Network-on-Chip Socket Protocol for Low-Latency Interconnect
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Solution Overview
Problem
Conventional network-on-chip (NoC) target Initiator Network Interface Units (NIUs) require extensive logic silicon area and deeper timing paths due to context array lookups, leading to slower and less efficient communication.
Innovation Solution
A transaction interface protocol is introduced with a transaction ID (TrID) signal in both request and response channels, allowing direct identification of context array entries, reducing logic complexity and latency by simplifying the interconnect topology between NoCs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional target NIUs perform context array look-up to identify the oldest entry matching the response packet sequence ID, then the system can maintain transaction ordering, but the logic silicon area increases and timing paths become deeper leading to slower operation
Solution Approach 1:
The initiator NIU pre-allocates a context array entry index to each request packet before it is sent. This index is included in the request packet itself, so when the response packet returns, the target NIU can directly use this pre-provided index to access the correct context array entry without performing a search. The preliminary assignment of the index eliminates the need for complex look-up logic at the target NIU.
Solution Approach 2:
The complex context array look-up logic is extracted from the target NIU and moved to the initiator NIU. The initiator NIU maintains the context array and performs the association between response packets and transactions using the pre-allocated indices. This extraction removes the burden of complex logic from the target NIU, reducing its silicon area and timing path depth.
2Reliability
If conventional target NIUs perform chained list look-up logic to deduce the oldest entry, then transaction responses can be correctly matched to requests, but the logic levels increase leading to longer and slower timing paths
Solution Approach 1:
The context array entry index is preliminarily assigned to each request packet at the initiator NIU before transmission. This pre-assignment creates a direct mapping between request and response packets that is carried within the packet itself. When the response packet arrives at the target NIU, the matching process becomes a simple index-based access rather than a multi-level chained list traversal, dramatically reducing logic levels and timing path length.
3Device complexity
If a simple index is used instead of context array look-up, then logic silicon area is reduced, but the ability to correctly identify and match transaction entries may be compromised
Solution Approach 1:
The initiator NIU pre-allocates unique context array entry indices to each outstanding request and embeds these indices in the request packets. This preliminary action ensures that when response packets return, the target NIU can reliably identify the corresponding transaction entry using the simple index without any ambiguity. The reliability of transaction entry identification is maintained because the index was assigned by the initiator NIU which has authoritative knowledge of the transaction state.
Solution Approach 2:
The context array entry index acts as an intermediary between the request packet and the response packet. Instead of requiring complex direct matching logic between packets, the index serves as a mediator that links the two packets to the correct context array entry. This intermediary simplifies the matching process while maintaining reliability, as the index provides a direct and unambiguous reference.
Data Source
AI summary
The invention is a transaction interface protocol wherein the interface protocol has a transaction identifier signal in each of the request and response channels. It is used between a target network interface unit (NIU) master and an initiator NIU slave that are directly connected through a transaction interface. The target NIU response channel uses the transaction ID signal to identify the entry in a context array associated with the corresponding request. The coupling of target NIU and initiator NIU enable the formation of an on-chip interconnect comprising multiple network-on-chip (NoCs) wherein the topology of the interconnect is simpler, smaller, faster, and has lower latency.