DEC-101 - Cypher Board INA219 I2C Address Aligned to PM Default (0x45 → 0x40)

Field	Value
Decision ID	DEC-101
Status	Decided
Date	2026-09-08
Author	Izzyonstage & GitHub Copilot
Amends	-
Related	DEC-098, DEC-099

Context

The system has two INA219 current/power monitors: the Power Module's U10 (5V_MAIN monitoring) and the Cypher Board's U2 (rotor-stack 3V3_ENIG monitoring). U10 sits at the INA219's default address 0x40 (A0/A1 both tied to GND — no address-strap components required). U2 was assigned 0x45, a non-default address requiring a specific A0/A1 strap combination per the INA219 datasheet's address table — a strap that was never actually documented at the pin level in the Cypher Board's own spec (unlike, e.g., the Cypher Board's U8 MCP23017, whose A2/A1/A0 strapping is explicitly called out).

0x45 was originally chosen purely to avoid an address collision with U10's 0x40, back when both devices were expected to share a single physical I²C bus. DEC-098/DEC-099 (2026-09-04) split the system onto six independent I²C bus instances, with the Power Module and Cypher Board now on entirely separate buses (I2C0 and I2C1 respectively) — the original collision concern no longer applies.

Decision

Change the Cypher Board's U2 (INA219) I²C address from 0x45 to 0x40, matching the Power Module's U10. Both devices use the INA219 default address configuration (A0/A1 both tied to GND), requiring zero address-strap components on either board.

Rationale

• Software simplification: identical driver/read code can now service both power monitors, with only the I²C bus/bank number differing between them (I2C0 for PM, I2C1 for Cypher).
• Hardware simplification: the INA219 default address requires no strap components at all — simpler and cheaper than implementing the specific non-default strap 0x45 would have required (which, in any case, had never been fully specified at the pin level on the Cypher Board).
• No collision risk: the two devices are on independent I²C buses (I2C0 vs I2C1), so duplicate addresses across boards are safe by construction.

Impact

• Cypher/Design_Spec.md §7 Power Telemetry: U2 address updated to 0x40, with a note explaining the deliberate same-address reuse across independent buses.
• Controller/Design_Spec.md §3 Telemetry & Logic, §3.1 I²C Bus Topology, FR-CTL-05: INA219 address references updated from 0x45 to 0x40.
• Power_Budgets.md: shunt-part note (§ INA219 shunt selection rationale) updated to reference the new 0x40 address on the Cypher Board's R1/U2.
• Not touched (pre-existing staleness, deferred per prior user direction — see plan.md): the legacy "Stator" INA219 references in Boards_Overview.md, Electrical_Design.md, Software/Linux_OS/Power_Management.md, Software/GUI_App/Design_Spec.md, and the GUI wireframe .drawio files still describe the pre-merge architecture at 0x45. These are already flagged for full rewrite under merge-update-top-level-docs (electronics docs) and the eventual software-documentation overhaul pass (out of scope until the full electronics design is merged), so are not corrected here to avoid inconsistent partial updates.