diff --git a/firmware/hackrf_usb/usb_queue.c b/firmware/hackrf_usb/usb_queue.c
index ed0e4a6e..91511885 100644
--- a/firmware/hackrf_usb/usb_queue.c
+++ b/firmware/hackrf_usb/usb_queue.c
@@ -131,10 +131,12 @@ static usb_transfer_t* endpoint_pop_transfer(
 void usb_transfer_schedule_wait(
 	const usb_endpoint_t* const endpoint,
 	void* const data,
-	const uint32_t maximum_length
+	const uint32_t maximum_length,
+        const transfer_completion_cb completion_cb
 ) {
         usb_transfer_t* const transfer = allocate_transfer();
         fill_in_transfer(transfer, data, maximum_length);
+        transfer->completion_cb = completion_cb;
         endpoint_add_transfer(endpoint, transfer);
         usb_endpoint_schedule_wait(endpoint, &transfer->td);
 }
@@ -142,11 +144,13 @@ void usb_transfer_schedule_wait(
 void usb_transfer_schedule_append(
 	const usb_endpoint_t* const endpoint,
 	void* const data,
-	const uint32_t maximum_length
+	const uint32_t maximum_length,
+        const transfer_completion_cb completion_cb
 ) {
         usb_transfer_t* const transfer = allocate_transfer();
         uint_fast8_t index = USB_ENDPOINT_INDEX(endpoint->address);
         fill_in_transfer(transfer, data, maximum_length);
+        transfer->completion_cb = completion_cb;
         // TODO: disable_interrupts();
         usb_transfer_t* tail = endpoint_transfers[index];
         for (; tail->next != NULL; tail = tail->next);
@@ -158,7 +162,7 @@ void usb_transfer_schedule_append(
 void usb_transfer_schedule_ack(
 	const usb_endpoint_t* const endpoint
 ) {
-	usb_transfer_schedule_wait(endpoint, 0, 0);
+  usb_transfer_schedule_wait(endpoint, 0, 0, NULL);
 }
 
 void usb_queue_transfer_complete(usb_endpoint_t* const endpoint)
diff --git a/firmware/hackrf_usb/usb_queue.h b/firmware/hackrf_usb/usb_queue.h
index 3963f15d..cec51fb3 100644
--- a/firmware/hackrf_usb/usb_queue.h
+++ b/firmware/hackrf_usb/usb_queue.h
@@ -40,13 +40,15 @@ void usb_endpoint_schedule(
 void usb_transfer_schedule_wait(
 	const usb_endpoint_t* const endpoint,
 	void* const data,
-	const uint32_t maximum_length
+	const uint32_t maximum_length,
+        const transfer_completion_cb completion_cb
 );
 
 void usb_transfer_schedule_append(
 	const usb_endpoint_t* const endpoint,
 	void* const data,
-	const uint32_t maximum_length
+	const uint32_t maximum_length,
+        const transfer_completion_cb completion_cb
 );
 
 void usb_transfer_schedule_ack(