Remove slave code when not using slave mode

Add a new twi_setSlaveMode call which actually attached the interrupts
to the slave pin change code onSdaChenge/onSclChange.  Don't attach
interrupts in the main twi_begin.

Because slave mode is only useful should a onoReceive or onRequest
callback, call twi_setSlaveMode and attach interrupts on the Wire
setters.

This allows GCC to not link in slave code unless slave mode is used,
saving over 1,000 bytes of IRAM in the common, master-only case.

Author: earlephilhower

cores/esp8266/core_esp8266_si2c.cpp

@@ -81,6 +81,9 @@ class Twi
     static void eventTask(ETSEvent *e);
     static void ICACHE_RAM_ATTR onTimer(void *unused);
 
+    // Allow not linking in the slave code if there is no call to setAddress
+    bool _slaveEnabled = false;
+
     // Internal use functions
     void ICACHE_RAM_ATTR busywait(unsigned char v);
     bool write_start(void);
@@ -124,6 +127,7 @@ class Twi
         }
     }
 
+
 public:
     void setClock(unsigned int freq);
     void setClockStretchLimit(uint32_t limit);
@@ -138,6 +142,7 @@ class Twi
     inline void ICACHE_RAM_ATTR reply(uint8_t ack);
     inline void ICACHE_RAM_ATTR stop(void);
     inline void ICACHE_RAM_ATTR releaseBus(void);
+    void enableSlave();
 };
 
 static Twi twi;
@@ -222,6 +227,8 @@ void Twi::setClockStretchLimit(uint32_t limit)
     twi_clockStretchLimit = limit * TWI_CLOCK_STRETCH_MULTIPLIER;
 }
 
+
+
 void Twi::init(unsigned char sda, unsigned char scl)
 {
     // set timer function
@@ -236,12 +243,6 @@ void Twi::init(unsigned char sda, unsigned char scl)
     pinMode(twi_scl, INPUT_PULLUP);
     twi_setClock(preferred_si2c_clock);
     twi_setClockStretchLimit(230); // default value is 230 uS
-
-    if (twi_addr != 0)
-    {
-        attachInterrupt(scl, onSclChange, CHANGE);
-        attachInterrupt(sda, onSdaChange, CHANGE);
-    }
 }
 
 void Twi::setAddress(uint8_t address)
@@ -250,6 +251,15 @@ void Twi::setAddress(uint8_t address)
     twi_addr = address << 1;
 }
 
+void Twi::enableSlave()
+{
+    if (!_slaveEnabled) {
+        attachInterrupt(twi_scl, onSclChange, CHANGE);
+        attachInterrupt(twi_sda, onSdaChange, CHANGE);
+        _slaveEnabled = true;
+    }
+}
+
 void ICACHE_RAM_ATTR Twi::busywait(unsigned char v)
 {
     unsigned int i;
@@ -1044,4 +1054,9 @@ extern "C" {
         twi.releaseBus();
     }
 
+    void twi_enableSlaveMode(void)
+    {
+        twi.enableSlave();
+    }
+
 };

cores/esp8266/twi.h

@@ -54,6 +54,7 @@ void twi_reply(uint8_t);
 //void twi_stop(void);
 void twi_releaseBus(void);
 
+void twi_enableSlaveMode(void);
 
 #ifdef __cplusplus
 }

libraries/Wire/Wire.cpp

@@ -274,10 +274,12 @@ void TwoWire::onReceive( void (*function)(int) ) {
 
 void TwoWire::onReceive( void (*function)(size_t) ) {
   user_onReceive = function;
+  twi_enableSlaveMode();
 }
 
 void TwoWire::onRequest( void (*function)(void) ){
   user_onRequest = function;
+  twi_enableSlaveMode();
 }
 
 // Preinstantiate Objects //////////////////////////////////////////////////////