First, the GFO pin is a general-purpose output pin and has a few potential configurations. The default is a simple general-purpose output that can be changed to either high or low via a SPI command to the SBC. This can be used to enable or inhibit an additional transceiver in case channel expansion is needed in the system, or just a general enable signal that can be utilized in multiple applications.

Figure 7-5 GFO Pin in Channel Expansion Application

However, the GFO pin is not just a standard output pin – this can also be used to communicate information back to the companion controller. Other functions this pin can perform are as follows: VCC1/VCC2/VEXCC electrical fault, watchdog interrupt, local wake up (LWU) request, bus wake up request (WUP), restart counter exceeded (indicated in standby mode), or a CAN bus fault. While all these functions can be accessed through SPI reads of interrupt registers and the nINT pin – the GFO pin can be configured for specific interrupts allowing the system to respond to specific fault cases without needing to rely on a SPI read to get the pertinent information resulting in a potentially quicker system reaction time.

The next pin that can focused on in the nRST pin. This pin can indicate to companion processor when the device is in restart mode – but this pin is not just an output this also acts as an input. Applying a low signal on this pin can cause the device to perform a reset that can reload the EEPROM and set all other registers to factory setting. This is very common to use this pin and connect directly to companion controller so that controller resets when the SBC resets.

Finally, the SW pin. This pin has a few uses – but the most notable one is putting the device into software development mode. When the pin is held in active state (default high, can be changed to low) watchdog errors does not result in mode changes. The reason this is tied to software development is that when prototyping with this device the last thing that can be implemented is the watchdog timing as this can greatly slow down development of other features. If watchdog is active and the system is not setup for this the subsequent missed watchdogs can cause the device to switch modes – which by default the device can be active for about 3-4 seconds before entering fail-safe mode if watchdog is active and controller is unconfigured for watchdog. Holding this pin high does not stop the watchdog from operating, so watchdog testing can be done in software development mode, but this can stop negative impacts from watchdog timer. In actual systems this pin is not held in active state if this was only used for the primary purpose. That being said – this pin can also act as a digital wake-up pin in addition to the analog WAKE pins offered by the device.