5.1. Rationale (looking at Nexus standard)

Nexus standard does NOT define any small connectors with focus on trace as Nexus defines message-based debug interface and it requires more pins than JTAG. Namely:

  • S26x 1-104068-2, Low performance trace (1 MDO signal).

  • S40x 1-104549-6, Low performance trace (6 MDO signals - labeled as "not recommended").

  • S50x 104549-7, Low performance trace (8 MDO signals).

As the smallest Nexus-recommended connector with reasonable trace bandwidth has 50 pins these are not practical as trace connectors.

So, it was decided that connectors defined by MIPI and Arm will be used for the RISC-V trace.

  • There are a lot of hardware trace probes, which are being used for debugging and tracing of Arm cores. Arm defines two standard connectors for trace:

    • Based on MIPI 20-pin connector (defined by MIPI) - this is for medium-performance tracing (4-bit, 100+ MHz double edge captures, max trace bandwidth 800Mbps or even higher for some high-performance trace probes).

    • Based on Mictor 38-pin connector (defined by MIPI) - this is for high-performance tracing (16-bit, up to 400MHz double edge, max trace bandwidth 12.8Gbps).

  • In July 2021 MIPI Alliance (following recommendations by Nexus TG group) released White Paper updating recommendations for debug and trace connectors and allowing 1/2/4-bit trace via MIPI20 connector.