Motorola is implementing TTP™ in support of the automotive industry approach to the development of “by wire” automobiles, which will rely on electronics and electric motors, rather than mechanical and hydraulic parts, to control steering, braking, suspension and transmission. The “by wire” systems are targeted to be highly dependable fault-tolerant electronics which will improve the capabilities, lower the overall cost, give higher reliability and reduce the car weight.
Compared to other leading communications protocols in cars today, e.g. Controller Area Network (CAN), TTP represents a completely different concept by being based on time-division multiple access (TDMA). Every action, transmission or reception of messages on the TTP bus is pre-defined, precisely timed and synchronized to a global notion of time, which every TTP node maintains continuously. The overriding philosophy is that everything is deterministic and known a priori and there are no surprises (e.g., events) such as in an event-driven protocol like CAN where every message transmission needs to arbitrate for the bus and the bus contention is resolved using a wired-and ID scheme.
The TTP module is based on a protocol engine that controls a serial data transfer on 2 separate channels (TTP Bus) which are connected between several TTP nodes. Each TTP module acts as a peripheral module for a host microcontroller. The handshake between the protocol engine and its host microcontroller is implemented as a RAM that can be accessed by the host microcontroller and the protocol engine. This RAM has a status/control section and a message section that stores the data that should be transferred through the TTP bus. A message descriptor list RAM (MEDL RAM) contains all the transferred data attributes,for example the address of the data, time of transfer within the TDMA cluster cycle etc. In order to synchronize between the nodes, the module includes a Time Control Unit (TCU) that controls the timing of data transfer, using timing compensation logic that synchronizes between the TTP nodes.
The TTP module supports both NRZ and MFM transmission codes, and uses bus guardians (BGs) to disable the transmission onto the TTP bus when the node is not allowed to transmit, in order to increase the fault tolerance of the system.
Motorola will initially be developing a TTP module that will be provided on a standalone TTP controller. The roadmap includes integrating the TTP module into several leading microcontroller families from Motorola, for example the 16-bit HC12 series and PowerPC®. We plan to present a detailed description of Motorola's TTP/C controller implementation.