diff --git a/Rakefile b/Rakefile
index 55df3ad..979d89f 100644
--- a/Rakefile
+++ b/Rakefile
@@ -143,13 +143,13 @@ task :doc => ["Doxyfile", "README.md"] do |t|
 end
 
 desc "compile source into object"
-rule '.o' => '.c' do |t|
-  sh "#{CC} #{cflags} #{archflags} -o #{t.name} -c #{t.source}"
+rule '.o' => ['.c', "#{LIBOPENCM3_LIB}/lib#{STM32F1_LIB}.a"] do |t|
+  sh "#{CC} #{cflags} #{archflags} -o #{t.name} -c #{t.prerequisites[0]}"
 end
 
 desc "generate dependencies"
-rule '.d' => '.c' do |t|
-  sh "#{CC} #{cflags} #{archflags} -MM -MF #{t.name} -c #{t.source}"
+rule '.d' => ['.c', "#{LIBOPENCM3_LIB}/lib#{STM32F1_LIB}.a"] do |t|
+  sh "#{CC} #{cflags} #{archflags} -MM -MF #{t.name} -c #{t.prerequisites[0]}"
 end
 
 desc "link binary"