We present a new reliable Network-on-Chip (NoC) suitable for Dynamically Reconfigurable Multiprocessors on Chip systems. The proposed NoC is based on routers performing online error detection of routing algorithm and data packet errors. Our work focuses on adaptive routing algorithms which allow to bypass faulty components or processor elements dynamically implemented inside the network. The proposed routing error detection mechanism allows to distinguish routing errors from bypasses of faulty components. The new router architecture is based on additional diagonal state indications and specific logic blocks allowing the reliable operation of the NoC. The main originality in the proposed NoC is that only the permanently faulty parts of the routers are disconnected. Therefore, our approach maintains a high run time throughput in the NoC without data packet loss thanks to a self-loopback mechanism inside each router.

1. Introduction

Nowadays, the trend for Embedded Systems is moving toward Multiprocessor Systems on Chip (MPSoC) in order to meet the requirements of real-time applications. The complexity of these Systems on Chip (SoC) is increasing and the communication medium is becoming a major issue in MPSoC. Generally, integrating a Network-on-Chip (NoC) in the SoC provides an effective way to interconnect several Processor Elements (PEs) or Intellectual Properties (IPs) (processors, memory controllers, etc.) [1]. The NoC medium features a high level of modularity, flexibility, and throughput. A NoC is constituted of routers and interconnections allowing the communications between the PEs and/or IPs. Communication on NoC relies on data packet exchanges. The paths for the data packets between a source and a destination through the routers are defined by the routing algorithm. Therefore, the paths that data packets are allowed to take in the network depend mainly on the adaptiveness permitted by the routing algorithm (partially or fully adaptive routing algorithm) which is locally applied in each router being crossed and for each data packet [2, 3].

Dynamically reconfigurable 2D Mesh NoCs (DyNoC, CuNoC, QNoC, ConoChi, etc.) are suitable for FPGA-based systems [1, 47]. Indeed, thanks to partial dynamic reconfiguration of FPGAs [8], the number and the position of the PEs or IPs implemented on the FPGA Chip being dynamically modified during runtime allow more adaptiveness of the MPSoC.

To achieve a reconfigurable NoC, an efficient dynamic routing algorithm of data packets is required. The goal is to preserve flexibility and reliability while providing high NoC performances in term of throughput. Figure 1 illustrates a dynamic reliable NoC. Figure 1(a) presents the communications between several IPs whereas Figures 1(b) and 1(c) depict a dynamic placement of an IP or the occurrence of a faulty node where bypasses determined by dynamic routing algorithm are required. Furthermore, faulty nodes or even faulty regions (areas in the network having a size larger than a tile) also make the communications in the networks harder, and even impossible for some routing algorithms and required fault-tolerant algorithms, as shown in Figure 1(c). Therefore, dynamic component placements and faulty nodes or regions are the main reasons why fault-tolerant or adaptive algorithms have been introduced and used in run-time dynamic NoCs [4]. Regarding adaptive or fault tolerant routing algorithms, several solutions have been proposed [9, 10]. Generally, these algorithms correspond to a modified XY routing algorithm allowing to bypass faulty or unavailable regions. In the case of adaptive routing algorithms based on the turn model [11], zones corresponding to already detected faulty nodes or unavailable regions in the NoC are defined. The neighbor routers of these zones must not send data packets toward these faulty routers or unavailable regions. To achieve that, chains or rings around the adjacent faulty nodes or regions are formed in order to delimit rectangular parts in the NoC covering all the faulty nodes or unavailable regions. In these chains or rings of switches, the routing tables are modified and differ from the standard tables related to the XY routing algorithm. These specific switches integrate in their tables additional routing rules allowing to bypass the faulty zones or regions dedicated to dynamic IP/PE instantiations, while avoiding starvation, deadlock, and livelock situations [11].

Regarding increasing complexity and the reliability evolution of SoCs, MPSoCs are becoming more sensitive to phenomena generating permanent, transient, or intermittent faults [12]. These faults can affect the data packet contents or generate routing errors. To detect these errors, specific error detection blocks are required in the network to locate the faulty sources. Moreover, permanent errors must be distinguished from transient errors. Indeed, permanent faulty parts of the NoC must be located precisely in order to be bypassed thanks to the adaptive routing algorithm. Consequently, the considered fault models used are Stuck-at faults for localization of permanent errors, and Bit-flip faults for transient errors [13].

