I2C的SCL和SDA的定义

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);
}