Docs: Add external trigger logging info

Author: nseidle

docs/configure_ports.md

@@ -19,7 +19,7 @@ By default, the **Radio** port is set to 57600bps to match the [Serial Telemetry
 The **Data** port on the RTK Facet, Express, and Express Plus is very flexible. Internally the **Data** connector is connected to a digital mux allowing one of four software-selectable setups. By default, the Data port will be connected to the UART1 of the ZED-F9P and output any messages via serial.
 
 * **NMEA** - The TX pin outputs any enabled messages (NMEA, UBX, and RTCM) at a default of 460,800bps (configurable 9600 to 921600bps). The RX pin can receive RTCM for RTK and can also receive UBX configuration commands if desired.
-* **PPS/Trigger** - The TX pin outputs the pulse-per-second signal that is accurate to 30ns RMS. This pin can be configured as an extremely accurate time base. The pulse length and time between pulses are configurable down to 1us. The RX pin is connected to the EXTINT pin on the ZED-F9P allowing for events to be measured with incredibly accurate nano-second resolution. Useful for things like audio triangulation. See the Timemark section of the [ZED-F9P Integration Manual](https://cdn.sparkfun.com/assets/learn_tutorials/1/8/5/7/ZED-F9P_IntegrationManual__UBX-18010802_.pdf) for more information.
+* **PPS/Trigger** - The TX pin outputs the pulse-per-second signal that is accurate to 30ns RMS. This pin can be configured as an extremely accurate time base. The pulse length and time between pulses are configurable down to 1us. The RX pin is connected to the EXTINT pin on the ZED-F9P allowing for events to be measured with incredibly accurate nano-second resolution. Useful for things like audio triangulation. See the [External Event Logging](#surveyor-data-port) section below and the Timemark section of the [ZED-F9P Integration Manual](https://cdn.sparkfun.com/assets/learn_tutorials/1/8/5/7/ZED-F9P_IntegrationManual__UBX-18010802_.pdf) for more information.
 * **I2C** - The TX pin operates as SCL, RX pin as SDA on the I2C bus. This allows additional sensors to be connected to the I2C bus.
 * **GPIO** - The TX pin operates as a DAC-capable GPIO on the ESP32. The RX pin operates as an ADC-capable input on the ESP32. This is useful for custom applications.
 
@@ -33,7 +33,7 @@ The **Data** port on the RTK Facet, Express, and Express Plus is very flexible.
 
 ## Data Port
 
-By default, the **Data** port is set to 460800bps and can be configured from 4800bps to 921600bps. The 460800bps baud rate was chosen to support applications where a large number of messages are enabled and a large amount of data is being sent. If you need to decrease the baud rate to 115200bps or other, be sure to monitor the MON-COMM message within u-center for buffer overruns. A baud rate of 115200bps and the NMEA+RXM default configuration at 4Hz *will* cause buffer overruns.
+By default, the **Data** port is set to 460800bps and can be configured from 4800bps to 921600bps. The 460800bps baud rate was chosen to support applications where a large number of messages are enabled and a large amount of data is sent. If you need to decrease the baud rate to 115200bps or other, be sure to monitor the MON-COMM message within u-center for buffer overruns. A baud rate of 115200bps and the NMEA+RXM default configuration at 4Hz *will* cause buffer overruns.
 
 [![Monitoring the COM ports on the ZED-F9P](https://cdn.sparkfun.com/r/600-600/assets/learn_tutorials/1/8/5/7/SparkFun_RTK_Express_-_Ports_Menu_MON-COMM_Overrun.jpg)](https://cdn.sparkfun.com/assets/learn_tutorials/1/8/5/7/SparkFun_RTK_Express_-_Ports_Menu_MON-COMM_Overrun.jpg)
 
@@ -47,4 +47,51 @@ Most applications do not need to plug anything into the **Data** port. Most user
 
 By default, the Data port is set to 460800bps and can be configured from 4800bps to 921600bps. 
 
-Note: The Data port does not output NMEA by default. The unit must be opened and the *Serial NMEA Connection* switch must be moved to 'Ext Connector'. See [Hardware Overview - Advanced Features](https://sparkfun.github.io/SparkFun_RTK_Firmware/hardware_rtk_surveyor/#advanced-features) for the location of the switch.
+Note: The Data port does not output NMEA by default. The unit must be opened and the *Serial NMEA Connection* switch must be moved to 'Ext Connector'. See [Hardware Overview - Advanced Features](https://sparkfun.github.io/SparkFun_RTK_Firmware/hardware_rtk_surveyor/#advanced-features) for the location of the switch.
+
+## External Event Logging
+
+![Three RTK Express with External Triggers](img/RTK%20Express%20with%20External%20Microphones.png)
+
+*Three RTK Expresses wired with external microphones*
+
+The external triggering system is a powerful feature enabling a variety of scientific applications. Above is three RTK Expresses wired with external microphones used in a 'popped balloon' audio triangulation experiment.
+
+The ZED-F9P has the ability to mark a detected event with +/-30 *nanosecond accuracy*. When enabled, an event on the RX pin (a low-to-high or high-to-low transition) on the DATA port, will trigger a message in the log with a very accurate timestamp. 
+
+* **Red** - 3.3V
+* **Green** - TX (output from the RTK device)
+* **Orange** - RX (input into the RTK device)
+* **Black** - GND
+
+[![Wires connected to a SparkFun USB C to Serial adapter](https://cdn.sparkfun.com/r/600-600/assets/learn_tutorials/2/1/8/8/SparkFun_RTK_Facet_-_Data_Port_to_USB.jpg)](https://cdn.sparkfun.com/assets/learn_tutorials/2/1/8/8/SparkFun_RTK_Facet_-_Data_Port_to_USB.jpg)
+
+For event logging, only the Black and Orange wires are needed. If you need to provide 3.3V to your system, the red wire can supply up to 600mA but we do not recommend sourcing more than 300mA.
+
+![Configuring the External Pulse and External Events](img/SparkFun%20RTK%20Ports%20PPS%20Config.png)
+
+*Configuring the External Pulse and External Events over WiFi*
+
+Be sure to select 'Enable External Event Logging' through the ports menu.
+
+Events within the log have the following format:
+
+> $GNTXT,numberOfSentences,sentenceNumber,CUSTOM_NMEA_TYPE_EVENT,triggerCount,towMsR,towSubMsR,accEst*CRC
+
+For example: 
+
+    $GNTXT,01,01,02,5,494326906,136292,31*74
+
+Where
+
+* $GNTXT: Custom NMEA text message
+* 01: numberOfSentences in this report
+* 01: sentenceNumber
+* 02: sentenceType - Externally triggered events are type 0x02
+* 5: triggerCount
+* 494326906: towMsR - Time Of Week of rising edge (ms)
+* 136292: towSubMsR - Millisecond fraction of Time Of Week of rising edge (ns)
+* 31: accEst - Accuracy estimate (ns)
+* 74: NMEA CRC
+
+The event timestamps can be analyzed to precisely coordinate or triangulate a past event. In the case of the three RTK Expresses with microphones, the three units' locations were known with RTK 14mm accuracy. The air temperature was taken to obtain the speed of sound. From these data points, we can solve for the location of a sound such as a popped balloon.