To protect the data packets against errors, Error Correcting Codes (ECC) are implemented inside the NoC components [14, 15]. Among the well-known solutions, three are usually applied for the communications in MPSoC through a NoC. First, the end-to-end solution requires to implement an ECC in each input port of the IPs or PEs of NoC [14]. The main drawback of this solution is its incapacity to locate the faulty components (PE, IP, router, connection, etc.) in the NoC. Consequently, it is inadequate for dynamic NoCs where the faulty or unavailable zones must be bypassed. Second, the hop-by-hop detection is based on the implementation of ECC in each input ports of the NoC switches. For instance, in a router of four communication directions (North, South, East, West), four ECC blocks are implemented. Therefore, when a router receives a data packet from a neighbor, the ECC block analyzes the content of the packet to verify the correctness of the data. This process allows to detect and correct data errors according to the efficiency of the ECC being used. Third, another proposed solution is the code disjoint [14]. In this approach, routers include one ECC in each input and output data ports. This solution localizes the error sources which can be either in the switches or on the data links between routers. For all these techniques, each ECC implemented in the routers of the network requires an additional cost in terms of logic area, latency of the data packets and power consumption.

Concerning the routing error detections and among the existing techniques able to detect faulty routing decisions, we find the analysis of the source and destination addresses presented in [16, 17]. When a router receives a data packet, it compares its own address with the destination and source addresses. Then, the router checks its position in the deterministic XY path in the NoC of the considered data packet. The router performing this control is able to conclude if the switch from which it received the packet made or not a routing error according to the correct XY path. The drawback of this technique is the impossibility to handle the bypass of faulty nodes or regions. It results that this solution cannot be applied in adaptive or fault-tolerant routing algorithms. Indeed, as specified in a turn model algorithm [11], the structure of the reconfigurable NoC may contain bypass areas in which the switches must be able to take different routing decisions than the XY routing algorithm. To handle message routing errors in dynamic networks, a new faulty switch detection mechanism is required for adaptive or fault-tolerant routing algorithms. The aim is defining a new mechanism allowing the bypasses of the faulty nodes or regions (statically or dynamically PEs/IPs placed).

In this paper, we present a new reliable dynamic NoC. The proposed NoC is a mesh structure of routers which are based on a new architecture to detect the adaptive routing errors. Our approach includes data packet error detections and corrections. The main originality of the proposed architecture is to localize precisely the error sources whereas using Switch-to-Switch error detections.

The remaining of this paper is organized as follows. Section 2 describes the architecture of the proposed reliable switch. Section 3 details the proposed routing error detection suitable for adaptive routing algorithms. Section 4 presents a specific self-loopback mechanism allowing to avoid losing data packets, maintain the performance of the NoC, and locate efficiency the permanent sources of data packet errors. Section 5 presents functional simulation validation while Section 6 gives several synthesis and performance results. Finally, conclusion and future works are given in Section 7.

2. Basic Concept of the RKT-Switch

We propose a new reliable NoC-based communication approach called RKT-NoC. The RKT-NoC represents a packet-switched network of intelligent independent reliable routers called RKT-switch. The architecture of the RKT-switch is depicted in Figure 2. The RKT-switch is characterized by an architecture of four directions (North, South, East, West) suitable for a 2D mesh NoC. The PEs or IPs can be connected directly to any side of a router. Therefore, there is no specific connection port for a PE or IP. Each port direction is composed of two unidirectional data bus (input and output ports). Each input port is associated with a fifo (buffers) and one routing logic block. The RKT-switch is based on store-and-forward switching technique. This technique is suitable for dynamically reconfigurable NoCs. Indeed, in our considered NoC, the PEs or IPs can be implemented by replacing one or several routers [6]. Therefore, by using the store-and-forward technique and when routers need to be reconfigured, just the emptying of the data packets stored in their buffers is required. Contrariwise, by using the wormhole switching technique [18], one data packet can be stored simultaneously in several routers. Consequently, the time required to empty all routers containing partial data packets and to reconstruct these packets before performing a reconfiguration is more significant.

