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UARTBus OSI-2: Network layer

2.1.3.1 Device and device group addresses

Every node receives every packets on bus. This makes possible to create broadcast addresses (0 is a general broadcast address) and device groups by using negative addresses for this purpose.

• 0 : common, special purpose broadcast address
• positive number: device address
• negative number: group address

2.1.3.2 Variable length addressing

Instead of using fixed int{8,16,32,64}_t for addressing, i've created a so called variable length addressing:

• If we choose int8_t, every packet has only two byte of overhead at the addressing, but only 128 group 1 broadcast and 127 device address available.
• If we choose int32_t, every packet has 8 byte overhead, but addresses can go at group to 2_147_483_648, one broadcast and 2_147_483_647 device address.

To balance between this extreme cases i've created an encoder/decoder that enables short address bytes when addresses are low (between -64 and +63) and increase address length when assign higher addresses.

Byte scheme:

• E: Extend: Address continued on the next byte. The in last address byte this bit is 0.
• S: Signum: sign of the whole number. Used only in the first byte. If set address is converted as: -(addr+1)
• An: address bit.

Example for one byte addresses:

Example for two byte addresses:

These types described as (s)vint_t (signed variable int) but it is not a real type which the compiler knows, just a notation. (There's an implementation for uvint_t if you use only unsigned values.)

2.1.3.3 Default packet scheme

The default packet scheme of the project:

• svint_t destination address
• svint_t source address
• uint8_t[n] payload
• uint8_t crc8 packet checksum