Add RTC example

examples/basic/rtc/.gitignore

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+.rebar3
+_*
+.eunit
+*.o
+*.beam
+*.plt
+*.swp
+*.swo
+.erlang.cookie
+ebin
+log
+erl_crash.dump
+.rebar
+logs
+_build
+.idea
+*.iml
+rebar3.crashdump
+*~

examples/basic/rtc/README.md

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+M5 RTC sample code
+==================
+
+![Sample code running on M5 Stick C Plus](./rtc.jpg)
+
+This sample code illustrates how to use RTC and perform synchronization with NTP.
+
+Usage
+-----
+
+- Install rebar3.
+- Copy `src/config.hrl-template` to `src/config.hrl`
+- Edit `src/config.hrl` with your Wifi credentials
+- Connect a M5 device with AtomVM (VM and library) preinstalled.
+- Compile and flash with:
+
+```
+rebar3 esp32_flash -p /dev/tty.usbserial-*
+```

examples/basic/rtc/rebar.config

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+{erl_opts, [debug_info]}.
+{deps, [
+    {atomvm_m5, {path, "../../../"}}
+]}.
+{plugins, [
+    rebar3_path_deps,
+    {atomvm_rebar3_plugin, {git, "https://github.com/atomvm/atomvm_rebar3_plugin.git", {branch, "master"}}}
+]}.

examples/basic/rtc/src/.gitignore

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+config.hrl

examples/basic/rtc/src/config.hrl-template

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+% Define Wifi settings
+
+-define(WIFI_SSID, "ssid").
+-define(WIFI_PSK, "psk").

examples/basic/rtc/src/rtc.app.src

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+{application, rtc,
+ [{description, "AtomVM M5 RTC example"},
+  {vsn, "0.1.0"},
+  {registered, []},
+  {applications,
+   [kernel,
+    stdlib
+   ]},
+  {env,[]},
+  {modules, []},
+  {licenses, ["MIT"]},
+  {links, []}
+ ]}.

examples/basic/rtc/src/rtc.erl

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+%% @doc RTC sample code
+-module(rtc).
+
+-export([start/0]).
+-include("config.hrl").
+
+start() ->
+    m5:begin_([{clear_display, true}, {output_power, false}, {internal_mic, false}]),
+    case m5_rtc:is_enabled() of
+        true ->
+            println("RTC found"),
+            display_rtc_time(),
+            println("Starting network"),
+            ok = start_network(),
+            wait_for_wifi();
+        false ->
+            io:format("RTC not found.\n"),
+            m5_display:print(<<"RTC not found.">>)
+    end.
+
+start_network() ->
+    Parent = self(),
+    WifiConfig = [
+        {sta, [
+            {connected, fun() -> Parent ! {wifi, connected} end},
+            {got_ip, fun(_Addr) -> Parent ! {wifi, got_ip} end},
+            {ssid, ?WIFI_SSID},
+            {psk, ?WIFI_PSK}
+        ]},
+        {sntp, [{host, "pool.ntp.org"}, {synchronized, fun(Timeval) -> Parent ! {ntp, Timeval} end}]}
+    ],
+    case network:start(WifiConfig) of
+        {ok, _Pid} ->
+            println("Network started"),
+            ok;
+        Error ->
+            println(io_lib:format("Network start failed:\n~p", [Error])),
+            Error
+    end.
+
+wait_for_wifi() ->
+    println("Wait for wifi"),
+    receive
+        {wifi, connected} ->
+            wait_for_ip();
+        Other ->
+            println(io_lib:format("Unexpected message:\n~p", [Other])),
+            wait_for_wifi()
+    after 10000 ->
+        io:format("Still waiting for wifi\n"),
+        wait_for_wifi()
+    end.
+
+wait_for_ip() ->
+    println("Wait for IP"),
+    receive
+        {wifi, got_ip} ->
+            wait_for_ntp();
+        Other ->
+            println(io_lib:format("Unexpected message:\n~p", [Other])),
+            wait_for_ip()
+    after 10000 ->
+        io:format("Still waiting for IP\n"),
+        wait_for_ntp()
+    end.
+
+wait_for_ntp() ->
+    println("Wait for NTP"),
+    receive
+        {ntp, Timeval} ->
+            println(io_lib:format("Synchronized:\n~p", [Timeval])),
+            set_rtc();
+        Other ->
+            println(io_lib:format("Unexpected message:\n~p", [Other])),
+            wait_for_ntp()
+    after 10000 ->
+        io:format("Still waiting for NTP\n"),
+        wait_for_ntp()
+    end.
+
+set_rtc() ->
+    io:format("Setting M5 RTC to : ~p\n", [erlang:universaltime()]),
+    m5_rtc:set_datetime(erlang:universaltime()),
+    display_rtc_time().
+
+display_rtc_time() ->
+    io:format("M5 RTC time is now : ~p\n", [m5_rtc:get_datetime()]),
+    DateTimeStr = iolist_to_binary(io_lib:format("RTC time:\n~p", [m5_rtc:get_datetime()])),
+    m5_display:println(DateTimeStr).
+
+println(IOData) ->
+    io:format("~s\n", [IOData]),
+    m5_display:println(IOData).