The data flow control used in our architecture is the Ack/Nack solution which can handle fault tolerant transmissions and minimize the impact of the considered Bit-flip model while increasing the robustness against SEU [19]. This solution relies on the retransmission of the packets which are received as faulty by a neighbor node. To perform one retransmission after that a data packet is sent to a node, a copy of the packet is locally saved until a Ack or Nack is received. If a neighbor router receives a flit containing an error which cannot be corrected by the ECC, a Nack is sent and the whole packet is retransmitted. Otherwise, an Ack is generated at full packet reception. More precisely, an Ack can be generated only when the totality of the data packet has been received and checked by the router. This allows to reduce the latency. For our proposed RKT-switch, the Hamming ECC is considered in order to provide a tradeoff between area overhead and the data error correction capacity compared to others ECC [15]. This choice allows to correct Single Event Upset (SEU) errors (one error bit in a flit) and detect Multiple Event Upset (MEU) errors (two error bits in a flit). Moreover, the Hamming code is more suitable for NoC based on Ack/Nack flow control than the parity bit check. Indeed, in case of Nack, the data packet latency is increased due to the required retransmission. However, with the Hamming ECC a SEU detection does not require a retransmission and then the throughput is preserved. The distinction between the temporary and transient errors is obtained thanks to a centralized journal, saving the transmission results, and a loopback output mechanism (see Section 4). Furthermore, our solution combined with the loopback mechanism and the novel centralized journal allows to locate errors, either in the bus connections or inside the switches (more details in the Section 4). The main advantage of this mechanism is to limit the resources overhead due to the considered Stuck-at model. In addition, our reliable structure is based on hop-by-hop detection allowing the robustness against the SEU and two bits MEU errors while maintaining a good tradeoff between the area overhead and the capacity to locate errors. Moreover, the IPs connected in the NoC also require to integrate the proposed blocks for reliability mechanism. Indeed, these error detection techniques (routing error detection and error correcting code) allow online error detection on the boundaries of the NoC.

3. Routing Error Detection

The proposed reliable switch incorporates an online routing fault detection. This approach allows the routing error detections for adaptive algorithm based on the well-known XY routing algorithm. The main difficulty to detect the errors is to distinguish a bypass of an unavailable component in the NoC due to the use of adaptive algorithm from a real routing error due to a faulty component of the NoC. Figure 3 illustrates the challenge for such error detections. Apart from an increase of the data packet latency, the consequence of the nondetection of routing errors is the loss of data packets which can be sent either to an already detected faulty router or toward an area performing a dynamic reconfiguration. To achieve a routing error detection, the proposed reliable router relies on diagonal node state indications, additional routing information in the header flits, and routing error detection blocks in each port (see Figure 2). The basic concept of our approach is as follows: each router receiving a data packet checks the correctness of the routing decision made by the previous crossed switch.

3.1. Elements Required for the Routing Error Detections
3.1.1. Diagonal Availability Indications

RKT-switch uses information links to indicate to its neighbors its availability status. We define as unavailable an input port which cannot receive data packets. To preserve the highest throughput of the NoC, our strategy is to disconnect only the faulty parts of the routers. Thereby, if an input port router is permanently faulty, it is disabled while maintaining active the others input ports in order to obtain a partial running switch. Contrariwise, if all input ports are faulty, the router is considered as unavailable. Similarly, we define as unavailable component, a component of the NoC which cannot receive data packets due to permanent faults or a partial dynamic reconfiguration.

The RKT-Switch indicates its availability status to the eight direct neighbor routers through the Diagonal Availability Indication (DAI) links. The network structure based on the DAI links is given in Figure 4. These DAI interconnections allow to check the correctness of the routing algorithm. Indeed, each router is able to control the availability status of the neighboring routers or components. For instance, in Figure 4, the router(𝑖,𝑗) can check the availability of the North and South input ports of the router(𝑖+1,𝑗+1). The network components (PEs or IPs) are not allowed to route data packets and are limited only to accept data packets intended for them. Moreover, the DAI interconnections are set when components are instantiated in the neighborhood of the considered router.

3.1.2. Journal of Error Localizations

Each routing error detection block of the input routers, owns three register journals (error journals) to keep the routing error detection results. These journals correspond to the routing logic blocks of the neighbor router connected to the considered input port. For example in the Figure 4, the West routing error detection block of the router(𝑖+1,𝑗+1) has three journals corresponding to the West, North, and South routing blocks of the router(𝑖,𝑗+1). Thanks to these error journals, the distinction between permanent and transient errors can be insured. In addition, from these journals, the location of the faulty routing algorithm blocks in the neighboring routers can be deduced. Therefore, a permanent error is considered if three successive routing error detections occur for a specific routing logic block.

3.1.3. Structure of the Information Fields in the Data Packets

A Sliding Gather Data (SGD) field is added in each header flit of the transmitted data packets. Figure 5 details the structure of a data packet. A Flit type bit is used to distinguish the header from the data flits. The SGD field contains the addresses of the previous and penultimate routers crossed. Therefore, each router receiving a data packet checks the SGD field and validates the routing choice made by the previous router. To achieve the routing validation, the SGD field is updated at each router crossed during the data packet transmissions. This update is done by each input buffer block which also requires an update of Hamming code due to the modification of the header flit of the data packets. In addition, a Unique Routing Path Indication (URPI) bit is added to the header of the data packets. This bit is set if a local router has only one routing output path available. This bit allows to avoid false detections. Our approach allows one solution adapted for the dynamic NoC-based fault-tolerant routing algorithm.

