Merge pull request adafruit#47 from stevenj/master

Proposal to allow a GPIO to hold the bootloader from running the app, if its in the correct state

README.md

@@ -188,6 +188,30 @@ Thus, it's best to set the USB ID to one for which there are no drivers.
 
 The bootloader sits at 0x00000000, and the application starts at 0x00002000 (SAMD21) or 0x00004000 (SAMD51).
 
+#### Bootloader Hold Switch
+
+A Board can be configured to sense the state of a GPIO and to hold the bootloader from running an application if it is in the appropriate state.  To enable this, add the following to your board configuration header file.
+
+```C
+// The Pin that will tell us to stay in the bootloader or not.
+#define HOLD_PIN PIN_PA02
+
+// Optional, define if a Pull up or pulldown is needed.
+#define HOLD_PIN_PULLUP
+//#define HOLD_PIN_PULLDOWN
+
+// What is the Hold state of the GPIO, 0 or 1.
+#define HOLD_STATE 1
+```
+
+Set `HOLD_PIN` to the appropriate GPIO and `HOLD_STATE` to the logic level which will hold the bootloader from running the application.
+
+The definition of _BOTH_ `HOLD_PIN` and `HOLD_STATE` triggers the inclusion of this feature. If either of these is undefined, this feature is not enabled.
+
+If an internal pullup/pulldown is required for the IO, it can be enabled with the _OPTIONAL_ `HOLD_PIN_PULLUP` or `HOLD_PIN_PULLDOWN` macros.  If neither are defined, then no pullup/pulldown will be enabled for the io pin.
+
+This switch is NOT dynamic.  Once the bootloader has sensed this pin and decided not to run the application, then a change in this IO will not, itself, then cause the Application to run, without also resetting the board.
+
 ## Code of Conduct
 
 This project has adopted the [Microsoft Open Source Code of Conduct](https://opensource.microsoft.com/codeofconduct/). For more information see the [Code of Conduct FAQ](https://opensource.microsoft.com/codeofconduct/faq/) or contact [[email protected]](mailto:[email protected]) with any additional questions or comments.

inc/uf2.h

@@ -265,6 +265,9 @@ void system_init(void);
 #define LED_TICK led_tick
 
 #define PINOP(pin, OP) (PORT->Group[(pin) / 32].OP.reg = (1 << ((pin) % 32)))
+#define PINIP(pin) (((PORT->Group[(pin) / 32].IN.reg) >> ((pin) % 32)) & 0x1)
+#define PINCFG(pin) (PORT->Group[(pin) / 32].PINCFG[(pin) % 32].reg)
+#define PINMUX(pin) (PORT->Group[(pin) / 32].PMUX[((pin) % 32)/2].reg)
 
 void led_tick(void);
 void led_signal(void);

src/main.c

@@ -94,6 +94,30 @@ extern int8_t led_tick_step;
 static void check_start_application(void) {
     uint32_t app_start_address;
 
+// Check if there is an IO which will hold us inside the bootloader.
+#if defined(HOLD_PIN) && defined(HOLD_STATE)
+    PORT_PINCFG_Type pincfg = {0};
+    pincfg.bit.PMUXEN = false;
+    pincfg.bit.INEN   = true;
+    pincfg.bit.DRVSTR = true;
+
+    PINOP(HOLD_PIN, DIRCLR);        // Pin is an input
+
+  #if defined(HOLD_PIN_PULLUP)
+    pincfg.bit.PULLEN = true;
+    PINOP(HOLD_PIN, OUTSET); // Pin is pulled up.
+  #elif defined(HOLD_PIN_PULLDOWN)
+    pincfg.bit.PULLEN = true;
+    PINOP(HOLD_PIN, OUTCLR); // Pin is pulled up.
+  #endif
+    PINCFG(HOLD_PIN) = pincfg.reg;
+
+    if (PINIP(HOLD_PIN) == HOLD_STATE) {
+        /* Stay in bootloader */
+        return;
+    }
+#endif
+
     /* Load the Reset Handler address of the application */
     app_start_address = *(uint32_t *)(APP_START_ADDRESS + 4);