diff --git a/lib/usb_cdcacm.c b/lib/usb_cdcacm.c
index 9314fc0..d4a34a9 100644
--- a/lib/usb_cdcacm.c
+++ b/lib/usb_cdcacm.c
@@ -403,7 +403,9 @@ void cdcacm_putchar(char c)
 		tx_buffer[(tx_i+tx_used)%LENGTH(tx_buffer)] = c; // overwrite old data
 	}
 	mutex_unlock(&tx_lock); // release lock
-	usbd_ep_write_packet(usb_device, 0x82, NULL, 0); // trigger tx callback
+	if (tx_used==1) { // to buffer is not empty anymore
+		usbd_ep_write_packet(usb_device, 0x82, NULL, 0); // trigger tx callback
+	}
 }
 
 /** @brief USB interrupt service routine called when data is received */