.

Shutdown Phase

Since the two signals S and PD must not be used during system operation, both are defined to be inactive.
Any module can initiate the shutdown phase by activation of PD.
All modules (including the initiating one) must then execute the following steps as soon as the activation of PD is detected:

  1. shutdown of high-level operation
    1. initiation of module shutdown
      As soon as the activation of PD is detected, each module activates S.
      The module, which initiated system shutdown by activating PD has to activate S as well, of course.
    2. shutdown of high-level operation (e.g. the operating system)
      Each module stops all computation in a safe manner, so it can be shut down without data loss or other issues.
      As soon as this is done, it deactivates S.
      In order to ensure that every module had a chance to detect the activation of PD, this step must take at least one millisecond.
    3. waiting for synchronization
      Each module waits for S to become inactive (all modules are done).
  2. system shutdown or restart
    1. evaluation of PD signal
      When S becomes inactive, the state of PD indicates whether the system shall shut down or restart.
      Hence, the initiating module, which activated PD, must have set it to the according state before it deactivated S.
      The implication of the PD state at this point is defined as follows:
      • active: A system shutdown is requested.
      • inactive: A system restart is requested.
    2. final shutdown or restart
      Depending on the evaluation of PD, each module reacts accordingly.
      • shutdown
        Each module completely stops itself and enters low-power mode.
      • restart
        If a restart was requested, each module starts with the first step of the startup phase.
        In order to minimize risk of errors, all modules can power off, except for a master node, which resets the whole system and forces a clean startup.

Again, a module which does not implement the protocol will cause no errors as long as it does not activate S or PD.
However, if such a module has its own power supply and does not enter low-power mode, it will unnecessarily draw energy and might not end up in a defined state as the rest of the system.