diff --git a/basic1.cpp b/basic1.cpp
index 780600b..700710b 100644
--- a/basic1.cpp
+++ b/basic1.cpp
@@ -88,7 +88,7 @@ void core1_entry() {
     while (1) {
         // Wait for refresh request
         if (refresh_requested && refresh_buffer && refresh_display) {
-            refresh_in_progress = true;
+            // refresh_in_progress is already set by Core 0 to lock the buffer
             
             // Get local copies for safe access
             LowLevelDisplay* display = refresh_display;
@@ -100,7 +100,7 @@ void core1_entry() {
             
             // Clear flags
             refresh_requested = false;
-            refresh_in_progress = false;
+            refresh_in_progress = false; // Unlock buffer for Core 0
         }
         
         // Small delay to avoid busy-waiting
@@ -124,6 +124,10 @@ bool refresh_screen_async(const uint8_t *buffer, LowLevelDisplay* display) {
         return false;
     }
     
+    // Lock the buffer immediately on Core 0 to prevent race condition
+    // Core 1 will unlock it (set to false) when done
+    refresh_in_progress = true;
+    
     // Queue refresh on Core 1
     refresh_buffer = buffer;
     refresh_display = display;