Hypothesis

Top-level functional decomposition of the Accordion Test Adapter R1, agreed during concept session 2026-06-08.

Bus topology (2026-06-09):

• UART · SPI: series daisy-chain through all module slots
• I2C: unique per slot — pin-strap selects which slot to address (1-of-4)

graph LR
    USBPD["USB PD
20V"]
    FTDI["FTDI USB-Serial
Vertical USB · REQ-003"]

    subgraph A2_LINK["Accordion A2 link"]
        CTOP["Com-Top
input connector"]
    end

    subgraph PWR["Power Management"]
        PM["20V distribution
5V / 3V3 / 1V8 generation
2.5V VREF = module 2.5V rail
Per-slot switches ≥1A
Shunt meas. 0–1A · Power LEDs"]
    end

    I2C_SW["I2C Slot Select
Pin-strap · DEC-002
1-of-4"]

    NTOP["N-Top"]
    SODIMM["SO-DIMM"]
    M2["M.2"]
    CTOP_OUT["Com-Top
reg + mini"]

    USBPD -->|"20V"| PM
    CTOP -->|"20V"| PM
    CTOP -->|"I2C"| I2C_SW
    I2C_SW -.->|"selects 1 slot"| NTOP & SODIMM & M2 & CTOP_OUT
    CTOP -->|"UART · SPI"| NTOP -->|"UART · SPI"| SODIMM -->|"UART · SPI"| M2 -->|"UART · SPI"| CTOP_OUT
    PM -->|"20V · 5V · 3V3 · 2.5V · 1V8"| NTOP & SODIMM & M2 & CTOP_OUT
    FTDI --> NTOP & SODIMM & M2 & CTOP_OUT

Key decisions reflected

• DEC-001: 20V from USB PD
• DEC-002: I2C slot select via pin-strap (1-of-4; I2C originates from A2 via Com-Top)
• DEC-003: Local 5V, 3V3, 1V8 generation
• DEC-004: All rails (20V, 5V, 3V3, 2.5V, 1V8) to every module slot, max 1A per slot
• DEC-005: 2.5V VREF (REQ-001) serves as the module slot 2.5V rail

Open questions

None — all FBD-level questions resolved.

Next step

DS.01 schematic capture.

Decision

The 20V power input to the Accordion Test Adapter shall come from USB PD. The Accordion A2 PSU module is not used as the power source.

Rationale

Com-Top + USB PD is sufficient; a dedicated PSU module adds unnecessary complexity and cost.

Rejected alternatives

• Accordion A2 PSU module — ruled out by engineer.

Follow-up

• Update SPECIFICATION.md scope section to reflect USB PD as power source.
• Update REQ for power input accordingly.

Decision

The I2C bus routing switch shall be implemented as a manual pin-strap (header + jumper), not a programmable IC switch.

Rationale

Simpler implementation; no firmware dependency; sufficient for bench/lab use case.

Rejected alternatives

• Programmable I2C switch IC — adds firmware complexity with no clear benefit for this use case.

Follow-up

• Capture as formal requirement in SPECIFICATION.md.

Decision

The adapter shall generate three local regulated supply rails on-board from the 20V USB PD input: 5V, 3V3, and 1V8.

Rationale

Module slots require standard logic supply voltages. Generating them locally avoids dependency on host equipment.

Rejected alternatives

None discussed — requirement stated directly by engineer.

Follow-up

• Capture as formal requirements in SPECIFICATION.md.
• Select regulator topology (LDO vs switching) during schematic capture.

Decision

Each module slot shall be supplied with all of the following rails: 20V, 5V, 3V3, 2.5V, 1V8. Maximum current per slot per rail shall not exceed 1A.

Rationale

Stated by engineer based on module interface requirements.

Rejected alternatives

None discussed.

Follow-up

• Update REQ-010: distribution covers all rails, not just 20V.
• Update REQ-011: power switches rated ≥1A.
• Update REQ-012: shunt current measurement range 0–1A per slot.
• Verify 2.5V: confirm whether 2.5V VREF (REQ-001) serves as the module slot supply rail or if a separate 2.5V power rail is needed.

Decision

The 2.5V supply distributed to module slots (REQ-010) is the same signal as the on-board 2.5V VREF (REQ-001). No separate 2.5V power regulator is needed.

Rationale

Confirmed by engineer.

Design note

The VREF source must be able to drive sufficient current for all connected module slots simultaneously. VREF IC selection shall account for total module load on the 2.5V rail. Verify during schematic component selection.

Requirement REQ-005 ("20V power input via USB PD") has no verifying test case. Add one with trace_testcase_create and link it (verified_by).

The 2.5V VREF (REQ-001) is distributed to all module slots as a supply rail (DEC-005). The VREF IC must be able to source the combined current of all module slots on this rail.

Verify during component selection: choose a VREF with adequate output current drive, or add a buffer stage if required.