Voltage

Voltage (VDD)

The baseline

DDR4's JEDEC-standard nominal operating voltage is 1.2V, encoded in every DDR4 module's SPD as a fixed baseline (byte 0x0B in the base config block) — it's not really a "setting" so much as the assumption the entire JEDEC speed-bin table (the CL/tRCD/tRP/tRAS values for each standard MT/s rating) was characterized against. Every DDR4 module is guaranteed to work at its base JEDEC speed at 1.2V; that guarantee is exactly what spd-matchtable's "Base SPD ceiling" is built from.

Why XMP profiles bump it to 1.35V

Running faster than a chip's 1.2V-characterized speed bin generally needs more electrical margin to keep signals clean and sense amplifiers fast enough at the tighter timings — raising VDD is the most direct way to buy that margin back. 1.35V has become the de facto standard voltage for XMP-rated DDR4 kits specifically because it's the point vendors settled on as "enough extra margin to hit a validated higher speed bin, without meaningfully compromising the chip's long-term reliability or heat output." This project's own reference kit demonstrates this concretely: its two dual-rank modules carry an embedded XMP profile rated 3200 MT/s CL16 at exactly 1.35V (decoded byte-for-byte and cross-checked in XMP Profiles Explained) — 1.35V isn't a marketing number here, it's a literal field encoded in the module's own SPD data that this tool reads directly.

Why more voltage isn't free, and this tool doesn't suggest any

Physically, higher VDD means:

• More heat. Power scales roughly with the square of voltage for a given current draw, so a jump from 1.2V to 1.35V is a non-trivial power increase, not a rounding error.
• Faster electromigration. Higher voltage across the same transistor geometries accelerates the physical migration of metal atoms in the chip's interconnects over time — a real, if slow, wear mechanism. This is why "how far can I push voltage" has diminishing and eventually negative returns, not a monotonic "more is safer" curve.
• A real ceiling before damage. DDR4 modules are not rated for arbitrary voltage; going meaningfully beyond what a kit's own vendor validated (routinely cited as roughly 1.4-1.45V as a soft practical ceiling for consumer DDR4, though this varies by IC and is not a universal constant) trades a small stability/frequency gain for real degradation risk.

This is exactly why spd-matchtable never recommends a voltage beyond what's literally present in a module's own SPD/XMP data. Its "Suggested starting point" cheat-sheet uses 1.2V at the SPD-guaranteed base ceiling specifically because that's the one voltage every installed module is JEDEC-certified for with zero inference involved — see Worst Case Methodology for how that ceiling itself is computed, and FAQ and Design Decisions for why the tool stops there rather than recommending a specific OC voltage for higher candidate speeds.