add more info in readme

README.md

@@ -202,6 +202,44 @@ As `0x1b02` is called from the main BSL code as part of the "mass-erase FRAM"
 command, it most likely implements this functionality, and hardly anything
 else.
 
+### Raspberry Pico NMI signal generator
+
+To generate the well-timed `NMI` signals, a Raspberry Pico is used. From a
+trigger signal from P1.4 to GP14, it will wait a specific amount of time, then
+lower GP15, which should be connected to NMI/#RST, for a few microseconds. This
+delay between the trigger and NMI is configured over a UART interface on P6.0
+and GP17. P1.5/GP16 is used as an "ack" signal from the Pico to the MSP430 to
+signal that the serial command has been received and processed. This is done to
+avoid having the MSP430 start sending a trigger signal before a new delay
+setting has been applied properly.
+
+Some other pins are used as handshaking: GP19 is connected to P3.2 to have the
+MSP430 wait until this pin is high to start tracing, which is needed because
+the NMI pin is also shared between the actual NMI signal, and the Spy-Bi-Wire
+debugging interface, which is used to upload new code. Using a switch to select
+which line (SBWTDIO or NMI/GP15) is connected to the NMI/#RESET pin, this can
+be mitigated. However, then P3.2 needs to be also pulled low for as long as the
+NMI/#RST pin is connected to SBWTDIO. For this, another switch can be used to
+have it toggle between GND and GP19/3V3.
+
+The Raspberry Pico code can be found in the `nmigen/` folder (not to be
+confused with the HDL language that had this name in the past).
+
+Full connection table:
+
+| MSP430 pin | Intermediate pin | Raspberry Pico pin |
+|:---------- |:---------------- |:------------------ |
+| GND        |                  | GND                |
+| P1.4       |                  | GP14               |
+| P1.5       |                  | GP16               |
+|            | SW1.LEFT         | GP19 or 3V3        |
+| P3.2       | SW1.MID          |                    |
+| GND        | SW1.RIGHT        | GND                |
+| P6.0       |                  | GP17               |
+|            | SW2.LEFT         | GP15               |
+| NMI/#RST   | SW2.MID          |                    |
+| SBWTDIO (from eZ-FET) | SW2.RIGHT |                |
+
 ## Proof of concept
 
 The code in `src/main.c` will dump the content of the BSL to `eUSCI_A0` in UART