AS8202

The AS8202 communications controller is an integrated device supporting serial communication according to the TTP/C specification. It performs all communications tasks such as reception and transmission of messages in a TTP^®  cluster  without interaction of the host  CPU.

TTP^®  provides mechanisms that allow the deployment in high-dependability distributed real-time systems. AS8202 provides following services:  
predictable transmission of messages with minimal jitter 
 fault-tolerant distributed clock synchronisation 
 consistent membership service with small delay 
 masking of single faults

The CNI (communication network interface) forms a temporal firewall. AS8202 decouples the controller network from the host subsystem by use of a dual ported RAM. This prevents the propagation of control errors. The interface to the host CPU is implemented as 16 bit wide non-multiplexed asynchronous bus interface.

TTP/C AS8202 follows a conflict-free media access strategy called time-division-multiple access (TDMA). This means, TTP/C deploys a time slot technique based on a global time which is  permanently synchronised. Each node is assigned a time slot in which AS8202 is allowed to perform transmit operation. The sequence of time slots is called TDMA round, a set of TDMA rounds
forms a cluster cycle. After one cluster cycle the operation of the network repeats.

The sequence of interactions forming the cluster cycle is defined in a static time schedule, called message-descriptor-list (MEDL). The definition of the MEDL of AS8202 conjunction with the global time determines the response time for a service request. The membership of all nodes in the network is evaluated by the communication controller. This information of AS8202 is presented in a consistent fashion to all correct cluster members. During operation, the status of every other node is propagated within one TDMA round. The MEDL of  AS8202 is loaded into the configuration memory when the system starts up.