diff --git a/src/GUI/MainWindow.cpp b/src/GUI/MainWindow.cpp
index ef7c39a..9798312 100644
--- a/src/GUI/MainWindow.cpp
+++ b/src/GUI/MainWindow.cpp
@@ -20,7 +20,9 @@ f32 MainWindow::getDPI(){
     return dpi;
 }
 
-void MainWindow::init(const char* window_title){
+void MainWindow::open(const char* window_title, UpdatingFunc _update){
+    update = _update;
+
     SDL_TRY(SDL_Init(SDL_INIT_EVERYTHING));
     SDL_version v;
     SDL_GetVersion(&v);
@@ -76,9 +78,9 @@ void MainWindow::init(const char* window_title){
 }
 
 // Wait, poll and handle events (inputs, window resize, etc.)
-void MainWindow::poll_events(u16& frame_updates_requested, bool wait){
+void MainWindow::poll_events(bool waitForEvent){
     SDL_Event event;
-    if(wait){
+    if(waitForEvent){
         // waits for first event in cpu-efficient way
         SDL_TRY(SDL_WaitEvent(&event) != 1); 
     }
@@ -87,8 +89,7 @@ void MainWindow::poll_events(u16& frame_updates_requested, bool wait){
         return;
     
     do {
-        if(ImGui_ImplSDL2_ProcessEvent(&event))
-            frame_updates_requested=2;
+        ImGui_ImplSDL2_ProcessEvent(&event);
         switch(event.type){
             case SDL_QUIT: {
                 close();
@@ -170,51 +171,34 @@ void MainWindow::draw_frame(){
     SDL_RenderPresent(sdl_renderer);
 }
 
-void MainWindow::startAndWait(){
+void MainWindow::startUpdateLoop(){
     if(loop_running)
-        throw UsefulException("loop is already running");
+        throw UsefulException("loop is running already");
 
-    // draw first frame
-    draw_frame();
-
-    u16 frame_updates_requested=1;
-    u64 prev_update_time_ms=SDL_GetTicks64();
+    nsec_t prev_update_time_ns = getMonotonicTimeNsec();
     loop_running=true;
     // main loop
     while(loop_running){
-        // waits for events
-        poll_events(frame_updates_requested, main_loop_wait_for_input);
+        poll_events(false);
+        
+        nsec_t update_time_ns = getMonotonicTimeNsec();
+        if(update_time_ns < prev_update_time_ns)
+            throw UsefulException("monotonic clock returned unexpected value");
+        f64 delta_time_s = (f64)(update_time_ns - prev_update_time_ns) / 1e9;
+        prev_update_time_ns = update_time_ns;
 
-        if(frame_updates_requested==0)
-        {
-            u64 update_time_ms=SDL_GetTicks64();
-            if(update_time_ms >= prev_update_time_ms + 1000/fps_min){
-                // if frame rate < fps_min then requests frame draw
-                // works only if main_loop_wait_for_input = false
-                frame_updates_requested=1;
-                prev_update_time_ms=update_time_ms;
-            }
-            else {
-                // skips frame rendering and waits to limit fps
-                u32 frame_delay_ms=1000/fps_max;
-                SDL_Delay(frame_delay_ms);
-                continue;
-            }
-        }
+        update(delta_time_s);
+        draw_frame();
 
-        // deaws requested number of frames
-        while(frame_updates_requested>0) {
-            draw_frame();
-            frame_updates_requested--;
+        nsec_t after_update_time_ns = getMonotonicTimeNsec();
+        nsec_t frame_delay_ns = (nsec_t)1e9 / fps_max - (after_update_time_ns - update_time_ns);
+        if(frame_delay_ns > 0){
+            std::cout<<"frameDelay: "<<frame_delay_ns / 1e9f<<std::endl;
+            SDL_Delay(frame_delay_ns / 1e6);
         }
     }
 
-    // Cleanup
-    MainWindow::destroy();
-}
-
-void MainWindow::close(){
-    loop_running=false;
+    destroy();
 }
 
 void MainWindow::destroy(){
@@ -226,6 +210,10 @@ void MainWindow::destroy(){
     SDL_Quit();
 }
 
