* `#define USE_I2C`
* For using I2C instead of Serial (defaults to serial)
+* `#define SOFT_SERIAL_PIN D0`
+ * When using serial, define this. `D0` or `D1`,`D2`,`D3`,`E6`.
+
# The `rules.mk` File
This is a [make](https://www.gnu.org/software/make/manual/make.html) file that is included by the top-level `Makefile`. It is used to set some information about the MCU that we will be compiling for as well as enabling and disabling certain features.