3.2. Application of the Routing Error Detections

RKT-switch allows online routing fault detections and can distinguish routing errors from dynamic bypasses due to the adaptive routing algorithm. If a router is surrounded by three unavailable neighbors, it become also unavailable. Indeed, as we use nonbouncing routers [20], if a data packet is sent to a router surrounded by three unavailable nodes, the packet cannot be routed. Several application examples illustrate the efficiency of the proposed routing error detections.

Figure 6 gives two examples of data packets routed from the router(𝑖1,𝑗1) toward the router(𝑖,𝑗1). In Figure 6(a), the router(𝑖,𝑗1) receives a data packet from its West neighbor router. It checks the correctness of the routing decision by the router(𝑖1,𝑗1). In this case, the router(𝑖1,𝑗1) respects the XY algorithm and no error is detected. In the Figure 6(b), the router(𝑖,𝑗) receives a data packet from the South direction. From the analysis of SGD field of this data packet and the penultimate router and destination addresses, the XY routing path between the router(𝑖,𝑗1) and the router(𝑖+1,𝑗1) is deduced. Consequently, the router(𝑖,𝑗) deduces that the router router(𝑖,𝑗1) do not respectes the XY routing algorithm. It results that the router(𝑖,𝑗) checks the DAI links of the router(𝑖+1,𝑗1). In the case where the router(𝑖+1,𝑗1) is unavailable, then the router(𝑖,𝑗1) makes a bypass decision and is not faulty (see Figure 6(a)). Otherwise, if the router(𝑖+1,𝑗1) is available, then the router(𝑖,𝑗1) makes a faulty routing decision (see Figure 6(b)). In Figure 6(b), the router(𝑖,𝑗) sets the error journal associated with the position of the routing logic block guilty of this routing error decision. This routing logic block is identified from the address of the penultimate node. More precisely, by checking the SGD field and identifying the penultimate router(𝑖2,𝑗2), the West routing logic block of the router(𝑖1,𝑗1) is deduced.

Figure 7 illustrates the role of the URPI bit. Here, the router(𝑖,𝑗) receives a data packet from the North direction according to the XY routing algorithm. By checking the DAI of the router(𝑖+1,𝑗+1), a bypass decision is deduced. However, the destination is located on the North direction compared to the previous router. According to the routing algorithm used, the bypass should have been by the North direction. The availability of the router(𝑖,𝑗+2) is deduced by checking the URPI in the SGD field. Indeed, if the URPI is activated, the router(𝑖,𝑗+1) had only one remaining path and then did not made a routing error. Contrariwise, if the URPI is not activated, the router(𝑖,𝑗+1) has two remaining available paths to route the data packets, and then it makes a routing error.

3.3. Principle of Routing Error Detection

XY based adaptive routing algorithms use the rules of the XY-algorithm [11] to route data packets into the network when there are available components. In case of an unavailable component, a specific routing path is locally chosen to bypass its position. When a router receives a data packet, it checks the correctness of the routing decision made by the previous node by applying the routing error detection algorithm depicted in Figure 8. If the address of the previous router (indicated in the SGD field) is the router itself, no routing error detection is performed due to a loopback of data packets (see Section 4). From the comparisons of addresses, the router checks if the previous routing decision has respected the XY routing algorithm. If it is respected, the previous decision is correct. Otherwise, the router defines if the previous decision is a bypass decision or a routing error. The detection algorithm requires to check the availability of the router in which the data packets should have pass according the XY algorithm. This control is performed thanks to the DAI links. If the router in the XY path is unavailable, the previous router makes a bypass decision. If it is available, the previous router makes a routing error. In this last case, the router adds a “1” in the error journal associated with the faulty routing logic block. The position of the faulty block is deduced from the address of the penultimate router in the SGD field. If three consecutive errors are performed by the faulty routing logic block, a permanent error is considered. In this case, a specific data packet is generated with as destination the faulty switch in order to inform of its failure. This specific data packet of one flit size indicates the faulty input port of the considered router that must be disconnected. The header flit is updated at each router crossed (addresses of the previous and penultimate routers). Consequently, the header flit requires to be coded by a Hamming coder. This block is implemented in the input buffer. By using a NoC based on data packets composed of several flits, it can happen to have one flit which is not following the header flit. In the proposed RKT-NoC, each flit has one bit indicating if the flit correspond at a header flit or a data flit as depicted in Figure 5. When a router receives the first flit of a data packet after the Hamming decoding, it checks if it is a header flit thanks to the flit type bit. If it is not a header flit, the flit is destroyed. Therefore, when the destination component receives a data packet, it counts the number of flits. If the number of the received flits does not match the number of flits indicated in the header flit, the packet is destroyed and a retransmission request is send to the emitter component.

