Wire: Expose default timeout settings in Wire.h

Previously, these were implicit in the default values of some global
variables. Now, the default timeout is defined in twi.h and exposed
through Wire.h, which:
 - Makes it easier to change later
 - Allows sketches to detect the default timeout value

Note that this definition is split between twi.h and Wire.h to ensure
that the default values are available as compile-time constants in twi.c
(which allows for more efficient initialization than e.g. writing these
values from the Wire constructor or from the begin() method, where the
latter has the extra downside of potentially overwriting values
previously set with setWireTimeout() if begin() is called again).
Additionally, since twi does not currently depend on Wire.h, defining
these values in Wire.h and using them in twi.c was not feasible without
breaking adding this extra dependency, so instead this commit defines
the values in twi and then re-exposes them in Wire.h (with the intention
of sketches referring to the Wire.h versions, not the twi.h versions).
See #362 for additional discussion.

Author: matthijskooijman

Diff for: libraries/Wire/src/Wire.h

@@ -25,6 +25,7 @@
 
 #include <inttypes.h>
 #include "Stream.h"
+#include "utility/twi.h"
 
 #define BUFFER_LENGTH 32
 
@@ -34,6 +35,10 @@
 // and clearWireTimeoutFlag()
 #define WIRE_HAS_TIMEOUT 1
 
+// When not configured, these settings are used for the timeout
+#define WIRE_DEFAULT_TIMEOUT TWI_DEFAULT_TIMEOUT
+#define WIRE_DEFAULT_RESET_ON_TIMEOUT TWI_DEFAULT_RESET_ON_TIMEOUT
+
 class TwoWire : public Stream
 {
   private:

Diff for: libraries/Wire/src/utility/twi.c

@@ -51,9 +51,9 @@ static volatile uint8_t twi_inRepStart;			// in the middle of a repeated start
 // and twi_do_reset_on_timeout could become true
 // to conform to the SMBus standard
 // http://smbus.org/specs/SMBus_3_1_20180319.pdf
-static volatile uint32_t twi_timeout_us = 0ul;
+static volatile uint32_t twi_timeout_us = TWI_DEFAULT_TIMEOUT;
 static volatile bool twi_timed_out_flag = false;  // a timeout has been seen
-static volatile bool twi_do_reset_on_timeout = false;  // reset the TWI registers on timeout
+static volatile bool twi_do_reset_on_timeout = TWI_DEFAULT_RESET_ON_TIMEOUT;  // reset the TWI registers on timeout
 
 static void (*twi_onSlaveTransmit)(void);
 static void (*twi_onSlaveReceive)(uint8_t*, int);

Diff for: libraries/Wire/src/utility/twi.h

@@ -34,6 +34,14 @@
   #define TWI_BUFFER_LENGTH 32
   #endif
 
+  #ifndef TWI_DEFAULT_TIMEOUT
+  #define TWI_DEFAULT_TIMEOUT 0
+  #endif
+
+  #ifndef TWI_DEFAULT_RESET_ON_TIMEOUT
+  #define TWI_DEFAULT_RESET_ON_TIMEOUT 0
+  #endif
+
   #define TWI_READY 0
   #define TWI_MRX   1
   #define TWI_MTX   2