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add more info in readme

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Triss 2022-04-15 02:38:45 +02:00
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@ -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