Time-Triggered Communication - Networked Embedded Systems - page 336

FIGURE . TTP monitoring node.

fault-tolerant TTP/C bus with  Mbps. In addition, the TTP Powernode provides interfaces to CAN
and TTP/A. As an operating system, TTPOS [Tan] with the fault-tolerance layer FT-COM stack
canbeused.
Another TTP/C-based product is the TTP monitoring node [TTTc] (c.f. Figure .), which
isalsobasedontheTTP-Ccontroller.heTTPmonitoringnodeisequippedwiththeMotorola
embedded Power PC processor MPCT, which features an on-chip fast Ethernet controller
(BASE-T), which can be used, e.g., for TCP/IP connections to a PC. The monitoring node
supports synchronous ( Mbps) and asynchronous (MFM— Mbps) bus interfaces for establishing
the TTP/C communication. For the synchronous bus interface, it employs a BASE-TX physical
layer with the media independent interface (MII) forming the bus toward the physical interfaces.
Furthermore, a PCMCIA slot for cards type I/II/III is available on the monitoring nodes.
The MPCT host CPU employs a PowerPC core and runs at  MHz. It is equipped with 
MBytes of RAM, and  MBytes of flash memory. The CNI memory of the TTP-C controller is
mapped into the address space of the MPCT host CPU.
The TTP monitoring node uses the embedded real-time Linux variant RTAI as its operation sys-
tem. RTAI [BBD + ] is short for “Real Time Application Interface”. It is a real-time Linux extension
developed by the Dipartimento di Ingegneria Aerospaziale Politecnico di Milano, which combines
a real-time hardware abstraction layer (RTHAL) with a real-time application interface for making
Linux suitable for real-time applications. RTAI introduces a real-time scheduler, which runs the con-
ventional Linux operating system kernel as the idle task, i.e., non-real-time Linux only executes when
there are no real-time tasks to run, and the real-time kernel is inactive.

14.5.2 TTP/A
TTP/A [EHK + ] is a member of the TTP protocol family that is intended for low-cost net-
works of sensors and actuators. TTP/A uses a structured name space as the source and sink of
all communication activities. This structured name space is called interface file system (IFS). An
IFS file is an indexed sequential file with up to  records. A record has a fixed length of  bytes
( bits).

14.5.2.1 Clock Synchronization

In TTP/A, communication is organized into rounds each consisting of several TDMA slots. A slot is
the unit for transmission of exactly one data byte transmitted via standard universal asynchronous
receiver transmitter (UART) format. Each communication round is started by the master with a so-
called fireworks byte. The fireworks byte serves as the epoch of the global time base and provides
a reference signal for clock synchronization. he fireworks byte enables slave nodes to synchronize
their local clocks. During startup, a fireworks byte with a regular bit pattern is used to establish initial
synchronization in conjunction with slave nodes containing imprecise on-chip oscillators.