Inter-Integrated Circuit Intranet Protocols: A library of protocols for dynamic routing and interfacing with I2C devices on a modular switched network.
It is advantageous to view a collection of devices on an I2C bus as a network - analogous to the Internet - under certain conditions:
- Dynamic Routing: Device reachability is subject to change. This enables physical “plug-and-play” functionality.
- Modularity: Devices can be added to the network as physical collections. This enables lifecycle management for physical collections of devices, and informs dynamic routing.
In order to achieve an intra-network of I2C devices, we propose a suite of protocols built on top of I2C which together form the I2C Intranet Protocol, along with a compatible hardware specification.
Well, from NXP (the current owners of Philips Semiconductor, the original filer of the I2C standard in 1980) we have this Pro/Con list for I2C among a variety of other serial communications protocols:
Pros:
- Simple
- Well known
- Universally accepted
- Plug & Play
- Large portfolio
- Cost effective
Cons:
- Limited speedSince I2C follows a controller-target topology, only the controller device needs to implement these protocols.
| OSI Layer | IP Implementation | I2CIP Equivalent |
|---|---|---|
| Physical | Ethernet, MAC Addresses | I2C SDA, SCL; addresses |
| Data Link | IP packets | I2C packets |
| Network | IP addresses, routing, switching | Multiplexers, FQA |
| Transport | TCP | Bottom-Up QOS 2 |
On an I2C bus, only one device may occupy any given 7-bit I2C address at a time. By splitting the I2C bus into many individually-selectable sub-busses/"subnetworks" using a multiplexer ("MUX"), one is able to communicate with devices that share I2C/device-address space independently on multiple subnets.
MUX bus 0 is reserved as the "default bus". SPRT EEPROM must be located on this bus.
MUX bus 7 is reserved as the "inactive bus". In order to forbid unintended communication, multiplexers are switched to this bus when not in use. NO devices can be located on this bus.
FQA defines 3 new prefixed address segments for addressing devices on an I2C Intranet. A necessary expansion on the I2C device-address system.
- The first 3 bits are reserved to indicate which of up to 8 I2C busses this Network is on.
LSB 13, LEN 3 - There are 8 reserved multiplexer addresses: 0x70 → 0x77. The following 3 bits are used to encode the “module address”.
LSB 10, LEN 3 - There are 8 busses available on each multiplexer. 3 bits are used to encode the bus.
LSB 7, LEN 3 - There are 7 bits of uniquely-addressable device-address space on each I2C bus.
LSB 0, LEN 7
FQA size: 16 bits. Together, segments 1 through 3 (bits 7 through 15) define the Subnet, while segment 4 (bits 0 through 6 still define the device-address.
Segment Extraction: (address >> LSB) & (0xFFFF >> (16 - LEN))
Visualized in binary: 0bNETMODBUSADDRESS
In decimal, like an IP address: N : M : B : ADR.
E.g. 0:3:1:043 (0b000:0 11:00 1:010 1011, 0x0668) indicates a device on I2C bus 0, module 3 (MUX address 0x73), MUX bus 1 (MUX instruction 0b00000010), device address 43 (0x2B).
| I2CIP Protocol | IP Equivalent | Function | Library Implementation | OSI Layer(s) |
|---|---|---|---|---|
| Device Lookup Protocol | ARP | Allows device groups to be looked up by ID. Informs "keep-alive" in a rebuild. | Routing Table (Hash Table): Device ID → FQA [ ] | Data Link, Network |
| Reverse DL Protocol | RARP | Allows device IDs to be looked up by FQA. | Routing Table (BST): FQA → Device ID | Data Link, Network |
| Network Scan Protocol | RIP | Builds the Routing Table. Uses SPRT EEPROM and any local SPRT to attempt address recognition (otherwise ignore). | Ping for modules + One-time local all-scan → Read EEPROM & One-time module all-scan → Build Routing Table | Data Link, Network |
| Bus Switching Protocol | SNMP | MUX bus switching | Write 1 << BUS to MUX |
Network |
Note: ARP and RARP are two different traversal methods of the same tree. This is done in order to minimize heap memory allocation (necessary on microcontroller architectures).
Multiplexers are used to split the I2C bus into many individually-selectable sub-busses, or simply busses. This allows for multiple devices to share I2C/device-address space independently on multiple subnets, enabling massive scalability of dynamic routing.
The I2C Multiplexer is a device that allows for the selection of one of 8 busses to be connected to the I2C bus. It is selectable by a 3-bit address, 0x70 to 0x77, and has 8 busses. Busses are selected in a bitwise fashion: 0b00000001 selects bus 0, 0b00000010 selects bus 1, etc. while disabling all other busses, and 0b00000000 disables all busses.
24LC32 16-bit 4kB EEPROM on Bus 0 at address
0x50stores a UTF-8-encoded JSON-formatted partial routing table (with decimal-encoded addresses).
A partial routing table (see below) indexes only the devices on a bus.
// Indexed by BUS
[
// Bus 0 (default bus)
{
// Array is used to list all occurrences of flexible-address devices
"eeprom" : [ 80 ], // Decimal encoding (0x50) - This is the EEPROM we are reading from
"mydevice" : [ 101, 43 ],
},
//Bus 1, etc. up to Bus 7 (makes 8 buses)
{ },
{
"eeprom" : [80], // A different eeprom, on bus 2
},
{ },
{ },
{ },
{ },
{ },
]Note: when flashed, all comments, whitespaces, and trailing commas MUST be removed. All strings must be double quotes.