Update Theory.md

Author: Nathan Seidle

Theory.md

@@ -1,4 +1,4 @@
-How I2C library works
+How I2C (aka DDC) communication works with a uBlox module
 ===========================================================
 
 When the user calls one of the methods the library will poll the Ublox module for new data. 
@@ -10,4 +10,18 @@ When the user calls one of the methods the library will poll the Ublox module fo
 * Otherwise, read number of bytes and process into NMEA, UBX, or RTCM frame.
 * If checksum is valid, flag frame as complete.
 
+This library was originally written to use the I2C interface but Serial has been implemented as well.
+
+How data is processed by this library
+===========================================================
+
+A method will call **sendCommand()**. This will begin waiting for a response with either **waitForACKResponse()** or **waitForNoACKResponse()** depending on the command we have sent (CFG commands generate an ACK where others like PVT do not).
+
+Once **waitForACKResponse()** or **waitForNoACKResponse()** is called the library will start checking the ublox module for new bytes. These bytes may be part of a NMEA sentence, an RTCM sentence, or a UBX packet. The library will file each byte into the appropriate container. Once a given sentence or packet is complete, the appropriate processUBX(), processNMEA() will be called. These functions deal with specific processing for each type.
+
+Note: When interfacing to a ublox module over I2C the **checkUbloxI2C()** will read all bytes currently sitting in the I2C buffer. This may pick up multiple UBX packets. For example, an ACK for a VALSET may be mixed in with an auto-PVT response. We cannot tell **checkUbloxI2C()** to stop once a give ACK is found because we run the risk of leaving bytes in the I2C buffer and loosing them. We don't have this issue with **checkUbloxSerial()**.
+
+**processUBX()** will check the CRC of the UBX packet. If validated, the packet will be marked as valid. Once a packet is marked as valid then **processUBXpacket()** is called to extract the contents. This is most commonly used to get the position, velocity, and time (PVT) out of the packet but is also used to check the nature of an ACK packet.
+
+Once a packet has been processed, **waitForACKResponse()/waitForNoACKResponse()** makes the appropriate decision what to do with it. If a packet satisfies the CLS/ID and characteristics of what **waitForACKResponse()/waitForNoACKResponse()** is waiting for, then it returns back to sendCommand. If the packet didn't match or was invalid then **waitForACKResponse()/waitForNoACKResponse()** will continue to wait until the correct packet is received or we time out. **sendCommand()** then returns with true/false depending on the success of **waitForACKResponse()/waitForNoACKResponse()**.