doc: added documentation for ethernet connection process

docs/ethernet.md

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+# Connection via Ethernet
+
+As we will delegate all the ML processing to the Jetson Nano, while keeping the control of the car and the Instrument Cluster on the Raspberry, we needed a way to efficiently communicate between them. As CAN has a maximum rate of 500kbs, we chose to try to connect via Ethernet. This document describes the steps performed to achieve it.
+
+## Setting up static IPs
+
+The static IPs for the devices are:
+
+* Raspberry Pi: 192.168.1.10/24
+* Jetson Nano: 192.168.20/24
+
+And the gateway IP is 192.168.1.1
+
+Both devices have the Network Manager installed, which eased the process. To configure the static IPv4, the following commands were used (raspberry pi example):
+
+```
+sudo nmcli c mod "Wired connection 1" ipv4.addresses 192.168.1.10/24 ipv4.method manual
+sudo nmcli con mod "Wired connection 1" ipv4.gateway 192.168.1.1
+sudo nmcli c down "Wired connection 1" && sudo nmcli c up "Wired connection 1"
+```
+
+After running these commands, pinging those IPs from either machine start working.
+
+## Installing ZeroMQ on Jetson Nano
+
+```
+echo 'deb http://download.opensuse.org/repositories/network:/messaging:/zeromq:/release-stable/xUbuntu_18.04/ /' | sudo tee /etc/apt/sources.list.d/network:messaging:zeromq:release-stable.list
+curl -fsSL https://download.opensuse.org/repositories/network:messaging:zeromq:release-stable/xUbuntu_18.04/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/network_messaging_zeromq_release-stable.gpg > /dev/null
+sudo apt update
+sudo apt install libzmq3-dev
+```
+
+## ZeroMQ Test
+
+Small POC programs were used just to test the functionality.
+
+### Publisher (Jetson Nano)
+
+```
+#include <zmq.h>
+#include <assert.h>
+
+int main(int argc, char **argv)
+{
+
+        // Connect to zmq
+        void *context = zmq_ctx_new ();
+        void *publisher = zmq_socket (context, ZMQ_PUB);
+        int rc = zmq_bind(publisher, "tcp://*:5556");
+        assert (rc == 0);
+        // Give some time for the subscribers to connect
+        sleep(1);
+        unsigned char message[4] = {1,0,0xE8,0x00};
+        rc = zmq_send(publisher, message, 4, 0);
+        assert(rc == 4);
+
+        zmq_close(publisher);
+        zmq_ctx_destroy(context);
+
+        return 0;
+}
+```
+
+### Subscriber (Raspberry Pi)
+
+```
+#include <stdio.h>
+#include <assert.h>
+#include <signal.h>
+#include <zmq.h>
+
+static volatile sig_atomic_t keep_running = 1;
+
+static void sig_handler(int _)
+{
+    (void)_;
+    keep_running = 0;
+}
+
+int main()
+{
+    signal(SIGINT, sig_handler);
+    signal(SIGTERM, sig_handler);
+
+    void *context = zmq_ctx_new();
+    void *subscriber = zmq_socket(context, ZMQ_SUB);
+    int rc = zmq_connect(subscriber, "tcp://192.168.1.20:5556");
+    assert(rc == 0);
+    rc = zmq_setsockopt(subscriber, ZMQ_SUBSCRIBE, "", 0);
+    assert(rc == 0);
+    printf("Waiting for messages\n");
+    unsigned char message[8];
+
+    while(keep_running)
+    {
+        rc = zmq_recv(subscriber, message, 8, 0);
+        if (rc == -1) continue;
+
+        // Extract speed
+        unsigned char speed = message[0];
+
+        // Extract unit
+        //unsigned char unit = message[1];
+
+        // Extract RPM (combine the two bytes)
+        unsigned short rpm = (message[3] << 8) | message[2];
+
+        // Print the extracted values
+        printf("\rSpeed: %03d km/h -- RPM:%04d", speed, rpm);
+        fflush(stdout);
+    }
+
+    zmq_close(subscriber);
+    zmq_ctx_destroy(context);
+
+    return 0;
+}
+```
+
+### Compilation
+
+To compile the programs run
+
+```
+gcc <filename> -o <output_name> -lzmq
+```