3 Local Wake Up (LWU)

Now with a basic understanding of trivial wake methods, it is important to understand wake up methods for when the primary regulator of the SBC (typically denoted VCC1) is off. Many SBC designers have this requirement in the system and is why many SBCs includes a discrete WAKE pin in which the primary function of the pin is to act as a local wake-up input that can be activated during the sleep mode of the SBC with or without primary power active. Many SBCs contain more than one WAKE pin; the TCAN28XX-Q1 family of devices includes three discrete WAKE inputs while the TCAN24xx-Q1 family includes four discrete WAKE inputs. This means that multiple wake up sources can be present in a design that forces the SBC out of sleep mode and into an operational state through these WAKE pins.

A wake signal input on the WAKE pins is referred to as a local wake up (LWU) because the source of the wake up is from a signal that is local to the SBC (within the same subsystem). While there can be various types of a wake-up signal, TI SBCs contain the following wake-up schemes: bidirectional edge detection, rising edge detection, falling edge detection, and a pulse detection. All edge detection works relatively similarly. If the wake pin crosses the appropriate threshold and the signal is held for the minimum amount of time (t_wake) a wake-up condition is sent to the SBC control block. So for a rising edge, that means the WAKE pin voltage goes from a low state to a high state and is held for at least t_wake before a wake-up signal is generated internally to the SBC. Falling edge is just the reverse of the rising edge, for example if the WAKE pin voltage goes from a high state to a low state and is held for at least t_wake before the wake-up signal is generated internally to the SBC. Bidirectional edge detection generates a wake-up signal for any high to low or low to high transition on the WAKE pin that meets the minimum timing requirements. An example of this can be shown by looking at the rising edge and falling edge local wake up signals from TI SBC TCAN2847-Q1 - however it applies to multiple SBC devices from TI.

Figure 3-1 Local Wake Up: Rising Edge Shown (TCAN2847-Q1)

Figure 3-2 Local Wake Up: Falling Edge Shown (TCAN2847-Q1)

Beyond edge detection, there is also pulse detection which is more involved than standard edge detection as there are now a few more specifications to consider. The standard threshold levels are the same for both edge and pulse detection; these are generally configurable within the SBC. See a specific SBC data sheet for more information on the specific SBC device thresholds. However, the directionality of the pulse must also be specified, either as a low to high to low pulse or a high to low to high pulse so the polarity of the pulse matters if pulse detection wake up is used. There are also timing considerations with pulse detection. There are three primary timing specifications that are used in pulse detection and the specifications are from shortest to longest t_wk_width_invalid, t_wk_width_min, and t_wk_width_max. These time ranges are generally configurable within the SBC. For a valid pulse to be detected the pulse must be >= t_wk_width_min and <= t_wk_width_max; if the pulse is < t_wk_width_min and > t_wk_width_invalid a wake up condition can be detected. This is observed when looking to the behavior of the TCAN2847-Q1 and similar devices that allow for pulse-based wake on the WAKE pins.

Figure 3-3 Local Wake Up: Pulse Detection (TCAN2847-Q1)

Note that these WAKE pins are high voltage tolerant and usually can withstand higher voltages than the supply pin of the SBC. be sure to check a specific device data sheet. While the above definitions define the most common use cases using the WAKE pins. There are a few of the uses that are further explained later in this application note.

The WAKE pins are generally simple to use. How does a designer use them in a production system? For this, look at a real-world example of an SBC that is positioned in a car door that must be woken up every time the car door opens or closes. Assume that a Hall-effect sensor is used to determine if the door is shut. For example, the sensor detects when the car door is closed and current does not flow through the sensor when the car door is shut showing a binary signal that states the condition of the door. For simplicity, assume the Hall effect sensor only has three pins (VCC, GND, and OUT) where OUT indicates if the door is closed or open. This is common of devices such as the TMAG5131-Q1 and can be used as the SBC wake-up source.

Figure 3-4 Hall Effect Sensor as Local Wake Source for SBC Diagram

In this simplified example the car door has a switch that conducts current when the door is closed and when the door is open, the circuit is an open circuit and not conduct current. This state of current and no current can be detected through the Hall effect sensor. The sensor output is open drain in the diagram but it does not have to be. This depends on the Hall effect sensor selected. If it is assumed that no current means a low output on the OUT pin and a detected current is a high output on the OUT pin, then what wake state must be used can be determined. Since this example is has the SBC wake up every time the door changes state and each door state is represented by either a high or a low logic level, the WAKE pin must be configured as bidirectional edge detection. Another important concept is the wake level configuration. Since this specific example uses a 5V Hall-effect sensor that means the maximum output is also going to be limited to around 5V. So the rising threshold must be lower than the maximum output voltage of the sensor. This is not a problem for an SBC if the proper wake configuration is implemented. The previous example is not the only way to use the SBC WAKE pin, but merely a common use case example of how a designer can implement wake into a system.