diff --git a/firmware/ice40-riscv/icE1usb/ice1usb_proto.h b/firmware/ice40-riscv/icE1usb/ice1usb_proto.h
index 5b65d76..8cf8439 100644
--- a/firmware/ice40-riscv/icE1usb/ice1usb_proto.h
+++ b/firmware/ice40-riscv/icE1usb/ice1usb_proto.h
@@ -20,6 +20,10 @@
 /*! returns a string describing the firmware version */
 #define ICE1USB_DEV_GET_FW_BUILD	0x02
 
+/*! performs an I2C register access (read/write depends on direction) */
+#define ICE1USB_DEV_I2C_REG_ACCESS	0x10
+
+
 enum e1usb_dev_capability {
 	/*! Does this board have a GPS-DO */
 	ICE1USB_DEV_CAP_GPSDO,