I2C，以及SMBUS

i2c协议: http://www.nxp.com/documents/user_manual/UM10204.pdf
SMBUS协议: http://www.smbus.org

two bus wires: SDA, SCL
addressable
Master/Slave
master-transmiter/master-receiver
multi-master bus
collision detection and arbitration

Standard-mode 100kbit/s
Fast-mode 400kbit/s
Fast-mode plus 1Mbit/s
High-speed mode 3.4Mbit/s
ultra Fast-mode 5Mbit/s ——单向8bit

each device uniq address
operate as a transmiter or receiver
Master initiates data transfer, generates the clock.
Any device addressed is a slave.

multi-master
arvitration procedure
依赖“线与”，开漏输出

SCL高电平时SDA有效
SCL低电平时SDA改变

START: SCL高电平期间SDA下降沿
STOP: SCL搞定平期间SDA上升沿
可以有repeat START

Byte, 8bits
后面跟ACK或NAK
ACK: 第9个始终SDA低
NAK: 第9个始终SDA高

START后面发送slave address
7bit地址，1bit方向位（0：写，1：读）

I2C规范4.2节介绍了SMBUS

驱动软件角度，SMBUS重点：第6章，重点关注6.5节。