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+Mode 3 provides a tool to scan for JTAG and SWD pinouts on a board.
+
+The implementation is lifted from [JTAGenum](https://github.com/cyphunk/JTAGenum/blob/master/JTAGenum.ino) and [SWDscan](https://github.com/szymonh/SWDscan/blob/master/src/main.cpp), for which we are very grateful.
+
+## Pinout
+
+Any pin marked as "test pin" below can be probed. Actual pins probed depends on the command used to scan, see [dmctl](https://git.lain.faith/sys64738/DapperMime-JTAG/wiki/dmctl#jtag-scan).
+
+### Raspberry Pico
+
+On the Raspberry Pico, 20 pins can be used:
+
+| Pin number | Usage          | Usage          | Pin number |
+|:---------- |:-------------- | --------------:| ----------:|
+| GP0        | stdio UART TX  |                | VBUS       |
+| GP1        | stdio UART RX  |                | VSYS       |
+| GND        | <ground> | <ground> | GND        |
+| GP2        | Test pin       |                | 3V3 EN     |
+| GP3        | Test pin       |                | 3V3 OUT    |
+| GP4        | Test pin       |                | ADC VREF   |
+| GP5        | Test pin       |                | GP28 / ADC2|
+| GND        | <ground> | <ground> | GND  / AGND|
+| GP6        | Test pin       |                | GP27 / ADC1|
+| GP7        | Test pin       |                | GP26 / ADC0|
+| GP8        | Test pin       |                | RUN        |
+| GP9        | Test pin       | Test pin       | GP22       |
+| GND        | <ground> | <ground> | GND        |
+| GP10       | Test pin       | Test pin       | GP21       |
+| GP11       | Test pin       | Test pin       | GP20       |
+| GP12       | Test pin       | Test pin       | GP19       |
+| GP13       | Test pin       | Test pin       | GP18       |
+| GND        | <ground> | <ground> | GND        |
+| GP14       | Test pin       | Test pin       | GP17       |
+| GP15       | Test pin       | Test pin       | GP16       |
+| <end>| <bottom> | <bottom> | <end>|
+
+## Usage
+
+This mode is controlled entirely through [dmctl](https://git.lain.faith/sys64738/DapperMime-JTAG/wiki/dmctl#jtag-scan).
+
+Time spent scanning is proportional to `n_pins_probed nCr n_pins_in_itf`, i.e. `n_pins_probed!/(n_pins_probed-n_pins_in_itf)!`. For JTAG, `n_pins_in_itf` is equal to 5, for SWD, it is 2. `n_pins_probed` depends on the `start` and `end` arguments passed to `dmctl`.