5 Cyclic Wake Up

Not all wake methods require external stimuli. Some SBC devices integrate an internal timer that can be used to wake up the SBC periodically. This idea is called cyclic wake-up: configure the integrated timers to wake up the SBC on regular repeating intervals. While timer specifics can vary across SBC device, devices in the TCAN28xx-Q1 or TCAN24xx-Q1 mid-range SBC families utilize a 10-bit timer. To understand how to utilize cyclic wake-up in a system one needs to understand how the operation varies between different SBC modes, how the wake methodology responds depending on device mode, as well as the potential options for configuration, and finally a brief note on why a cyclic wake-up scheme can be employed in a system.

In general, most TI SBCs have a few different modes. These can be grouped as either transitional modes which are in-between states that cannot be directly switched to; these include initialization state at device power up and the restart state that the device must transition to enter standby. Beyond the transitory states there are generally also 3 operational modes, which are generally standby, normal, and sleep mode. In standby mode the device is on and functional but communication is not occurring. This is the wait/idle mode that the device enters after power up. When the SBC enters standby mode a long window watchdog timer starts which must be triggered correctly to stay in standby mode. Otherwise, the device restarts and potentially enters a fail-safe or sleep mode. Normal mode is where the device is fully functional including bus communication. Sleep mode is a low power mode where most of the SBC is shut down but can be woken up if there is bus communication or a system function the device must complete. Finally, some SBCs also include a fail-safe mode which is similar to sleep mode except that there are more restrictions about how the device returns to standby mode, including removing any faults that caused the device to move into fail-safe mode. These modes are important to understand because the cyclic wake-up functionality varies depending on the SBC mode. To help illustrate this the TCAN2847-Q1 mid-range SBC device is used to highlight common ways that the cyclic timer functions depending on the SBCs operational mode.

Cyclic wake is available for use in the normal, standby, sleep, and fail-safe modes of the SBC device, but the function varies slightly depending on SBC mode. In normal and standby modes cyclic wake functions the same. After selecting either integrated timer1 or timer2 and the timer on-time is set, the timer starts at the beginning of the ON timer1/2 programmed by the interrupt pin (nINT pin on the TCAN2847-Q1) pulls low for the duration of the programmed on-time. The SBC ignores the first on-time period but every subsequent period uses the interrupt pin to alert the host processor that there has been an interrupt flag generated. In general, using cyclic wake up in this way is not as common as using cyclic wake up in sleep, but this is still possible. In sleep mode the general set up and configuration is the same as standby and normal modes – but the key difference is how the device reacts to the on-time period. When the timer enters the on-time period the SBC wakes up, transitions to restart mode, when VCC1 is available, this generates an interrupt flag to acknowledge the wake up, and then transition to standby mode. Upon entrance into standby mode, the long window watchdog timer starts and the host processor must correctly trigger the watchdog timer or else the device returns to sleep mode and waits for the next on-time period of for the timer1/2.

Cyclic wake can also be used to wake a device from a fail-safe mode; however, the configuration options are typically more limited. Fail-safe mode is like sleep mode except that the device cannot exit the mode unless the fault that caused fail-safe mode is cleared or the system is power cycled. When looking at the TCAN28xx-Q1 mid-range family of SBC devices, cyclic wake up is available in fail-safe mode but the on-time period options are restricted to 500ms, 1s, or 2s. When in fail-safe mode and the cyclic wake up timer enters the ON period, the device checks to see if the fault has cleared; if not, the process repeats until the Sleep Wake Error (SWE) timer expires and the device transitions to sleep mode.

So why does cyclic wake used in a system where there are multiple other wake methods? The answer varies with use case, but one of the most common implementations is to use cyclic wake in sleep mode. This gives the designer a way to check system status periodically while in a low power or sleep mode without requiring an event to take place on the local WAKE pins. It also gives a controlled way to exit fail-safe mode by essentially polling to check if the fault is present, still without direct intervention from the host MCU or controller to poll. While in standby and normal modes the cyclic wake can be used to alert the host MCU or processor of time passing, which the host MCU or processor could use to modify the configuration of the SBC device while in standby and normal modes – a modification could be simply turning on the CAN transceiver to enable bus communication.