From b9b988fff407090860350df5516d119f4bc8df07 Mon Sep 17 00:00:00 2001
From: "Dr.-Ing. Amilcar do Carmo Lucas" <amilcar.lucas@iav.de>
Date: Thu, 8 Jul 2021 13:54:25 +0200
Subject: [PATCH] AP_FETtecOneWire: Do not write to the UART buffer if the
 previous transfer did not complete yet

Required for stable operation on F4 processors
---
 libraries/AP_FETtecOneWire/AP_FETtecOneWire.cpp | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/libraries/AP_FETtecOneWire/AP_FETtecOneWire.cpp b/libraries/AP_FETtecOneWire/AP_FETtecOneWire.cpp
index e5d8a92209..9a5a2dc5b1 100644
--- a/libraries/AP_FETtecOneWire/AP_FETtecOneWire.cpp
+++ b/libraries/AP_FETtecOneWire/AP_FETtecOneWire.cpp
@@ -798,6 +798,15 @@ void AP_FETtecOneWire::update()
         return;
     }
 
+#if defined(STM32F4)
+    if (_uart->tx_pending()) {
+        // there is unsent data in the send buffer,
+        // do not send more data because FETtec needs a time gap between frames
+        _period_too_short++;
+        return;
+    }
+#endif
+
 #if HAL_AP_FETTEC_CONFIGURE_ESCS
     // run ESC configuration state machines if needed
     if (_running_mask != _motor_mask) {