# Open Chiplet Atlas

> The Open Chiplet Atlas® (OCA®) Ecosystem, developed by Tenstorrent, defines open, royalty-free standards that let chiplets from different vendors integrate plug-and-play in a single System in Package (SiP). It has three parts: the OCA System Architecture specification, the AXI over UCIe (AoU) protocol specification, and the OCA Harness, an open-source reference implementation (RTL, DV, docs).

Key facts:

- The OCA architecture is full stack, organized into five layers (top to bottom): Software, System, Protocol, Transport, Physical. Most other chiplet standards stop at the physical, transport, or protocol layer.
- It is built on existing industry standards: UCIe and BoW for high-speed die-to-die transport, I3C/I2C as auxiliary transport, AXI/CHI bus protocols, IEEE Std 1838 for test access.
- OCA-defined protocols and interfaces include OCCP (Open Chiplet Configuration Protocol, sideband), OCMI (Open Chiplet Management Interface, software), and OCSP (Open Chiplet Safety Protocol).
- Profiles (currently Automotive, Data Center, and Data Center using Bunch of Wires) select required/optional functional domains and fix parameter values; chiplets that conform to the same profile are guaranteed to interoperate.
- No license fees or royalties. All specifications are drafts under active development. The latest versions are OCA System Architecture v0.8.0 (2026-06-25) and AoU v0.8 (2026-07-09). The website labels the OCA release "0.8.0" and its PDF cover says "Version 0.8"; both refer to the same release. Dates in this file use the YYYY-MM-DD format.
- Earlier documents (including the 2025 white paper) use the older name "Open Chiplet Architecture"; it refers to the same standard.
- Specifications are published as PDFs. `/specifications/{oca|aou}/latest` always redirects to the newest version.
- Feedback on the specifications goes to feedback@openchipletatlas.org.
- [llms-full.txt](https://www.openchipletatlas.org/llms-full.txt) is a consolidated text of four key resources: the home page, the white paper, and the latest OCA and AoU specifications. It is converted to Markdown from the PDFs, keeps tables as Markdown tables, and replaces diagrams with captions, PDF page links and descriptions. It omits the license appendices, earlier versions, translated pages and the Harness docs.

## Specifications

- [OCA System Architecture Specification v0.8.0 (2026-06-25)](https://www.openchipletatlas.org/specifications/oca/latest): Latest version. PDF, 93 pages. Chiplet roles and topology; System Layer (communication channels, addressing, boot, system management, system time, security, interrupts, debug, test, FuSa, RAS); Protocol and Transport Layers; Software Layer; Automotive, Data Center, and Data Center (BoW) profiles; terminology
- [AXI over UCIe (AoU) Protocol Specification v0.8 (2026-07-09)](https://www.openchipletatlas.org/specifications/aou/latest): Latest version. PDF, 75 pages. Bridges non-coherent AXI networks-on-chip across UCIe-connected chiplets: flit packing, message structures, credit-based flow control, overheads and efficiency, interface state management and initialization, extensions, profiles (Basic, HBF)
- [OCA White Paper, Revision 1.0 (2025-06-30)](https://www.openchipletatlas.org/whitepaper): PDF, 7 pages. Short introduction to the motivation, the five-layer architecture, design principles, and integration examples

## OCA Harness

- [OCA Harness documentation](https://docs.tenstorrent.com/tt-oca-harness/): Open-source chiplet framework implementing the OCA architecture: security, boot, system management, debug, test, and die-to-die integration. Includes getting-started, technical reference, integrator, and programmer guides
- [tt-oca-harness on GitHub](https://github.com/tenstorrent/tt-oca-harness): RTL, DV, documentation, and register/IP-XACT tooling
- [tt-oca-harness-aou on GitHub](https://github.com/tenstorrent/tt-oca-harness-aou): AXI over UCIe bridge RTL
- [OCA Harness discussions](https://github.com/tenstorrent/tt-oca-harness/discussions): Community Q&A

## Optional

- [OCA System Architecture Specification v0.7.6](https://www.openchipletatlas.org/specifications/oca/0.7.6): Previous version (2026-02-10). Older versions: [0.7.5](https://www.openchipletatlas.org/specifications/oca/0.7.5) (2025-12-23), [0.7.3](https://www.openchipletatlas.org/specifications/oca/0.7.3) (2025-12-10), [0.7](https://www.openchipletatlas.org/specifications/oca/0.7) (2025-09-22), [0.6](https://www.openchipletatlas.org/specifications/oca/0.6) (2025-08-22)
- [AoU Specification v0.7.1](https://www.openchipletatlas.org/specifications/aou/0.7.1): Previous version (2026-03-10). Older version: [0.6](https://www.openchipletatlas.org/specifications/aou/0.6) (2025-11-03)
- [Tenstorrent](https://tenstorrent.com): The company that developed OCA