More conservative memory monitoring

Add function to maintain a  minimum value seen of free memory.  Add call to it in DCCWaveform interrupt handler (assumed to be the likely worst case for stack usage).  Report this minimum value in main loop.

CommandStation-EX.ino

@@ -127,7 +127,7 @@ void loop()
 #if ENABLE_FREE_MEM_WARNING
   static int ramLowWatermark = 32767; // replaced on first loop 
 
-  int freeNow = freeMemory();
+  int freeNow = updateMinimumFreeMemory();
   if (freeNow < ramLowWatermark)
   {
     ramLowWatermark = freeNow;

DCCWaveform.cpp

@@ -23,6 +23,7 @@
 #include "DCCWaveform.h"
 #include "DCCTimer.h"
 #include "DIAG.h"
+#include "freeMemory.h"
  
 
 DCCWaveform  DCCWaveform::mainTrack(PREAMBLE_BITS_MAIN, true);
@@ -203,6 +204,9 @@ void DCCWaveform::interrupt2() {
   if (remainingPreambles > 0 ) {
     state=WAVE_MID_1;  // switch state to trigger LOW on next interrupt
     remainingPreambles--;
+    // Update free memory diagnostic as we don't have anything else to do this time.
+    // Allow for checkAck and its called functions using 22 bytes more.
+    updateMinimumFreeMemory(22); 
     return;
   }
 

freeMemory.cpp

@@ -1,5 +1,6 @@
 /*
  *  © 2020, Harald Barth
+ *  © 2021, Neil McKechnie
  *  
  *  This file is part of Asbelos DCC-EX
  *
@@ -30,6 +31,9 @@ extern char *__malloc_heap_start;
 #endif
 
 
+static int minimum_free_memory = 32767;
+
+
 int freeMemory() {
   char top;
 #if defined(__arm__)
@@ -40,3 +44,12 @@ int freeMemory() {
 #error bailed out alredy above
 #endif
 }
+
+// Update low ram level.  Allow for extra bytes to be specified
+// by estimation or inspection, that may be used by other 
+// called subroutines.
+int updateMinimumFreeMemory(unsigned char extraBytes) {
+  int spare = freeMemory()-extraBytes;
+  if (spare < minimum_free_memory) minimum_free_memory = spare;
+  return minimum_free_memory;
+}

freeMemory.h

@@ -1,5 +1,6 @@
 /*
  *  © 2020, Harald Barth
+ *  © 2021, Neil McKechnie
  *  
  *  This file is part of DCC-EX
  *
@@ -20,4 +21,5 @@
 #ifndef freeMemory_h
 #define freeMemory_h
 int freeMemory();
+int updateMinimumFreeMemory(unsigned char extraBytes=0);
 #endif