Arbitration methods and I2C significance

Arbitration methods for gaining control of the system bus

Arbitration decides which of several requesting devices becomes bus master.

1. Priority arbitration: an arbiter with a Req/Ack line pair per device grants the bus to the highest-priority request. Priority may be fixed or rotating (round robin).
2. Daisy-chain arbitration: devices share one Req line; the Grant is passed through the devices in series. The first device in the chain that has requested keeps the grant, so priority depends on position. Simple, but unfair and fault-sensitive.
3. Network-oriented (distributed) arbitration: there is no central arbiter. Each device senses the bus and sends when it is idle; collisions are detected and resolved, e.g. CSMA/CD in Ethernet, or bitwise arbitration by identifier priority in CAN (the lower ID wins), or the wired-AND SDA/SCL arbitration in I2C.
4. Polling / centralised arbiter variants: the arbiter polls devices in turn (round-robin).

Significance of the I2C protocol

I2C (Inter-Integrated Circuit) is a two-wire synchronous serial bus (SDA data and SCL clock) developed by Philips for connecting low-speed peripherals to a processor.

• Only two wires for many devices, saving pins and board space.
• Multi-master and multi-slave capability, with 7-bit (or 10-bit) addresses to identify slaves.
• Built-in acknowledge bit after each byte gives reliable transfer.
• Built-in arbitration: if two masters transmit together, the one sending a 0 while the other sends 1 wins (wired-AND), without losing data.
• Speeds: 100 kbps standard, 400 kbps fast, 3.4 Mbps high-speed.
• Used for EEPROMs, RTC, sensors, ADC/DAC, LCD drivers in embedded systems. Limits: slow compared with SPI and short distance.