variants: nano_connect: 1200 touch support

Signed-off-by: Gilberto Conti <g.conti@arduino.cc>

Author: gibix

variants/arduino_nano_rp2040_connect_rp2040/variant.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) Arduino s.r.l. and/or its affiliated companies
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include "Arduino.h"
+
+#define NANO_RP2040_LED_PIN 6
+
+void _on_1200_bps() {
+    /*
+     * Call reset_usb_boot() directly via the RP2040 ROM lookup table.
+     * This avoids depending on rom_reset_usb_boot from libpico (not
+     * exported to LLEXT sketches). The ROM is always at 0x00000000:
+     *   0x14: 16-bit offset of the function table
+     *   0x18: 16-bit offset of the table lookup function
+     * 'UB' is the 2-byte code for reset_usb_boot (ROM_FUNC_RESET_USB_BOOT).
+     */
+    typedef void *(*rom_lookup_fn)(uint16_t *table, uint32_t code);
+    typedef void __attribute__((noreturn)) (*reset_usb_boot_fn)(uint32_t, uint32_t);
+
+    rom_lookup_fn lookup = (rom_lookup_fn)(uintptr_t)(*(volatile uint16_t *)0x18u);
+    uint16_t *table      = (uint16_t *)(uintptr_t)(*(volatile uint16_t *)0x14u);
+
+    reset_usb_boot_fn fn = (reset_usb_boot_fn) lookup(table, ('U' | ('B' << 8)));
+    fn(1u << NANO_RP2040_LED_PIN, 0u);
+}