Implement double reset detection

Closes #5

src/CMakeLists.txt

@@ -20,6 +20,7 @@ file(RENAME fsdata.c my_fsdata.c)
 
 add_executable(${PROGRAM_NAME}
         main.cpp
+        double_reset_detector.cpp
         )
 target_compile_definitions(${PROGRAM_NAME} PRIVATE
         WIFI_SSID=\"${WIFI_SSID}\"

src/double_reset_detector.cpp

@@ -0,0 +1,58 @@
+// Double reset logic based on pico_bootsel_via_double_reset.c from pico sdk
+
+#include "pico.h"
+#include "pico/time.h"
+#include "pico/bootrom.h"
+#include "pico/binary_info.h"
+#include "pico/stdlib.h"
+
+namespace double_reset_detector {
+
+bool double_reset_detected = false;
+
+// 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 500
+#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 AP mode if detected
+ *
+ * This function is registered to run automatically before main(). The
+ * algorithm is:
+ *
+ *   1. Check for magic token in memory; enter AP 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
+    magic_location[0] = 0;
+    double_reset_detected = true;
+}
+
+#endif
+} // namespace double_reset_detector

src/main.cpp

@@ -7,26 +7,12 @@
 
 #include "lwipopts.h"
 #include "ssi.h"
-
-void run_server() {
-    httpd_init();
-    ssi_init();
-    printf("Http server initialized.\n");
-    // infinite loop for now
-    for (;;) {}
+namespace double_reset_detector {
+    extern bool double_reset_detected;
 }
 
-int main() {
-    stdio_init_all();
-
-    if (cyw43_arch_init()) {
-        printf("failed to initialise\n");
-        return 1;
-    }
-    // turn on LED to distinguish from BOOTSEL mode
-    cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, 1);
-
-    cyw43_arch_enable_sta_mode();
+void connect_to_wifi() {
+     cyw43_arch_enable_sta_mode();
     // this seems to be the best be can do using the predefined `cyw43_pm_value` macro:
     // cyw43_wifi_pm(&cyw43_state, CYW43_PERFORMANCE_PM);
     // however it doesn't use the `CYW43_NO_POWERSAVE_MODE` value, so we do this instead:
@@ -43,6 +29,39 @@ int main() {
         auto ip_addr = cyw43_state.netif[CYW43_ITF_STA].ip_addr.addr;
         printf("IP Address: %lu.%lu.%lu.%lu\n", ip_addr & 0xFF, (ip_addr >> 8) & 0xFF, (ip_addr >> 16) & 0xFF, ip_addr >> 24);
     }
-
+}
+
+void run_server() {
+    connect_to_wifi();
+    httpd_init();
+    ssi_init();
+    printf("Http server initialized.\n");
+    // infinite loop for now
+    for (;;) {}
+}
+
+void setup() {
+    if (double_reset_detector::double_reset_detected) {
+        printf("Double reset detected!");
+        // just blink the LED for now
+        while (true) {
+            cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, 1);
+            sleep_ms(500);
+            cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, 0);
+            sleep_ms(500);
+        }
+    }
+}
+
+int main() {
+    stdio_init_all();
+
+    if (cyw43_arch_init()) {
+        printf("failed to initialise\n");
+        return 1;
+    }
+    // turn on LED to distinguish from BOOTSEL mode
+    cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, 1);
+    setup();
     run_server();
 }