+void MainWindow::close(){
+    loop_running = false;
+}
+
 void MainWindow::draw_bg_window(){
     const ImGuiDockNodeFlags dockspace_flags =
             ImGuiDockNodeFlags_PassthruCentralNode;
@@ -292,13 +280,12 @@ void MainWindow::draw_bg_window(){
 
 void MainWindow::draw_debug_window(){
     ImGuiIO& io = ImGui::GetIO();
-
     ImGui::Begin("Debug Options");
-    ImGui::ColorEdit3("clear_color", (float*)&clear_color);
-    ImGui::Checkbox("main_loop_wait_for_input", &main_loop_wait_for_input);
-    ImGui::Text("Application average %.3f ms/frame (%.2f FPS)", 1000.0f / io.Framerate, io.Framerate);
-    ImGui::Checkbox("Demo Window", &show_demo_window);
-    ImGui::Checkbox("Metrics/Debug Window", &show_metrics_window);
+        ImGui::ColorEdit3("clear_color", (float*)&clear_color);
+        ImGui::InputInt("fps_max", &fps_max);
+        ImGui::Text("Application average %.3f ms/frame (%.2f FPS)", 1000.0f / io.Framerate, io.Framerate);
+        ImGui::Checkbox("Demo Window", &show_demo_window);
+        ImGui::Checkbox("Metrics/Debug Window", &show_metrics_window);
     ImGui::End();
 
     if (show_demo_window)
diff --git a/src/GUI/MainWindow.hpp b/src/GUI/MainWindow.hpp
index 7e375b6..ad0a2b2 100644
--- a/src/GUI/MainWindow.hpp
+++ b/src/GUI/MainWindow.hpp
@@ -3,6 +3,7 @@
 #include <SDL.h>
 #include <imgui.h>
 #include "../std.hpp"
+#include "../time.hpp"
 
 /// converts hex color to float vector
 #define RGBAHexToF(R8,G8,B8,A8) ImVec4(((u8)35)/255.0f, ((u8)35)/255.0f, ((u8)50)/255.0f, ((u8)255)/255.0f)
@@ -13,30 +14,32 @@ namespace ougge::GUI {
 
 #define default_font "DroidSans"
 
+using UpdatingFunc = void (*)(f64 deltaTime);
+
 class MainWindow {
 public:
     ImVec4 clear_color = RGBAHexToF(35,35,50,255);
     f32 default_font_size = 14.0f;
-    u8 fps_min = 30;
-    u8 fps_max = 60;
+    int fps_max = 30;
+    // called on each frame
+    UpdatingFunc update = nullptr;
 
 private:
-    bool loop_running=false;
-    bool main_loop_wait_for_input=true;
+    bool loop_running = false;
     bool show_demo_window = false;
     bool show_metrics_window = false;
     SDL_Window* sdl_window = nullptr;
     SDL_Renderer* sdl_renderer = nullptr;
 
 public:
-    void init(const char* window_title);
-    void startAndWait();
+    void open(const char* window_title, UpdatingFunc update);
+    void startUpdateLoop();
     void close();
-    f32 getDPI();
 
 private:
     void destroy();
-    void poll_events(u16& frame_updates_requested, bool wait);
+    f32 getDPI();
+    void poll_events(bool waitForEvent);
     void draw_frame();
     void draw_ui();
     void draw_debug_window();
diff --git a/src/main.cpp b/src/main.cpp
index 540b7ea..1e862dd 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -8,15 +8,16 @@
 
 using namespace ougge;
 
-int main(int argc, const char** argv){
-    if(setlocale(LC_CTYPE, "C.UTF-8")!=0)
-        std::cerr<<"\e[93msetlocale failed!\n";
+void update(f64 deltaTime){
+    std::cout<<"deltaTime: "<<deltaTime<<std::endl;
+}
 
+int main(int argc, const char** argv){
     try {
         Resources::init();
         GUI::MainWindow w;
-        w.init("ougge");
-        w.startAndWait();
+        w.open("ougge", update);
+        w.startUpdateLoop();
     }
     catch(const std::exception& e){
         std::cerr<<"Catched exception: "<<e.what()<<std::endl;
diff --git a/src/time.cpp b/src/time.cpp
new file mode 100644
index 0000000..318b234
--- /dev/null
+++ b/src/time.cpp
@@ -0,0 +1,19 @@
+// posix version definition to use clock_gettime
+#ifndef _XOPEN_SOURCE 
+    #if __STDC_VERSION__ >= 199901L
+        #define _XOPEN_SOURCE 600
+    #else
+        #define _XOPEN_SOURCE 500
+    #endif
+#endif
+
+#include "time.hpp"
+#include <ctime>
+#include "UsefulException.hpp"
+
+nsec_t getMonotonicTimeNsec(){
+    struct timespec t;
+    if(clock_gettime(CLOCK_MONOTONIC, &t) != 0)
+        throw UsefulException("clock_gettime(CLOCK_MONOTONIC) error");
+    return (nsec_t)t.tv_sec * 1e9 + (nsec_t)t.tv_nsec;
+}
diff --git a/src/time.hpp b/src/time.hpp
new file mode 100644
index 0000000..6c6bb6f
--- /dev/null
+++ b/src/time.hpp
@@ -0,0 +1,10 @@
+#pragma once
+
+#include "std.hpp"
+
+/// nanoseconds
+typedef i64 nsec_t;
+
+/// can be used to measure delta time
+///@return time from some moment in nanoseconds.
+nsec_t getMonotonicTimeNsec();