diff --git a/stm32/aioc-fw/Src/cos.h b/stm32/aioc-fw/Src/cos.h
new file mode 100644
index 0000000..d15cee0
--- /dev/null
+++ b/stm32/aioc-fw/Src/cos.h
@@ -0,0 +1,16 @@
+#ifndef COS_H_
+#define COS_H_
+
+#include <stdint.h>
+#include "stm32f3xx_hal.h"
+#include "usb_hid.h"
+
+static inline void COS_SetState(uint8_t state)
+{
+    LED_SET(0, state & 0x01 ? 1 : 0);
+
+    USB_HIDSendButtonState(state & 0x01 ? USB_HID_BUTTON_VOLDN : 0x00);
+    USB_SerialSendLineState(state & 0x01 ? USB_SERIAL_LINESTATE_DCD : 0x00);
+}
+
+#endif /* COS_H_ */