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https://github.com/portasynthinca3/cant

Shared bus for DIY smart homes
https://github.com/portasynthinca3/cant

avr cant-bus smarthome

Last synced: about 3 hours ago
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Shared bus for DIY smart homes

Host: GitHub
URL: https://github.com/portasynthinca3/cant
Owner: portasynthinca3
Created: 2022-05-11T19:50:57.000Z (over 2 years ago)
Default Branch: master
Last Pushed: 2022-05-11T19:51:36.000Z (over 2 years ago)
Last Synced: 2024-11-06T03:32:21.307Z (about 2 months ago)
Topics: avr, cant-bus, smarthome
Language: C
Homepage:
Size: 22.5 KB
Stars: 0
Watchers: 1
Forks: 0
Open Issues: 0
Metadata Files:
- Readme: README.md

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README

# CAN'T
**CAN Alternative, Not Terrible** (WIP) is a shared decentralized bus intended for use in DIY smart homes with cheap MCUs.
- **PHY** level: wired, asynchronous, half-duplex. AVR implementation tested with **100kbps** @8MHz (internal RC oscillator).
- **APP** level: up to 256 devices, devices react to others' state change broadcasts, as opposed to direct control messages.

## Directories
- `avr`: sample AVR implementation
- `tmp_sensor`: KiCad project for a DS18B20-based temperature sensor

## Progress
- [x] PHY specification laid out
- [x] APP specification laid out
- [x] AVR transmitter
- [ ] AVR receiver
- [ ] AVR bridge to USB
- [ ] ESP32 bridge to WiFi
- [ ] Mobile app
- [ ] PHY specification consolidated
- [ ] APP specification consolidated

## PHY specification
Properly terminated and biased RS-485 twisted pair.

### States
The line has three states:
- **`L`**: One device drives the line, D+ is low, D- is high.
- **`H`**: One device drives the line, D+ is high, D- is low.
- **`Z`**: No devices drive the line, biasing must force this state into an **`L`**.

Data is encoded using NRZI:
- **`T`**: **`L`**->**`H`** or **`H`**->**`L`** (transition), signals a logic **`1`**.
- **`N`**: **`L`**->**`L`** or **`H`**->**`H`** (no transition), signals a logic **`0`**.

Special conditions:
- **`NNNNNN`** (no transition for 6 bit times) signals the end of a transmission.
- Four consecutive **`N`** s must be followed by an additional **`T`** (bit stuffing), even if one is already coming up, to maintain synchronization.

### Frame
An individual transmission is called a **frame**. Frames include, in order:
- **Sync signal**: An **`HL`** at the rate of the transmission, which it has to maintain during the rest of the frame. For a 100kbps transmission the bit time is 10 microseconds.
- **Data**: NRZI-encoded with bit stuffing, as explained above. The length (in bits) must be divisible by 8. Bits within bytes are sent MSB first, LSB last.
- **Checksum**: encoded just like the data. Reference implementation in `avr/src/cant.c` in function `cant_crc`.
- **Post-transmission silence**: **`NNNNNN`** (no transition for 6 bit times), signifies the end of the transmission.
- **Release**: **`ZZ`**, the transmitter releases the bus, allowing the biasing network to force it into an **`L`**. Other transmitters aren't allowed to begin transmitting during this time interval.

### Congestion resolution
Since there's only one shared bus, only one device can talk at the same time. Here's the congestion resolution algorithm:
- Before considering transmission, pick the priority (**`0`** - normal, **`1`** - low). This value is internal to the device and never shared on the bus. Each device should also have a pre-configured constant 8-bit address.
- Just before taking control of the bus, check time since the last **`T`** state. It must be >= 8 bits at the speed of the last transmission, or the default configured speed if none occurred beforehand. If the time is less than 8 bits, the current transmission has not yet been completed, and the transmitter should wait until it completes.
- Wait for `(address*(priority+1))/4` bits. It can be observed that the lower the address and higher the priority, the sooner the device will try to write again. It is therefore a good idea to give important devices like intrusion detectors low address numbers.
- If the line is still busy, wait again.
- Otherwise, start transmitting.

## APP specification
(all multibyte fields are big endian)

### First three bytes
- `[0]`: source (transmitter) address
- `[1]`: 5 MSB = packet type, rest are reserved
- `[2]`: target address for targeted packet types, any value otherwise

Sections below mention the packet types by their name, numeric value and whether they're (B)roadcast or (T)argeted.

### State Change (0B), State Transmit (1B), Force Switch (4T)
These three packet types use the same field format, but the semantics are different:
- `State Change` is broadcast every time the state changes.
- `State Transmit` is broadcast at regular intervals irregardless of changes in the state, preferably about every 1 to 5 seconds.
- `Force Switch` is sent from one device to another asking for a force state switch, overriding its internal rules. If the change was successful, the target should broadcast a `State Change`.

Fields:
- `[3]`: 5 MSB = device type, 3 LSB = configuration parameter related to type. Device types:
- `0`: Switch or switch group; `parameter`: number of switches; `[4]`: switch states, arranged from LSB for switch 0 to MSB for switch 7.
- `1`: Brightness controller; `[4]`: value.
- `2`: Light bulb; `parameter[LSB]`: supports brightness control; `[4]`: brightness. For on-off lamps, values above 128 are interpreted as "turned on", and "turned off" for values below 128.
- `3`: Temperature sensor; `[5:4]`: centidegrees celsius above -273C

### Reset Request (2T)
Requests a device reset.
`[3][LSB]`: enter bootloader for firmware download?

### Program Block (3T)
Piece of firmware to be flashed by the bootloader.
`[4:3]`: page number, interpretation depends on device architecture.
`rest`: page data.

### Program Response (5T)
Sent by the device in bootloader mode in response to a `Program Block`.
`[3]`: status:
- `0`: OK, page flashed
- `1`: transmission error
- `2`: write failed
- `3`: page missed
`[5:4]` if status==3: first missed page number