diff --git a/const.hpp b/const.hpp
index 7d9666e..8b75758 100644
--- a/const.hpp
+++ b/const.hpp
@@ -80,6 +80,7 @@
 static constexpr auto SPD_BYTE_6 = 6;
 static constexpr auto SPD_BYTE_12 = 12;
 static constexpr auto SPD_BYTE_13 = 13;
+static constexpr auto SPD_BYTE_18 = 18;
 static constexpr auto SPD_BYTE_BIT_0_3_MASK = 0x0F;
 static constexpr auto SPD_BYTE_MASK = 0xFF;