diff --git a/docs/efm8.rst b/docs/efm8.rst
index 19d8b6b..8e7e2ec 100644
--- a/docs/efm8.rst
+++ b/docs/efm8.rst
@@ -3,5 +3,9 @@
 
 ----------
 
+The protocol is documented in AN945_.
+
 .. automodule:: efm8
    :members:
+
+.. _AN945: https://www.silabs.com/documents/public/application-notes/an945-efm8-factory-bootloader-user-guide.pdf