I2C Slave Implementation

[me-no-dev]

This pull request adds support for I2C Slave to the ESP32 Arduino Framework

Fixes: #118

[platypii]

This PR added an overload for Wire.begin which is now causing errors downstream in Heltec's BMP180.cpp.

.pio/libdeps/heltec_wifi_lora_32_V2/Heltec ESP32 Dev-Boards/src/BMP180.cpp:13:29: error: call of overloaded 'begin(int&, int, int)' is ambiguous
   Wire.begin(sda, 12, 100000);
                             ^
In file included from .pio/libdeps/heltec_wifi_lora_32_V2/Heltec ESP32 Dev-Boards/src/BMP180.h:5,
                 from .pio/libdeps/heltec_wifi_lora_32_V2/Heltec ESP32 Dev-Boards/src/BMP180.cpp:1:
.platformio/packages/framework-arduinoespressif32/libraries/Wire/src/Wire.h:79:10: note: candidate: 'bool TwoWire::begin(int, int, uint32_t)'
     bool begin(int sda=-1, int scl=-1, uint32_t frequency=0); // returns true, if successful init of i2c bus
          ^~~~~
.platformio/packages/framework-arduinoespressif32/libraries/Wire/src/Wire.h:80:10: note: candidate: 'bool TwoWire::begin(uint8_t, int, int, uint32_t)'
     bool begin(uint8_t slaveAddr, int sda=-1, int scl=-1, uint32_t frequency=0);
          ^~~~~

I plan to file an issue and a PR in that repo, but the problem is I can't actually figure out how to disambiguate between these two overloaded constructors. How do I even tell C++ which to prefer?

It seems like it would have been better to add the uint8_t slaveAddr argument to the END of the Wire.begin function, to avoid breaking lots of code in the wild?

[me-no-dev]

the easiest way is to cast the arguments, like Wire.begin((int)SDA, (int)SCL);, but the best would be to use:

Wire.setPins(SDA, SCL);
Wire.begin();
Wire.setClock(frequency); // if necessary

I intend to look into this and see if I can find a better way to resolve it.

[platypii]

Thanks for looking into it. Actually though, casting the first argument to (int) doesn't work. GCC still thinks the int and uint8_t are ambiguous. The only way I was able to get it to compile was to cast the third argument to unsigned:

-  Wire.begin(13, 12, 100000);
+  Wire.begin(13, 12, 100000U);