Add BOOTSEL, allowing BOOTSEL use as a button

Since every board has a bootsel button, allow end users to read its
state with a simple `if (BOOTSEL)`.  Uses code from the pico-examples.

cores/rp2040/Arduino.h

@@ -59,7 +59,6 @@ void delay(unsigned long);
 void delayMicroseconds(unsigned int us);
 unsigned long millis();
 
-
 #ifdef __cplusplus
 } // extern "C"
 #endif
@@ -68,6 +67,7 @@ unsigned long millis();
 #include "SerialUSB.h"
 #include "SerialUART.h"
 #include "RP2040.h"
+#include "Bootsel.h"
 
 // Template which will evaluate at *compile time* to a single 32b number
 // with the specified bits set.

cores/rp2040/Bootsel.cpp

@@ -0,0 +1,58 @@
+/**
+ * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+#include <Arduino.h>
+#include "pico/stdlib.h"
+#include "hardware/gpio.h"
+#include "hardware/sync.h"
+#include "hardware/structs/ioqspi.h"
+#include "hardware/structs/sio.h"
+
+// This example blinks the Pico LED when the BOOTSEL button is pressed.
+//
+// Picoboard has a button attached to the flash CS pin, which the bootrom
+// checks, and jumps straight to the USB bootcode if the button is pressed
+// (pulling flash CS low). We can check this pin in by jumping to some code in
+// SRAM (so that the XIP interface is not required), floating the flash CS
+// pin, and observing whether it is pulled low.
+//
+// This doesn't work if others are trying to access flash at the same time,
+// e.g. XIP streamer, or the other core.
+
+static bool __no_inline_not_in_flash_func(get_bootsel_button)() {
+    const uint CS_PIN_INDEX = 1;
+
+    // Must disable interrupts, as interrupt handlers may be in flash, and we
+    // are about to temporarily disable flash access!
+    uint32_t flags = save_and_disable_interrupts();
+
+    // Set chip select to Hi-Z
+    hw_write_masked(&ioqspi_hw->io[CS_PIN_INDEX].ctrl,
+                    GPIO_OVERRIDE_LOW << IO_QSPI_GPIO_QSPI_SS_CTRL_OEOVER_LSB,
+                    IO_QSPI_GPIO_QSPI_SS_CTRL_OEOVER_BITS);
+
+    // Note we can't call into any sleep functions in flash right now
+    for (volatile int i = 0; i < 1000; ++i);
+
+    // The HI GPIO registers in SIO can observe and control the 6 QSPI pins.
+    // Note the button pulls the pin *low* when pressed.
+    bool button_state = !(sio_hw->gpio_hi_in & (1u << CS_PIN_INDEX));
+
+    // Need to restore the state of chip select, else we are going to have a
+    // bad time when we return to code in flash!
+    hw_write_masked(&ioqspi_hw->io[CS_PIN_INDEX].ctrl,
+                    GPIO_OVERRIDE_NORMAL << IO_QSPI_GPIO_QSPI_SS_CTRL_OEOVER_LSB,
+                    IO_QSPI_GPIO_QSPI_SS_CTRL_OEOVER_BITS);
+
+    restore_interrupts(flags);
+
+    return button_state;
+}
+
+__Bootsel::operator bool() {
+    return get_bootsel_button();
+}
+
+__Bootsel BOOTSEL;

cores/rp2040/Bootsel.h

@@ -0,0 +1,29 @@
+/*
+ * Simple BOOTSEL reader object
+ *
+ * Copyright (c) 2021 Earle F. Philhower, III <earlephilhower@yahoo.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+#pragma once
+
+class __Bootsel {
+public:
+    __Bootsel() { }
+    operator bool();
+};
+
+extern __Bootsel BOOTSEL;

libraries/rp2040/examples/Bootsel/Bootsel.ino

@@ -0,0 +1,17 @@
+/* Simple sketch to do something on a BOOTSEL press */
+/* Releaed into the public domain */
+
+void setup() {
+  Serial.begin(115200);
+  delay(5000);
+  Serial.println("I dare you to hit the BOOTSEL button...");
+}
+
+int c = 0;
+void loop() {
+  if (BOOTSEL) {
+    Serial.printf("\a\aYou pressed BOOTSEL %d times!\n", ++c);
+    // Wait for BOOTSEL to be released
+    while (BOOTSEL) { delay(1); }
+  }
+}

libraries/rp2040/keywords.txt

@@ -0,0 +1 @@
+BOOTSEL	KEYWORD1