diff --git a/builder/frameworks/_embed_files.py b/builder/frameworks/_embed_files.py
index 0375cbd..75d1a86 100644
--- a/builder/frameworks/_embed_files.py
+++ b/builder/frameworks/_embed_files.py
@@ -103,7 +103,9 @@ def embed_files(files, files_type):
 
 
 def transform_to_asm(target, source, env):
-    return [join("$BUILD_DIR", s.name + ".S") for s in source], source
+    files = [join("$BUILD_DIR", s.name + ".S") for s in source]
+    env.AppendUnique(PIOBUILDFILES=files)
+    return files, source
 
 env.Append(
     BUILDERS=dict(