3.4. Simulations

A 3 × 3 RKT-NoC with 3 IPs as illustrated in the Figure 9 has been simulated in ModelSim environment. The data packets contain 2 flits. In this simulation, we focus on the routing error detection block located in the West port of the router(3,2). The simulation results are shown in Figure 10. At the Event 1, the error journal corresponding to the West routing block of the neighbor router(2,2) already has one error detected. Similarly, the North error journal has detected two errors in the last two routing error detections. At Event 2, a header flit is analyzed. By checking the information in the header flit, we deduce the destination is the 𝐼𝑃(4,1) and packet has been emitted by the 𝐼𝑃(2,4). The previous router is the router(2,2) and the penultimate router is router(2,3). The data packet is received by the router(3,2) from the West direction input port. Next, the routing error detection algorithm is applied. As the XY routing path has not been respected, the availability of the router(1,1) is checked via the diagonal_SW signal. As this router is unavailable, the North routing logic block of the router(2,2) did not made a fault, and its corresponding error journal is set to “0.” At Events 3 and 4, two data packets are received. Both are emitted from the router(0,2) and are destined to the router(3,4). For these data packets, the XY algorithm has not been respected. However, the router(2,3) which has transmitted these data packets is available. Consequently, a routing error is detected and a second “1” is added in the West error journal corresponding to the faulty routing block. The three previous routing error detections were positives, then a permanent routing error is deduced in the West direction input port of the router(2,2). This information is send to the control logic which sends a specific data packet to the neighbor in order to disconnect the faulty part of the router(2,2).

4. Loopback Block

In dynamic reconfigurable NoC, the components of the network can change during operation as illustrated in Figure 1. Indeed, the number and the position of the PEs and IPs in the NoC can be dynamically modified in order to meet the requirements of the application or to replace a detected faulty IP. To realize dynamic reconfiguration in the 2D Mesh NoC implemented in the FPGA, Partial Reconfigurable Regions  (PRRs) have to be defined inside the FPGA [21]. These PRRs are regions where Partial Reconfigurable Modules  (PRMs) are implemented. PRMs represent electronic circuits of functional units which are defined by specific partial-bitstreams and can be placed according to the application [21]. In practice, these PRMs correspond the PEs or IPs which are implemented and placed inside the dynamic NoC, as illustrated in Figure 1.

In a reliable NoC, faulty routers are run time isolated. If we consider a permanent faulty router which cannot be corrected, this router is permanently disabled and will never be used again. Similarly, during the reconfiguration of a PRR, no packet can be send inside the area being reconfigured. Thus, these active PRRs in the NoC are dynamically isolated. However, these isolations can lead to data packets losses and increase packet latency. These drawbacks occur when routers contain data packets in their output buffers while the neighboring nodes become unavailable due to dynamic implementations or permanent fault detections. Thereby, these data packets remain stored in the output routers. Consequently, data packets are either partially stored until the end of the reconfiguration (dynamic implementation case) or trapped and lost in the permanently faulty node detection case. To overcome these drawbacks, the proposed RKT-switch contains output buffer blocks associated with loopback blocks as described in Figure 2. The role of each loopback block is to empty the buffers of each output port by a looping back mechanism of data packets in the input port of the router (more details in the Section 4.3). It results that the looped back packets are re-routed toward another output port of the router and allow to avoid the data packets to be trapped. Figure 11 illustrates the role of a loopback block. A PE or IP emitter send toward a destination IP data packets according the XY routing algorithm. Suddenly, if we consider the router(1,3) unavailable, the data packets remaining in the West output of the router(2,3) are looped back and re-routed toward its South output. This mechanism allows then to route these stored data packets to the destination. Therefore, as the router(1,3) is identified like unavailable, the follow data packets coming from the East input port are directly routed toward the South port by using the dynamic routing algorithm. Furthermore, one main advantage of using our proposed loopback block associated with a centralized journal of errors and the switch-to-switch data error detections (see Figure 2), is the precise location and distinction of the data error sources. Therefore, we can locate precisely if the data errors are on the data bus or in the input or output ports, and if the faults are permanent or transient.

4.1. Localization of Data Packet Errors

To locate the errors and to distinguish permanent and transient errors, a centralized journal of data packet errors is implemented as described in the Figure 2. This block is composed of journals corresponding to the input and output ports. These journals are shift registers with three bits of depth.

