diff --git a/firmware/cpld/README b/firmware/cpld/README
new file mode 100644
index 00000000..8f9e7bac
--- /dev/null
+++ b/firmware/cpld/README
@@ -0,0 +1,9 @@
+The primary CPLD image is: sgpio_if/default.xsvf
+
+This is a binary file built from HDL source in sgpio_if.  You do not need
+Xilinx tools unless you want to make your own modifications.
+
+To update the CPLD, first update the firmware, libhackrf, and hackrf-tools.
+Then:
+
+$ hackrf_cpldjtag -x sgpio_if/default.xsvf