diff --git a/src/mame/drivers/supertnk.cpp b/src/mame/drivers/supertnk.cpp
index eed2371bbe6..78578b779ae 100644
--- a/src/mame/drivers/supertnk.cpp
+++ b/src/mame/drivers/supertnk.cpp
@@ -94,6 +94,8 @@ CRU lines:
 >b12    Unknown, maybe some special-hardware sound effect or lights blinking (?)
 >b13    Unknown, maybe some special-hardware sound effect or lights blinking (?)
 
+XTAL (on CPU board) is marked 20.790 on one PCB, 22.118 on another.
+
 ***************************************************************************/