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Synthèse de haut-niveau de contrôleurs ultra-faible consommation ...

Synthèse de haut-niveau de contrôleurs ultra-faible consommation ...

tel-00553143, version 1

tel-00553143, version 1 - 6 Jan 2011 108 Proposed system model and design-flow for SM synthesis shows however some inherent problems. For example, often the tasks to be executed in a WSN node are smaller w.r.t. the overhead incurred for switching between tasks. Also, each process requires its own stack space in memory, which does not fit properly with the stringent memory constraints of sensor nodes. 5.1.3 Event-driven approach For these reasons, a relatively different execution model seems more appropriate. The idea is to take into account the reactive nature of a WSN node. In this case, the system essentially waits for any event to happen where an event can be the availability of data from a sensor, the arrival of a packet, or the expiration of a timer and reacts to these events by performing certain tasks. This approach is called event-driven execution paradigm. Since all the events are destined for small tasks to be performed, the whole WSN application can be presented in the form of Task Flow Graphs (TFGs). Such an approach is presented in Figure 5.2 (c) where different application and control tasks, present in a system, communicate with each other through events. 5.2 System-level execution model We use event-driven paradigm as the system-level execution model in our approach. To give an example, Figure 5.3 shows the TFGs of a lamp switching application, where a transmitting node demands a receiving node to switch on/off its lamp if a button is pressed at transmitter end. Figure 5.3 (a) shows the TFG for receive mode when a node waits for a signal from the transmitter and switches the lamp, whereas Figure 5.3 (b) presents the TFG for transmit node where a node waits for push-button event and sends a signal to the receiver to switch the lamp. This application involves several tasks such as data transmission, data reception, wait for acknowledgment, and timer, push-button and lamp switching APIs. All these control-oriented tasks are spread across different layers of the communication stack and involve further sub-tasks associated to them. For instance, beacon and data packets transmission and reception involve physical layer functions that exchange data between I/O peripherals of the MCU and the RF transceiver using SPI-protocol. The control-flow itself follows a simplified version of RICER, a low-power MAC protocol [84]. In typical WSN node, such tasks are handled by an MCU and corresponding OS that provides support for multi-tasking features. In our proposed approach, all of the tasks present in such TFGs are executed on dedicated hardware resources (i.e. the hardware micro-tasks). Figure 5.4 presents a system-level view of a WSN node platform based on our proposed approach. Such a system consists of: � A set of power-gated hardware micro-tasks accessing shared resources (e.g. peripherals (RF, sensor) and memories (gated/non-gated)). Each of these hardware micro-tasks is able to perform a specific task such as temperature sensing, processing data, sending data to SPI-interface etc.

tel-00553143, version 1 - 6 Jan 2011 System-level execution model 109 Ext.Event T receiveData() Timer 100 ms MT1 MT2 data_Received MT5 turnOff() MT3 light_Switched timeOut_NoData ack_Sent lightOn() MT4 WakeUp Beacon rIdx, rIdy sendBeacon() beacon_Sent sendAck() Received Data tIdx, tIdy Ack Frame tIdx, tIdy Received Data Previous Data LED ack_notOK not_received_Beacon MT11 turnOff() Ext.Event B counter() wait() Push Button MT6 MT7 MT8 wait_Done ack_OK MT9 Counter period wait_Beacon rIdx, rIdy receiveBeacon() received_Beacon data_Sent MT10 Wait time Sent Data tIdx, tIdy rIdx, rIdy sendData() receiveAck() (a) TFG for receive mode (b) TFG for transmit mode Figure 5.3: TFGs presenting the tasks running in a lamp switching application. Int. Event A En_A Micro-task A Sensor (e.g. : heat) En_MEM_A Int. Event B I/O Port Radio chip (e.g. : CC2420) Micro-task Micro-task Vdd B C Vdd Local Memory A’ I/O Port En_B Ex. Event B Monitor Vdd Vdd Vdd Vdd Flash Memory G Ex. Event A Int. Event C En_C En_MEM_B Int. Event D Local Memory B’ En_D Micro-task D Figure 5.4: System-level view of a micro-task based WSN node architecture

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