diff --git a/.gitignore b/.gitignore
index 782b932..07f5511 100644
--- a/.gitignore
+++ b/.gitignore
@@ -33,3 +33,5 @@ extras/tests/simavr_based/test_seq*/Makefile
 extras/tests/esp32_hw_based/*.log
 library.properties
 examples
+extras/gen_pmf_const/PoorManFloat.cpp
+extras/gen_pmf_const/main
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1603209..4a9c1a9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ TODO:
 
 pre-0.31.2:
 - Move constants out of PoorManFloat.h
+- Fix pmfl constant used by SAM Due aka 21MHz
 
 0.31.1:
 - Fix for issue #280: stopMove() is interrupted if followed by update of speed/acceleration
diff --git a/extras/gen_pmf_const/Makefile b/extras/gen_pmf_const/Makefile
new file mode 100644
index 0000000..9a37a60
--- /dev/null
+++ b/extras/gen_pmf_const/Makefile
@@ -0,0 +1,17 @@
+CPPFLAGS=-I../../src
+
+../../src/PoorManFloatConst.h: main
+	./main >../../src/PoorManFloatConst.h
+
+main: main.o PoorManFloat.o
+
+main.o:	main.cpp
+
+PoorManFloat.o: PoorManFloat.cpp
+
+PoorManFloat.cpp: ../../src/PoorManFloat.cpp
+	cp ../../src/PoorManFloat.cpp PoorManFloat.cpp
+
+clean:
+	rm -f PoorManFloat.cpp
+	rm -f main main.o PoorManFloat.o
diff --git a/extras/gen_pmf_const/README.md b/extras/gen_pmf_const/README.md
new file mode 100644
index 0000000..234f74e
--- /dev/null
+++ b/extras/gen_pmf_const/README.md
@@ -0,0 +1,4 @@
+# generate pmf constants
+
+Those constants are stored in PoorManFloatConst.h and are autogenerated
+by invoking make. This builds main from main.cpp.
diff --git a/extras/gen_pmf_const/main.cpp b/extras/gen_pmf_const/main.cpp
new file mode 100644
index 0000000..c452da4
--- /dev/null
+++ b/extras/gen_pmf_const/main.cpp
@@ -0,0 +1,63 @@
+#include <stdio.h>
+
+#include <PoorManFloat.h>
+
+void out(const char *name, pmf_logarithmic value) {
+	printf("#define %s ((pmf_logarithmic)0x%04x)\n", name, value);
+}
+
+int main() {
+	puts("// Autogenerated by extras/gen_pmf_const/main");
+	puts("// DO NOT EDIT");
+	puts("#ifndef POORMANFLOATCONST_H");
+	puts("#define POORMANFLOATCONST_H");
+	puts("");
+	puts("#include <PoorManFloat.h>");
+	puts("");
+
+	pmf_logarithmic x;
+
+	x = pmfl_from((uint8_t)1);
+	out("PMF_CONST_1", x);
+
+	x = pmfl_shr(pmfl_from((uint8_t)3), 1);
+	out("PMF_CONST_3_DIV_2", x);
+	
+	x = pmfl_multiply(pmfl_from((uint32_t)16e6), pmfl_from((uint32_t)8e6));
+	out("PMF_CONST_128E12", x);
+	
+	x = pmfl_from((uint32_t)16e6);
+	out("PMF_CONST_16E6", x);
+	
+	x = pmfl_from((uint16_t)500);
+	out("PMF_CONST_500", x);
+	
+	x = pmfl_from((uint16_t)1000);
+	out("PMF_CONST_1000", x);
+	
+	x = pmfl_from((uint16_t)2000);
+	out("PMF_CONST_2000", x);
+	
+	x = pmfl_from((uint16_t)32000);
+	out("PMF_CONST_32000", x);
+	
+	x = pmfl_divide(pmfl_from((uint32_t)16e6), pmfl_sqrt(pmfl_from((uint8_t)2)));
+	out("PMF_CONST_16E6_DIV_SQRT_OF_2", x);
+
+	x = pmfl_from((uint32_t)21e6);
+	out("PMF_CONST_21E6", x);
+	
+	x = pmfl_from((uint16_t)42000);
+	out("PMF_CONST_42000", x);
+	
+	x = pmfl_divide(pmfl_from((uint32_t)21e6), pmfl_sqrt(pmfl_from((uint8_t)2)));
+	out("PMF_CONST_21E6_DIV_SQRT_OF_2", x);
+	
+	x = pmfl_square(pmfl_from((uint32_t)21e6));
+	x = pmfl_shr(x, 1);
+	out("PMF_CONST_2205E11", x);
+
+	puts("#endif");
+	
+	return 0;
+}
diff --git a/extras/tests/pc_based/test_03.h b/extras/tests/pc_based/test_03.h
index 35fa20d..d76ca81 100644
--- a/extras/tests/pc_based/test_03.h
+++ b/extras/tests/pc_based/test_03.h
@@ -22,7 +22,7 @@ bool perform_test() {
       {42000, 1, false, PMF_CONST_42000},
       {14849242, 1, false, PMF_CONST_21E6_DIV_SQRT_OF_2},
       {16000000, 2, true, PMF_CONST_128E12},  // (16e6)^2 / 2
-      {22100000, 2, true, PMF_CONST_2205E11}  // (21e6)^2 / 2
+      {21000000, 2, true, PMF_CONST_2205E11}  // (21e6)^2 / 2
   };
   uint16_t l1;
   pmf_logarithmic p1;
diff --git a/src/PoorManFloat.h b/src/PoorManFloat.h
index 3ec2c0f..60debf6 100644
--- a/src/PoorManFloat.h
+++ b/src/PoorManFloat.h
@@ -5,6 +5,7 @@
 typedef int16_t pmf_logarithmic;
 
 #define PMF_CONST_INVALID ((pmf_logarithmic)0x8000)
+#define PMF_CONST_MAX ((pmf_logarithmic)0x7fff)
 
 pmf_logarithmic pmfl_from(uint8_t x);
 pmf_logarithmic pmfl_from(uint16_t x);
diff --git a/src/PoorManFloatConst.h b/src/PoorManFloatConst.h
index 6845308..d39ab32 100644
--- a/src/PoorManFloatConst.h
+++ b/src/PoorManFloatConst.h
@@ -1,9 +1,10 @@
+// Autogenerated by extras/gen_pmf_const/main
+// DO NOT EDIT
 #ifndef POORMANFLOATCONST_H
 #define POORMANFLOATCONST_H
 
 #include <PoorManFloat.h>
 
-#define PMF_CONST_MAX ((pmf_logarithmic)0x7fff)
 #define PMF_CONST_1 ((pmf_logarithmic)0x0000)
 #define PMF_CONST_3_DIV_2 ((pmf_logarithmic)0x012c)
 #define PMF_CONST_128E12 ((pmf_logarithmic)0x5dba)
@@ -16,5 +17,5 @@
 #define PMF_CONST_21E6 ((pmf_logarithmic)0x30a5)
 #define PMF_CONST_42000 ((pmf_logarithmic)0x1eb7)
 #define PMF_CONST_21E6_DIV_SQRT_OF_2 ((pmf_logarithmic)0x2fa5)
-#define PMF_CONST_2205E11 ((pmf_logarithmic)0x5f96)
+#define PMF_CONST_2205E11 ((pmf_logarithmic)0x5f4a)
 #endif