Add pico_bootsel_via_double_reset library, fix #87

src/rp2_common/CMakeLists.txt

@@ -36,6 +36,7 @@ if (NOT PICO_BARE_METAL)
     # NOTE THE ORDERING HERE IS IMPORTANT AS SOME TARGETS CHECK ON EXISTENCE OF OTHER TARGETS
     pico_add_subdirectory(boot_stage2)
 
+    pico_add_subdirectory(pico_bootsel_via_double_reset)
     pico_add_subdirectory(pico_multicore)
     pico_add_subdirectory(pico_unique_id)
 

src/rp2_common/pico_bootsel_via_double_reset/CMakeLists.txt

@@ -0,0 +1,10 @@
+add_library(pico_bootsel_via_double_reset INTERFACE)
+
+target_sources(pico_bootsel_via_double_reset INTERFACE
+        ${CMAKE_CURRENT_LIST_DIR}/pico_bootsel_via_double_reset.c
+        )
+
+target_link_libraries(pico_bootsel_via_double_reset INTERFACE
+        pico_bootrom
+        pico_time
+        )

src/rp2_common/pico_bootsel_via_double_reset/pico_bootsel_via_double_reset.c

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2021 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include "pico.h"
+#include "pico/time.h"
+#include "pico/bootrom.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, GPIO to use as bootloader activity LED when BOOTSEL mode is entered via reset double tap (or -1 for none), type=int, default=-1, group=pico_bootsel_via_double_reset
+#ifndef PICO_BOOTSEL_VIA_DOUBLE_RESET_ACTIVITY_LED
+#define PICO_BOOTSEL_VIA_DOUBLE_RESET_ACTIVITY_LED -1
+#endif
+
+// 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, 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
+ *
+ * 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.
+ */
+
+// 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)];
+
+/*! \brief Check for double reset and enter BOOTSEL mode if detected
+ *  \ingroup pico_bootsel_via_double_reset
+ *
+ * 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;
+    uint32_t led_mask = PICO_BOOTSEL_VIA_DOUBLE_RESET_ACTIVITY_LED >= 0 ?
+        1u << PICO_BOOTSEL_VIA_DOUBLE_RESET_ACTIVITY_LED : 0u;
+    reset_usb_boot(
+        led_mask,
+        PICO_BOOTSEL_VIA_DOUBLE_RESET_INTERFACE_DISABLE_MASK
+    );
+}
+
+#endif

test/kitchen_sink/CMakeLists.txt

@@ -27,6 +27,7 @@ target_link_libraries(kitchen_sink_libs INTERFACE
     hardware_xosc
     pico_bit_ops
     pico_bootrom
+    pico_bootsel_via_double_reset
     pico_divider
     pico_double
     pico_fix_rp2040_usb_device_enumeration