Big project: interfaces

Two chips that talk to each other need a common language. For people that is a protocol on paper, for chips a list of addresses with a fixed meaning. Without that list, sooner or later one chip writes the temperature where another expects the battery level.

Defining memory addresses#

For a few values one Logic Memory per value is enough. When there are many, a data bus pays off: the stack of a housing of its own, which all chips read and write via fixed addresses. get reads an address, put writes it, and the chip fetches the ReferenceId of the bus housing via lbn:

define GEHAEUSE HASH("StructureCircuitHousing") # IC housing
lbn bus GEHAEUSE HASH("Datenbus") ReferenceId Average
get version bus 0          # address 0: interface version
get value bus 3            # address 3: power status
put bus 2 status           # address 2: only life support writes here

Documenting the protocol#

Every address gets a line in a table that sits in every chip involved as a header comment or lives in the library next to the scripts:

Three columns are mandatory: who writes, what it means and which values are allowed. Anything outside the range is treated as an error by the reader.

Versions#

Sooner or later you change the protocol: a field is added, a meaning changes. Then old and new chips run together for a while. So that this does not go wrong silently, the interface version sits at a fixed place. Every chip compares it with the version it understands itself and only executes commands if both match:

seq ok version SCHNITTSTELLE     # NaN or wrong version: 0
select command ok command 0  # no command without the matching version

If the version does not match, the chip goes into the safe state, reports the error and writes its own version into its status. That way the central unit sees immediately which chip still speaks the old language.