arm: add uart echo app

arm/main.cc

@@ -1,13 +1,55 @@
 #include <cstdint>
 
+#include "xuartlite.h"
+
 struct Gpio {
     volatile uint32_t data;
 };
 
 #define gpio0 ((Gpio*) 0x40000000)
 
-int main() {
-    gpio0->data = 42;
+namespace {
 
-    while (true) {}
+constexpr uintptr_t kUart0BaseAddress = 0x40001000;
+XUartLite uart0;
+XUartLite_Config uart0_config = {
+    .DeviceId = 0,
+    .RegBaseAddr = kUart0BaseAddress,
+    .BaudRate = 115200,
+    .UseParity = false,
+    .DataBits = 8,
+};
+
+[[maybe_unused]]
+void sleep(int ms) {
+    for (int i = 0; i < ms; i++) {
+        // sleep for 1 ms
+        for (int j = 0; j < 50000; j++) {
+            asm("");
+        }
+    }
 }
+
+}  // namespace
+
+int main() {
+    int recv = 0;
+    gpio0->data = 37;
+
+    XUartLite_CfgInitialize(&uart0, &uart0_config, uart0_config.RegBaseAddr);
+
+    while (true) {
+        uint8_t c;
+        int received_bytes = XUartLite_Recv(&uart0, &c, 1);
+        if (received_bytes < 1) {
+            continue;
+        }
+        recv++;
+        gpio0->data = recv & 0xff;
+        XUartLite_Send(&uart0, &c, 1);
+        if (c == '\r') {
+            c = '\n';
+            XUartLite_Send(&uart0, &c, 1);
+        }
+    }
+}