diff --git a/misoclib/soc/__init__.py b/misoclib/soc/__init__.py
index 217ce1378..ef7c45454 100644
--- a/misoclib/soc/__init__.py
+++ b/misoclib/soc/__init__.py
@@ -165,7 +165,7 @@ class SoC(Module):
     def get_csr_regions(self):
         return self._csr_regions
 
-    def add_constant(self, name, value):
+    def add_constant(self, name, value=None):
         self._constants.append((name, value))
 
     def get_constants(self):
diff --git a/misoclib/soc/cpuif.py b/misoclib/soc/cpuif.py
index 626bdd603..256612f02 100644
--- a/misoclib/soc/cpuif.py
+++ b/misoclib/soc/cpuif.py
@@ -92,7 +92,10 @@ def get_csr_header(regions, constants, with_access_functions=True):
 
     r += "\n/* constants */\n"
     for name, value in constants:
-        r += "#define " + name + " " + str(value) + "\n"
+        r += "#define " + name
+        if value is not None:
+            r += " " + str(value)
+        r += "\n"
 
     r += "\n#endif\n"
     return r