The RKT-switch uses the Ack/Nack data flow control. When a data packet is transmitted to a neighbor node, one copy of the data is stored until the acknowledgement is received. If no error occurred during the transmission (reception of an Ack), a “0” is added in the journal related to the input port of the get-in direction and the output port. If an uncorrectable error is detected by the neighbor, a retransmission is performed by generating a Nack. If the Ack/Nack data flow control of the considered router receives successively three Nacks from a neighbor, we consider a permanent error of the data packet sent by the considered router. Consequently, the data packet is looped back. Indeed, if three transmissions of one data packet failed, then the neighbor detects an uncorrectable error due to a permanent error. After the loopback, the data packet is checked by the input ECC. If no error is detected, we can conclude the errors were on the data bus. The error happened three consecutive times on the bus (i.e., three Nacks), then we can conclude a permanent error on the data bus. If an uncorrectable error is detected by the ECC, the error can be either on the bus, on the input port, or in the output port. The data packet is destroyed and a “1” is added in the input and output journals. Table 1 shows the correlation of the results of the data error detection allowing to deduce the position of the error (input or output block, or data bus), and if the error is permanent or transient. Indeed, if three consecutive errors occur in the same input or output journal, the centralized journal of data packet errors concludes a permanent error in the corresponding direction. The data packets being looped back after being checked by the ECC are controlled by the routing error detection block. However, this block finds in the SGD field that the previous router address is similar to its own address, deduces a loopback, and does not perform the routing error detection algorithm. When a permanent fault has been detected in a router, the faulty part of the NoC needs to be isolated. The part to isolate has been located precisely by using the journal and the loopback with the switch-to-switch error detections and can be either in the input port, output port, or data bus. If the error is in the input port, the router will locally activate the unavailable horizontal availability link of the faulty input port, and the two associated DAI links. In this way, the neighbor component connected to the faulty port will not send new data packet in this direction, and the DAI flags will indicate to the diagonal neighbors the possibility to bypass its position. If the error is on the data bus or in the output port, the router who detects the permanent error must indicate to the neighbor to activate its DAI. To indicate which port need to be disconnected, the router detecting the permanent fault send a data packet to the destination of the neighbor router. This data packet based on one flit contains the address of the destination router, the direction of the port to disconnect and the Hamming bits to protect the data. The router must not send this special flit in the direction that have been detected as faulty. Consequently, the data packet is generated in the input port corresponding to the direction of the faulty neighbor. As we consider nonbouncing routers, the data packets cannot be routed in the same direction as the get-in port. Therefore, the routing logic block will made a bypass routing and the packet will be send to the correct router.

4.2. Avoiding Data Packet Losses

The RKT-switch uses buffers in each output port as depicted in the Figure 2. This switch is suitable for dynamic reliable NoC in which the components of the network can be modified in run time. If a PRR is dynamically reconfigured, no data packet can be sent to the reconfigurable area until the end of the reconfiguration process. Similarly, if a NoC router is becoming permanently faulty, the data packets need to bypass its position. However, if the output buffers are full while the neighbor router becomes unavailable (dynamic reconfiguration or permanent fault), the packets are trapped in the output port. In this case, the data packets are looped back inside the considered router in order to find a nonbusy or nonfaulty routing path. Therefore, the data packets are routed again toward a new output port of the switch, allowing to avoid the loss of the data packets. By emptying the output buffer, we reduce the data packet latency in the case where a neighbor router performing a partial reconfiguration. Similarly, the data packets are saved if the neighbor router is permanently unavailable. In the case of a loopback decision and emptying of the output buffer, the result of the ECC analysis is not used to disconnect the output port of the router. Indeed, the router require to distinguish a loopback due to an unavailable neighbor from a loopback request resulting of three consecutive Nack receptions. More precisely, if the ECC detects no error in the data packets after three consecutive Nacks, a permanent error is considered and located on the data bus.

4.3. Architecture of the Loopback Block

Figure 2 depicts the logic architecture of the loopback block. A loopback output block is implemented in the four ports of the router as illustrated in the Figure 2. The architecture of the loopback output block is depicted in Figure 12. The logic control block checks the availability of the neighbor router in order to transmit the data packets (data_request_in signal). If no loopback is required, one semicrossbar connects the buffer to the data_out signal in order to send the data packets toward the neighbouring router and activates the data_request_out signal. Next, a multiplexor connects the input data bus to the data_in bus. If a loopback is required in case where either a neighbor router is unavailable or an output block request occurs after three Nack receptions, the logic control block configures the semi-crossbar block to send the considered data packet on the data_loopback bus. Therefore, the data packet is looped back inside the router and will be considered as a new packet into the input port of the router. During this step and to avoid the reception of a new data packet from the neighbor switch, the occ_out signal is activated.

