Allow double-reset to jump to USB bootloader

Call `rp2040.enableDoubleResetBootloader()` anywhere in the code to
enable the check.  W/o that call, the checker will be linked in.

See #892

Author: earlephilhower

cores/rp2040/RP2040Support.h

@@ -314,6 +314,8 @@ class RP2040 {
         reboot();
     }
 
+    static void enableDoubleResetBootloader();
+
     void wdt_begin(uint32_t delay_ms) {
         watchdog_enable(delay_ms, 1);
     }

cores/rp2040/sdkoverride/pico_bootsel_via_double_reset.c

@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2021 Raspberry Pi (Trading) Ltd.
+ *
+ * Hacked by EFP3 to allow disabling unless requested
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include "pico.h"
+#include "pico/time.h"
+#include "pico/bootrom.h"
+#include "pico/binary_info.h"
+
+// PICO_CONFIG: PICO_BOOTSEL_VIA_DOUBLE_RESET_TIMEOUT_MS, Window of opportunity for a second press of a reset button to enter BOOTSEL mode (milliseconds), type=int, default=200, group=pico_bootsel_via_double_reset
+#ifndef PICO_BOOTSEL_VIA_DOUBLE_RESET_TIMEOUT_MS
+#define PICO_BOOTSEL_VIA_DOUBLE_RESET_TIMEOUT_MS 200
+#endif
+
+// PICO_CONFIG: PICO_BOOTSEL_VIA_DOUBLE_RESET_ACTIVITY_LED, Optionally define a pin to use as bootloader activity LED when BOOTSEL mode is entered via reset double tap, type=int, min=0, max=29, group=pico_bootsel_via_double_reset
+
+// PICO_CONFIG: PICO_BOOTSEL_VIA_DOUBLE_RESET_INTERFACE_DISABLE_MASK, Optionally disable either the mass storage interface (bit 0) or the PICOBOOT interface (bit 1) when entering BOOTSEL mode via double reset, type=int, min=0, max=3, default=0, group=pico_bootsel_via_double_reset
+#ifndef PICO_BOOTSEL_VIA_DOUBLE_RESET_INTERFACE_DISABLE_MASK
+#define PICO_BOOTSEL_VIA_DOUBLE_RESET_INTERFACE_DISABLE_MASK 0u
+#endif
+
+/** \defgroup pico_bootsel_via_double_reset pico_bootsel_via_double_reset
+ *
+ * When the 'pico_bootsel_via_double_reset' library is linked, a function is
+ * injected before main() which will detect when the system has been reset
+ * twice in quick succession, and enter the USB ROM bootloader (BOOTSEL mode)
+ * when this happens. This allows a double tap of a reset button on a
+ * development board to be used to enter the ROM bootloader, provided this
+ * library is always linked.
+ */
+
+#if !PICO_NO_BI_BOOTSEL_VIA_DOUBLE_RESET
+bi_decl(bi_program_feature("double reset -> BOOTSEL"));
+#endif
+
+// Doesn't make any sense for a RAM only binary
+#if !PICO_NO_FLASH
+static const uint32_t magic_token[] = {
+        0xf01681de, 0xbd729b29, 0xd359be7a,
+};
+
+static uint32_t __uninitialized_ram(magic_location)[count_of(magic_token)];
+
+/* Check for double reset and enter BOOTSEL mode if detected
+ *
+ * This function is registered to run automatically before main(). The
+ * algorithm is:
+ *
+ *   1. Check for magic token in memory; enter BOOTSEL mode if found.
+ *   2. Initialise that memory with that magic token.
+ *   3. Do nothing for a short while (few hundred ms).
+ *   4. Clear the magic token.
+ *   5. Continue with normal boot.
+ *
+ * Resetting the device twice quickly will interrupt step 3, leaving the token
+ * in place so that the second boot will go to the bootloader.
+ */
+
+/*static*/ void __attribute__((constructor)) boot_double_tap_check(void) {
+    for (uint i = 0; i < count_of(magic_token); i++) {
+        if (magic_location[i] != magic_token[i]) {
+            // Arm, wait, then disarm and continue booting
+            for (i = 0; i < count_of(magic_token); i++) {
+                magic_location[i] = magic_token[i];
+            }
+            busy_wait_us(PICO_BOOTSEL_VIA_DOUBLE_RESET_TIMEOUT_MS * 1000);
+            magic_location[0] = 0;
+            return;
+        }
+    }
+    // Detected a double reset, so enter USB bootloader
+    magic_location[0] = 0;
+#ifdef PICO_BOOTSEL_VIA_DOUBLE_RESET_ACTIVITY_LED
+    const uint32_t led_mask = 1u << PICO_BOOTSEL_VIA_DOUBLE_RESET_ACTIVITY_LED;
+#else
+    const uint32_t led_mask = 0u;
+#endif
+    reset_usb_boot(
+        led_mask,
+        PICO_BOOTSEL_VIA_DOUBLE_RESET_INTERFACE_DISABLE_MASK
+    );
+}
+
+#endif

docs/rp2040.rst

@@ -70,3 +70,12 @@ int rp2040.getTotalHeap()
 Returns the total heap that was available to this program at compile time (i.e.
 the Pico RAM size minus things like the ``.data`` and ``.bss`` sections and other
 overhead).
+
+Bootloader
+----------
+
+void rp2040.enableDoubleResetBootloader()
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Add a call anywhere in the sketch to ``rp2040.enableDoubleResetBootloader()`` and
+the core will check for a double-tap on reset, and if found will start the USB
+bootloader.

keywords.txt

@@ -54,6 +54,8 @@ SerialPIO	KEYWORD2
 setFIFOSize	KEYWORD2
 setPollingMode	KEYWORD2
 
+enableDoubleResetBootloader	KEYWORD2
+
 OUTPUT_2MA	LITERAL1
 OUTPUT_4MA	LITERAL1
 OUTPUT_8MA	LITERAL1