diff --git a/litedram/phy/s7ddrphy.py b/litedram/phy/s7ddrphy.py
index 7a27dac..4f39a20 100644
--- a/litedram/phy/s7ddrphy.py
+++ b/litedram/phy/s7ddrphy.py
@@ -76,7 +76,7 @@ class S7DDRPHY(Module, AutoCSR):
         iodelay_tap_average = {
             200e6: 78e-12,
             300e6: 52e-12,
-            400e6: 39e-12,
+            400e6: 39e-12, # Only valid for -3 and -2/2E speed grades
         }
 
         half_sys8x_taps = math.floor(tck/(4*iodelay_tap_average[iodelay_clk_freq]))