[sblib] Consider acknowledge frames in now renamed sendingTelegram

Formerly, `Bus::sendingTelegram()` returned `true` iff there was a data
frame ("telegram") to send. However, several call sites actually need
to know whether there is any frame to send, i.e. both data and
acknowledge frames are relevant.

To address this requirement, rename `sendingTelegram` to `sendingFrame`
and make it consider acknowledge frames as well.

Also provide some reasoning for the condition in `BcuBase::loop()`, so
hopefully we do not need to wonder about it again.

examples/example-txstresstest/src/app_main.cpp

@@ -96,7 +96,7 @@ BcuBase* setup()
  */
 void loop(void)
 {
-    if (!bcu.bus->sendingTelegram() && ((TEST_DELAY_MS <= 0) || (sendTimeout.expired())))
+    if (!bcu.bus->sendingFrame() && ((TEST_DELAY_MS <= 0) || (sendTimeout.expired())))
     {
         sendTestTelegram();
     }

sblib/inc/sblib/eib/bus.h

@@ -101,11 +101,11 @@ class Bus
     void timerInterruptHandler();
 
     /**
-     * Test if there is a telegram being sent.
+     * Test if there is a frame being sent (data frame or acknowledge frame).
      *
-     * @return True if there is a telegram to be sent, false if not.
+     * @return True if there is a frame to be sent, false if not.
      */
-    bool sendingTelegram() const;
+    bool sendingFrame() const;
 
     /**
      * Test if there is a received telegram in bus.telegram[].
@@ -289,9 +289,9 @@ inline void Bus::maxSendBusyRetries(int retries)
     sendBusyRetriesMax = retries;
 }
 
-inline bool Bus::sendingTelegram() const
+inline bool Bus::sendingFrame() const
 {
-    return sendCurTelegram != nullptr;
+    return sendCurTelegram != nullptr || sendAck != 0;
 }
 
 inline bool Bus::telegramReceived() const

sblib/src/eib/bcu_base.cpp

@@ -47,7 +47,18 @@ void BcuBase::loop()
     bus->loop();
     TLayer4::loop();
 
-    if (bus->telegramReceived() && !bus->sendingTelegram() && (userRam->status() & BCU_STATUS_TRANSPORT_LAYER))
+    // We want to process a received telegram only if there is nothing to send because:
+    //
+    //     1) Processing the telegram can cause a response telegram, e.g. a T_ACK in
+    //        connection-oriented Transport Layer messages, and we need to have an empty
+    //        buffer to be able to store and send such responses.
+    //
+    //     2) When debugging, it's crucial to only stop in safe states, i.e. only when
+    //        there is nothing to send, not even an acknowledge frame. Otherwise, the
+    //        Bus timer is configured to pull the bus low (send a 0 bit) for some time
+    //        and the MCU continues timer operation, even when a breakpoint is active.
+    //
+    if (bus->telegramReceived() && !bus->sendingFrame() && (userRam->status() & BCU_STATUS_TRANSPORT_LAYER))
     {
         processTelegram(bus->telegram, (uint8_t)bus->telegramLen); // if processed successfully, received telegram will be discarded by processTelegram()
     }
@@ -66,7 +77,7 @@ void BcuBase::loop()
     }
 
     // Rest of this function is only relevant if currently able to send another telegram.
-    if (bus->sendingTelegram())
+    if (bus->sendingFrame())
     {
         return;
     }

sblib/src/eib/bcu_default.cpp

@@ -123,7 +123,7 @@ void BcuDefault::loop()
     BcuBase::loop(); // check processTelegram and programming button state
 
     // Rest of this function is only relevant if currently able to send another telegram.
-    if (bus->sendingTelegram())
+    if (bus->sendingFrame())
     {
         return;
     }

sblib/src/eib/bus.cpp

@@ -190,7 +190,7 @@ bool Bus::canPause(bool waitForTelegramSent)
     //   cannot pause as we must retry within a certain time frame.
     // * have not sent the telegram yet, it depends on @ref waitForTelegramSent whether the user
     //   wants us to pause or not.
-    if (sendingTelegram())
+    if (sendCurTelegram != nullptr)
     {
         if (repeatTelegram)
             return false;
@@ -219,7 +219,7 @@ void Bus::prepareTelegram(unsigned char* telegram, unsigned short length) const
  *
  * Is called from within the BCU-loop method. Is blocking if there is no space
  * in the Telegram buffer (as we have only one buffer at BCU level, the check for buffer free is
- * in the BCU-loop on bus.sendingTelegram())
+ * in the BCU-loop on bus.sendingFrame())
  *
  * Send a telegram. The checksum byte will be added at the end of telegram[].
  * Ensure that there is at least one byte space at the end of telegram[].