diff --git a/README.md b/README.md
index 53ae245..e7f5698 100644
--- a/README.md
+++ b/README.md
@@ -5,4 +5,5 @@ https://blog.naver.com/eziya76/221188701172
 
 Reference : 
  - https://blog.domski.pl/using-fatfs-with-hal/
+ - http://elm-chan.org/docs/mmc/mmc_e.html
  - http://www.dejazzer.com/ee379/lecture_notes/lec12_sd_card.pdf