# Controller A A-2 USB Write-Size Qualification — 2026-07-28

Status: complete
Type: evidence
Updated: 2026-08-01
Next Action: none; retain as predecessor evidence for the qualified 32 KiB broad baseline
Parent: work/wp/completed/wp-characterize-usb-filex-write-size-durability.md

## Identity and Guards

- Controller A assembly/J-Link: `1751136 / 69664082`
- Diagnostic S-record SHA-256:
  `772773E39DF0F58396175E85A3FE25878F5F1036D2F39711913387D45EF105EF`
- Controller SD raw CID:
  `035344535330384780976F0084018559`
- USB specimen: PNY A-2, serial `071C49DE246A1F97`
- USB baseline: newly formatted FAT32, label `A-CTRL-2`, UUID `EFA2-EFED`

The specimen was intentionally empty. WAV fixtures are unrelated to this
deterministic write-size diagnostic and were not copied onto the clean
baseline.

## Controller Results

| Tier | FileX limit | Exact bytes | Rows | Calls | File-write ticks | FileX status |
| --- | ---: | ---: | ---: | ---: | ---: | ---: |
| 1 MiB | 16 KiB | 1,048,576 | 16,384 | 64 | 157 | 0 |
| 1 MiB | 32 KiB | 1,048,576 | 16,384 | 32 | 89 | 0 |
| 1 MiB | 64 KiB | 1,048,576 | 16,384 | 16 | 74 | 0 |
| 7 MiB | 16 KiB | 7,340,032 | 114,688 | 448 | 1,820 | 0 |
| 7 MiB | 32 KiB | 7,340,032 | 114,688 | 224 | 696 | 0 |
| 7 MiB | 64 KiB | 7,340,032 | 114,688 | 112 | 389 | 0 |

Every controller record reported complete, exact target bytes, successful
close/flush/rename/readback verification, and overall status zero.

For the sustained tier, 32 KiB reduced measured file-write ticks by about
61.8% and was about 2.6 times as fast as 16 KiB. The 64 KiB case reduced them
by about 78.6% and was about 4.7 times as fast as 16 KiB.

## Independent Pi Verification

- All three 1 MiB files were byte-for-byte identical:
  `6ab78b4dde26efd91a20aa003198a13d794b1cd7276f3cc05880f944f56966b3`
- All three 7 MiB files were byte-for-byte identical:
  `885418a583a8ebf3d9e0c0f0811ca821fb82ee99a25ef6573a0a49f01fc82b1d`
- Exact sizes and row counts matched the controller contract.
- Read-only host comparison passed for each tier.
- Unmounted `fsck.vfat -n -v` passed after both tiers.
- A-2 was safely unmounted at closeout.

## Disposition

The first physical specimen establishes that 16, 32, and 64 KiB all sustain
the full 7 MiB target without a detected publication or persistence error.
This does not establish the final production cap from one clean specimen.

Use 32 KiB as the conservative successor candidate: it captures a material
speed gain while preserving twice the request-size margin relative to the
passing 64 KiB boundary. Retain 64 KiB as a demonstrated performance result,
not the production selection, until removal/reinsertion, reboot, and
additional-specimen qualification provide comparable evidence.

Follow-up testing also passed 96 KiB on this specimen. See
`work/evidence/controller-a-a2-usb-write-size-96k-qualification-20260728.md`.
That result does not change the conservative production recommendation without
additional lifecycle and multi-specimen evidence.
