diff --git a/fuzzers/fuzzer.mk b/fuzzers/fuzzer.mk
index a606f757..0e52ffda 100644
--- a/fuzzers/fuzzer.mk
+++ b/fuzzers/fuzzer.mk
@@ -7,7 +7,6 @@ all: database
 $(SPECIMENS_OK):
 	mkdir -p build
 	bash ${XRAY_DIR}/utils/top_generate.sh $(subst /OK,,$@)
-	touch $@
 
 run:
 	$(MAKE) clean
diff --git a/utils/genheader.sh b/utils/genheader.sh
index 6e766ff2..611cda7d 100644
--- a/utils/genheader.sh
+++ b/utils/genheader.sh
@@ -15,7 +15,6 @@ test $# -ge 1 || exit 1
 test ! -e "$SPECDIR"
 export SPECDIR=$1
 
-rm -rf "$SPECDIR"
 mkdir -p "$SPECDIR"
 cd "$SPECDIR"
 
diff --git a/utils/top_generate.mk b/utils/top_generate.mk
new file mode 100644
index 00000000..9b8812e3
--- /dev/null
+++ b/utils/top_generate.mk
@@ -0,0 +1,26 @@
+all: OK
+
+OK: generate.ok
+	touch OK
+
+# Some projects have hard coded top.v, others are generated
+top.v.ok:
+	if [ -f ${FUZDIR}/top.py ] ; then python3 ${FUZDIR}/top.py >top.v; fi
+	touch top.v.ok
+
+vivado.ok: top.v.ok ${FUZDIR}/generate.tcl
+	${XRAY_VIVADO} -mode batch -source ${FUZDIR}/generate.tcl
+	test -z "$(fgrep CRITICAL vivado.log)"
+	touch vivado.ok
+
+design_bits.ok: vivado.ok
+	\
+        for x in design*.bit; do \
+            ${XRAY_BITREAD} -F ${XRAY_ROI_FRAMES} -o $${x}s -z -y $$x ; \
+        done
+	touch design_bits.ok
+
+generate.ok: design_bits.ok ${FUZDIR}/generate.py
+	python3 ${FUZDIR}/generate.py
+	touch generate.ok
+
diff --git a/utils/top_generate.sh b/utils/top_generate.sh
index af0409fd..0e960d53 100644
--- a/utils/top_generate.sh
+++ b/utils/top_generate.sh
@@ -6,18 +6,5 @@ set -ex
 
 export FUZDIR=$PWD
 source ${XRAY_GENHEADER}
-
-# Some projects have hard coded top.v, others are generated
-if [ -f $FUZDIR/top.py ] ; then
-    python3 $FUZDIR/top.py >top.v
-fi
-
-${XRAY_VIVADO} -mode batch -source $FUZDIR/generate.tcl
-test -z "$(fgrep CRITICAL vivado.log)"
-
-for x in design*.bit; do
-	${XRAY_BITREAD} -F $XRAY_ROI_FRAMES -o ${x}s -z -y $x
-done
-
-python3 $FUZDIR/generate.py
+make -f $XRAY_DIR/utils/top_generate.mk