diff --git a/mcu_utils/fw/README.md b/mcu_utils/fw/README.md
index b78329771af..f0d48cccbc3 100644
--- a/mcu_utils/fw/README.md
+++ b/mcu_utils/fw/README.md
@@ -1,3 +1,8 @@
 Warning!
 fancyblink and stm32f4-stock firmware doesn't work on our
 STM32F407G-DISC board.
+
+stm32f4_usb_cdc.bin is a number crunching fmw
+that communicates with PC using USB virtual serial port.
+You can cat /dev/ttyACM0 or 1
+to see its messages.
diff --git a/mcu_utils/fw/stm32f4_usb_cdc.bin b/mcu_utils/fw/stm32f4_usb_cdc.bin
new file mode 100755
index 00000000000..faeee847d09
Binary files /dev/null and b/mcu_utils/fw/stm32f4_usb_cdc.bin differ