Add multicore support with setup1/loop1 (#113)

Support running code on the second core by adding a setup1() and/or
a loop1() routine to a sketch.  These functions operate exactly like
the normal Arduino ones, and anything they call will be run on
the second core automatically.

Add a simple multicore example.

cores/rp2040/main.cpp

@@ -20,6 +20,7 @@
 
 #include <Arduino.h>
 #include <pico/stdlib.h>
+#include <pico/multicore.h>
 
 extern void setup();
 extern void loop();
@@ -29,6 +30,21 @@ extern void loop();
 void initVariant() __attribute__((weak));
 void initVariant() { }
 
+
+// Optional 2nd core setup and loop
+extern void setup1() __attribute__((weak));
+extern void loop1() __attribute__((weak));
+static void main1() {
+    if (setup1) {
+        setup1();
+    }
+    while (true) {
+        if (loop1) {
+            loop1();
+        }
+    }
+}
+
 extern "C" int main() {
 #if F_CPU != 125000000
     set_sys_clock_khz(F_CPU / 1000, true);
@@ -45,8 +61,12 @@ extern "C" int main() {
     DEBUG_RP2040_PORT.begin();
 #endif
 
+    if (setup1 || loop1) {
+        multicore_launch_core1(main1);
+    }
+
     setup();
-    while (1) {
+    while (true) {
         loop();
         if (arduino::serialEventRun) {
             arduino::serialEventRun();

docs/index.rst

@@ -34,6 +34,8 @@ For the latest version, always check https://github.com/earlephilhower/arduino-p
 
    Ported/Optimized Libraries <libraries>
 
+   Multicore Processing <multicore>
+
    Using Pico-SDK <sdk>
 
    Licenses <license>

docs/multicore.rst

+Multicore Processing
+====================
+
+The RP2040 chip has 2 cores that can run independently of each other, sharing
+peripherals and memory with each other.  Arduino code will normally execute
+only on core 0, with the 2nd core sitting idle in a low power state.
+
+By adding a ``setup1()`` and ``loop1()`` function to your sketch you can make
+use of the second core.  Anything called from within the ``setup1()`` or
+``loop1()`` routines will execute on the second core.
+
+See the ``Multicore.ino`` example in the ``rp2040`` example directory for a
+quick introduction.

libraries/rp2040/examples/Multicore/Multicore.ino

+// Demonstrates a simple use of the setup1()/loop1() functions
+// for a multiprocessor run.
+
+// Will output something like, where C0 is running on core 0 and
+// C1 is on core 1, in parallel.
+
+// 11:23:07.507 -> C0: Blue leader standing by...
+// 11:23:07.507 -> C1: Red leader standing by...
+// 11:23:07.507 -> C1: Stay on target...
+// 11:23:08.008 -> C1: Stay on target...
+// 11:23:08.505 -> C0: Blue leader standing by...
+// 11:23:08.505 -> C1: Stay on target...
+// 11:23:09.007 -> C1: Stay on target...
+// 11:23:09.511 -> C0: Blue leader standing by...
+// 11:23:09.511 -> C1: Stay on target...
+// 11:23:10.015 -> C1: Stay on target...
+
+// Released to the public domain
+
+// The normal, core0 setup
+void setup() {
+  Serial.begin();
+  delay(5000);
+}
+
+void loop() {
+  Serial.printf("C0: Blue leader standing by...\n");
+  delay(1000);
+}
+
+// Running on core1
+void setup1() {
+  delay(5000);
+  Serial.printf("C1: Red leader standing by...\n");
+}
+
+void loop1() {
+  Serial.printf("C1: Stay on target...\n");
+  delay(500);
+}