之前一直认为I2C的SCL和SDA针脚是可以改为其他模拟输入的针脚，正好晚上闲来无事，翻了翻源码，终于让我找到了定义的位置：pins_arduino.h

这个文件中定义了所有针脚的宏及对应关系：

static const uint8_t SS   = 10;

static const uint8_t MOSI = 11; static const uint8_t MISO = 12; static const uint8_t SCK  = 13;static const uint8_t SDA = 18; static const uint8_t SCL = 19; static const uint8_t LED_BUILTIN = 13; static const uint8_t A0 = 14; static const uint8_t A1 = 15; static const uint8_t A2 = 16; static const uint8_t A3 = 17; static const uint8_t A4 = 18; static const uint8_t A5 = 19; static const uint8_t A6 = 20; static const uint8_t A7 = 21;从这里能够看出，对于特殊针脚的一些定义，其中逻辑针脚是接着led的13继续定义的。而SDA和SCL与A4和A5定义为同一针脚，从这里可以看出，系统默认的就是A4和A5了，如果想换其他针脚，可以对应修改。

在twi.c中对SCL和SDA的直接引用，下面红色部分代码：

void twi_init(void)

{   // initialize state   twi_state = TWI_READY;   twi_sendStop = true; // default value   twi_inRepStart = false;      // activate internal pullups for twi.  digitalWrite(SDA, 1);  digitalWrite(SCL, 1);   // initialize twi prescaler and bit rate   cbi(TWSR, TWPS0);   cbi(TWSR, TWPS1);   TWBR = ((F_CPU / TWI_FREQ) - 16) / 2;   /* twi bit rate formula from atmega128 manual pg 204   SCL Frequency = CPU Clock Frequency / (16 + (2 * TWBR))   note: TWBR should be 10 or higher for master mode   It is 72 for a 16mhz Wiring board with 100kHz TWI */   // enable twi module, acks, and twi interrupt   TWCR = _BV(TWEN) | _BV(TWIE) | _BV(TWEA); }