DOC: Add developer documentation

doc/DEV_NOTES.md

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+Test Points
+-----------
+
+The PT100 PCB has 15 test points, numbered T1 to T15.
+
+NOTE: T1, T2, T4, T5, T6, T7, T8, T9, T10, T11 are referenced to `GND` (T3). T4, T5, T6, T7, T8, T9, T10, T11 are 3.3V level signals.
+
+NOTE: T12, T13, T15 are referenced to `GNDISO` (T14). T12, T13, T15 are 5V level signals.
+
+* T1 - `5V`
+    * USB 5V DC power rail
+    * Used for MCU USB circuitry and digital isolator power supply
+
+* T2 - `3V3`
+    * Regulated 3.3V DC power rail
+    * Used for MCU core and ports and EEPROM
+
+* T3 - `GND`
+    * Ground for PC connected circuitry (MCU/EEPROM)
+
+* T4 - `SBWTDIO`
+    * MCU debug/programming interface, connects to SBW RST on TI LaunchPad ez-FET header
+
+* T5 - `SBWTCK`
+    * MCU debug/programming interface, connects to SBW TST on TI LaunchPad ez-FET header
+
+* T6 - `GPIO1`
+    * General purpose I/O for debugging, pin P4.4 on MCU, hardware serial port on MCU (TX)
+    * Currently outputs a LOW when interrupts are enabled, and outputs a HIGH when interrupts are disabled (during reception of byte during timer operation)
+
+* T7 - `GPIO2`
+    * General purpose I/O for debugging, pin P4.5 on MCU, hardware serial port on MCU (RX)
+    * Currently toggles state when the `MCU_DATA_IN_3V3` signal is being sampled
+
+* T8 - `MCU_DATA_OUT_3V3`
+    * Controls transistor for DATA signal
+    * `MCU_DATA_OUT_3V3` LOW = transistor off = DATA line pulled HIGH (`5VISO`) by pull up resistor
+    * `MCU_DATA_OUT_3V3` HIGH = transistor on = DATA line pulled LOW (`GNDISO`) by transistor
+
+* T9 - `MCU_DATA_IN_3V3`
+    * Level shifted version of `KFD_DATA_IN`
+
+* T10 - `MCU_SENSE_OUT_3V3`
+    * Controls transistor for SENSE signal
+    * `MCU_SENSE_OUT_3V3` LOW = transistor off = SENSE line pulled HIGH (`5VISO`) by pull up resistor
+    * `MCU_SENSE_OUT_3V3` HIGH = transistor on = SENSE line pulled LOW (`GNDISO`) by transistor
+
+* T11 - `MCU_SENSE_IN_3V3`
+    * Level shifted version of `KFD_SENSE_IN`
+
+* T12 - `KFD_DATA_IN`
+    * Target DATA signal
+
+* T13 - `KFD_SENSE_IN`
+    * Target SENSE signal
+
+* T14 - `GNDISO`
+    * Target isolated ground
+
+* T15 - `5VISO`
+    * Target isolated 5V DC power rail
+
+External Logic Analyzer
+-----------------------
+
+A external logic analyzer can be connected to monitor the TWI signals independent of the KFDtool. The TWI physical protocol is close to a UART signal, and therefore UART protocol decoders of logic analyzers can be used. The following screenshots are of the Saleae logic analyzer software.
+
+* Ground - connected to T3
+* Channel 00 "GPIO1" - connected to T6
+* Channel 01 "GPIO2" - connected to T7
+* Channel 02 "DATA_OUT" - connected to T8
+* Channel 03 "DATA_IN" - connected to T9
+* Channel 04 "SENSE_OUT" - connected to T10
+* Channel 05 "SENSE_IN" - connected to T11
+* Trigger - Channel 03 "DATA_IN" falling edge
+
+![image](pic/logic_data.png)
+
+Async serial (UART) analyzer settings:
+
+![image](pic/logic_settings.png)