diff --git a/firmware/freq_table.md b/firmware/freq_table.md
new file mode 100644
index 0000000..d39b93b
--- /dev/null
+++ b/firmware/freq_table.md
@@ -0,0 +1,17 @@
+Frequency table
+==============
+
+calculation like:
+
+```
+rate = 8000000 / (3 * 64)
+offset = 50
+
+print "const uint16_t freq_table[] = {"
+
+for i in range(100):
+	n = int(440.0 / rate * 2 ** ((i - offset) / 12.0) * 2 ** 16 + 0.5)
+	if n < 0x10000 / 2:
+		print "/*0x%02x*/\t%d," % (i, n)
+print "};"
+```