5. Simulation Results

To validate functionally the RKT-NoC communication approach, we simulated a 3 × 3 NoC connected to 12 communication modules. These simulations detailed the steps for the permanent error locations of the data packets based on the proposed approach. The RKT-Switch is configured to use data packets composed of 4 flits, and each input buffer can contain two data packets. Each communication module sends and receives data packets to the module connected to the opposite side of the network. Figure 13(a) illustrates the topology of the simulated network and the disposition of the modules.

Figure 14 is a snapshot of the simulation results. It can be seen at event 1 that all the modules send a data packet simultaneously. At event 2, the reception of all the data packets is also simultaneously. Indeed, all the data packets crossed the same number of routers, and they did not make a bypass.

The second simulation case represents a RKT-NoC in which an input block of the router(2,2) is permanently faulty. The simulated fault is two bits stuck at “1” in the West input buffer. Figure 15 is a snapshot of the simulation results of the error detection. At event 1, all the modules are sending a data packet. Here, the centralized journal of the router(2,2) already contains two errors for the west input. At event 2, the router(2,2) receives a data packet in its West input. This packet is sent to the East neighbor at event 3. The neighbor router(3,2) generates three consecutive Nacks due to an error which cannot be corrected by the Hamming ECC. The data packet is looped back, and the result of the error detection performed in the ECC of the East input is positive. This result is stored in the centralized journal at event 5. The three last results for the West input were positive. The router concludes to a permanent error and activates its unavailable flags to the West, North-West, and South-West. All the data packets have been received at event 4 excepted one.

The next part of the simulation is presented in the Figure 16. At event 1, all the modules send one data packet. At event 2, all the data packets are received excepted one which is received at event 3. This is due to a bypass of the router(2,2) for the data packet sent from the IP1. Indeed, this data packet crossed two more routers increasing the latency as illustrated in Figure 13(b).

These simulation results show one of the main advantages of the RKT-NoC which is the possibility to detect on run-time permanent errors, and to disconnect partially the faulty blocks of the switches. In these simulations, only one input port of the router(2,2) has been disconnected from the network. This allows to keep as high as possible the number of available paths, and then to maintain the throughput.

6. Synthesis Results and Performance Evaluation

6.1. FPGA Synthesis Results

The results are obtained by considering of RKT-Switches configured to data packets of 4 flits and where each input buffer can contain two data packets. Table 2 gives the synthesis results in terms of Slices Registers, Slices LUTs and maximal working frequency for different data bus size and FPGA technology (Virtex V, Virtex VI, and Virtex VII Xilinx FPGA). We note that the 32-bits RKT-Switch requires 4340 Registers, 6542 LUTs and can operate up to 459.6 MHz on the Virtex VII FPGA technology. The operating frequencies decrease depending of the occupancy rate of the implementation design in the FPGA chip. Table 4 gives the logic resources required for each block of the proposed reliability concept of the router and depending to the data bus size. These blocks can be directly used in others NoC routers based on XY routing algorithms in order to integrate some parts of the proposed mechanism by taking into account the area overhead. We observe that the main area overhead is obtained for the output buffer block corresponding ot 46% of the router in Xilinx Virtex VI FPGA. We have also synthesized several RKT-NoC sizes in Xilinx Virtex VI technology. These results are given in the Table 3 in terms of Slices Registers and Slices LUTs. Moreover, an estimation of the power consumption is given through the Xilinx XPower Estimator tool [22]. The results demonstrate that our proposed architecture can be efficiency implemented with FPGA technology. It can be stated that an attractive tradeoff between high speed and low logic resources has been achieved. Indeed, our 4×4   NoC implementation on a Xilinx Virtex-VI device uses only 201074 Slices-LUTs for a 64 bits bus size. We note from the results in the Table 3 that the power consumption overhead generated by the proposed detection and diagnosis mechanism is limited and suitable for the FPGA implementation.

6.2. Performance Evaluation
6.2.1. Flit Injection Rate

The Packet Injection Rate (PIR) is the number of data packets that can be send in one clock cycle. For instance, if an IP has a PIR of 0.5, that means it can send 50 data packets in 100 clock cycles. The Flit Injection Rate (FIR) is the value of the PIR multiplied by the number of flits in each data packet. We have estimated the FIRmax by simulating different sizes of NoC: 1×1, 3×3 and 4×4. Each RKT-NoC is connected to the maximum number of communication modules. The FIRmax is obtained when the network is working without unavailable component, and when the modules send and receive data packets only to the neighbor located at the opposite side of the network. Indeed, by sending packets with this traffic, no case of congested router can occur. The FIRmax has been estimated to 0.369 for any RKT-NoC size and any number of IPs.

6.2.2. Latency

In a 𝑛×𝑛 RKT-NoC, the minimal latency (latencymin) to cross the network from source to destination is defined by (1). N𝑅𝐾𝑇 and latency𝑅𝐾𝑇min are the number of switches crossed and the minimal latency to cross one switch, respectively. The following equation takes into account the additional clock cycles of the Ack/Nack data flow control used for our reliable switches. This data flow control technique requires two clock cycles latency per router transmission: Latencymin=𝑁𝑅𝐾𝑇latency𝑅𝐾𝑇min+𝑁𝑅𝐾𝑇2.(1) In real-time applications, the data packet latency depends on the traffic network. We have evaluated the average latency for different sizes of RKT-NoC: 1×1,3×3 and 4×4. To evaluate the latency, we have simulated these NoCs surrounded by the maximum number of communication modules: 4 modules for the 1×1 NoC, 12 modules for the 3×3 NoC, and 16 modules for the 4×4 NoC. Each module sends and receives data packets. The destinations of the data packets are generated randomly. The emission of data packets are performed at the maximum PIR. Table 5 gives the minimal and maximal average latencies for each NoC size in terms of clock cycles and timing (ns) by considering the maximum working frequency obtained with Xilinx Virtex V, VI, and VII FPGAs. It results that, for a 3×3 RKT-NoC, the minimum and maximum average latencies are 73 and 129 clock cycles, respectively.

6.2.3. Throughput

The maximum throughput of a RKT-Switch depends on the data bus width of 𝑛 bits, the working frequency 𝑓, and the FIR. The throughputmax is given by (2) by considering the maximum number of connected IPs of 4: Throughputmax=4𝑛FIRmax𝑓.(2) If we consider a data bus size of 64 bits in which 4 IPs are connected, and the maximum operating frequency of the RKT-Switch in the Virtex VII FPGA technology, the Throughputmax is 41.72 Gbps. Figure 17 gives the Throughputmax of one RKT-Switch for different data bus widths and operating frequencies. The theoretical maximum throughput of one RKT-NoC is defined by the Throughputmax of one router multiplied by the number of switches in the NoC.

6.3. Robustness Evaluation

To evaluate the robustness of the system against Single Event Upsets (SEUs), we have simulated the synthesizable architecture of a 6×6 RKT-NoC surrounded by 24 IPs generating random-traffic. This simulation has been realized for 3 different FIR: 0.13, 0.2 and maximum rate. For each FIR, we have injected SEUs at different frequency as specified in the Table 6. For instance, the frequency 15 means one SEU is injected in each 15 clock cycles. The position of the SEU in the network is random. For each configuration, 2.4 billion data packets have been transmitted. Table 6 shows the number of packet lost for the different simulation configurations. We can see that for the maximum FIR and a SEU frequency of 25, only 8 data packets are lost. We can clearly show that the number of packet lost increases with the FIR. Indeed, the number of data packets waiting in buffers when a neighbor is busy increases with the FIR. Thus, when data packets are waiting in a buffer, we have a probability to have two consecutive SEUs on the same data packet. We use the Hamming correcting code which can only correct one bit error and detect two errors. If a data packet receives one SEU in an input or output buffer and another one on the data bus, the retransmission protocol (i.e., Ack/Nack) saves the data packet. However, if a second SEU occurs also in input or output buffer, the data packet is lost. The same tendency is observed when increasing the SEU frequency or/and the FIR (see the Table 6).

7. Conclusion

In this paper, we have proposed a new reliable switch architecture for 2D mesh NoCs. This reliable architecture integrates a new online routing detection suitable for dynamic routing algorithms. Our detection approach allows to distinguish a routing error from a routing bypass decision of faulty nodes in the network and locates the specific faulty routing block. The MPSoC data packets are protected by using Error Correcting Code (ECC) of Hamming and a switch-to-switch error detection. Furthermore, the proposed loopback block in each output ports of the proposed switch allows to distinguish and locate precisely the data error sources (input or output ports, data bus between nodes, etc.). One originality of our solution is that with proposed reliable NoC, only the faulty parts of the switches are disconnected allowing to maintain online the throughput of the NoC. The FPGA synthesis of the proposed reliable NoC shows attractive performances since it leads to design with small logic area, satisfactory throughput rates, and low latency while providing an efficient online errors detection suitable for NoC-based reconfigurable systems.