Rules the schematic and layout follow for the minimal board (core only, 2 layers). They combine
“toy hardware” guidance with current practice. Each rule says adopted, adapted (right idea, applied
differently, with the reason) or rejected (with the reason).
| Rule |
Status |
How it’s met |
| ESD protection on user-reachable conductors |
adapted |
One USB ESD part (USBLC6-2SC6) at the USB-C connector, the only port a user plugs things into often. The jack gets passive protection only (22 Ω earpiece series resistor, AC coupling, 1 kΩ on the detect line); the owner ruled out extra ESD networks. Revisit if EVT ESD tests fail. |
| Series resistance between anything user-facing and a GPIO |
adapted |
Jack insertion contact: 1 kΩ to IO4. Keys and the hook go straight to GPIOs (owner decision): an MX switch’s contacts sit inside its housing and are not user-reachable. USB D+/D- go to the native USB pins behind the ESD part (USB needs them direct). RESET and BOOT are internal pinhole buttons. |
| Overcurrent protection |
adapted |
No fuse: a compliant USB source limits VBUS, there is no battery, and the LDO current-limits and shuts down on over-temperature (DESIGN.md §2). |
| Reverse-polarity protection |
rejected |
USB-C is keyed; VBUS and GND are on fixed pins. |
| Toy-safety standards (ASTM F963, EN 71-1, EN IEC 62115) |
adopted as a design input |
No sharp edges, screws needed to open the base, captive keycaps (§4). |
| Rule |
Status |
How it’s met |
| Respect strapping pins |
adopted |
The S3’s straps are GPIO0, GPIO3, GPIO45, GPIO46 (not the original ESP32’s 0/2/5/12/15). check_pin_table proves nothing can pull GPIO45/46 high at reset and GPIO0 has no pull-down. |
| Octal PSRAM pins |
adopted |
GPIO35-37 unconnected (N16R8), enforced by the pin table. |
| Antenna clearance |
adapted |
WROOM-1U with a plug-in U.FL antenna (owner): no board-edge keep-out; the antenna itself goes ≥ 15 mm from metal in the enclosure. |
| Use an expander when GPIOs run out |
not needed |
12 keys, the hook, I2S, I2C, SPI, ringer and LED use 32 of the 33 usable GPIOs directly. |
| Rule |
Status |
How it’s met |
| Decoupling per the datasheets |
adopted |
Module 22 µF + 100 nF (WROOM-1 p41), codec per ES8311 p4, LDO in/out per SGM2212 p10. Layout: the capacitor sits between the pin and its GND via, ≤ 2 mm away. |
| I2C pull-ups sized for the bus |
adopted |
One pair of 4.7 kΩ; one device on the bus. |
| “Single-point grounding” |
rejected in favour of a ground pour |
Split grounds create slots that return currents detour around. On 2 layers: the bottom mostly GND, stitched to top GND pours; placement keeps the mic path away from USB, I2S clocks and the piezo drive. |
| Rule |
Status |
How it’s met |
| Replaceable mechanical inputs |
adopted |
MX switches in hot-swap sockets for the keys and the hook. |
| Captive keycaps |
adopted |
Keycap skirt wider than the lid hole, inserted from inside: no small part to pull off. |
| Hardware RC debounce on every key |
rejected |
Metal-contact MX switches; firmware debounce (5-10 ms) is reliable and adds no parts. |
| Display: drop-in, not soldered |
adopted |
A ready-made module on a pin header (cable or jumpers). |
| Mounting holes |
adopted |
4 × M3 near the corners and near the connectors, so plug forces go into the case. |
| Through-hole where force is applied |
adopted |
USB-C with through-hole shell tabs, the jack’s locating pegs, THT header. |
| Clear silkscreen |
adopted |
Key legends, connector labels, J3 pin order, the owner’s signature logo, “Open Lounge Phone”, “CERN-OHL-S-2.0”, revision. |
| Stackup |
adopted |
2 layers, 1.6 mm, the cheapest process at every fab. |
| Net class |
Width |
Clearance |
Notes |
| Default signal |
0.2 mm (0.15 min) |
0.15 mm |
|
| I2S, I2C, SPI |
0.2 mm |
0.2 mm |
I2S clocks short, over GND |
| USB D+/D- |
pair, matched within 0.15 mm |
— |
full speed; see BOARD_REQUIREMENTS BR-21 |
| Audio (mic, earpiece) |
0.25 mm |
0.3 mm |
short, guarded by GND |
| VBUS, 3V3 |
≥ 0.5 mm or pour |
0.2 mm |
|
| Vias |
0.3 mm drill / 0.6 mm pad |
|
≥ 2 vias per power transition |
make build passes; KiCad DRC 0 errors, 0 unconnected items.- A written review against the installed
kicad, emc and pcb-layout-review skills
(hardware/tools/install-skills.sh), every finding fixed or explained.
- Rendered top/bottom/3D images inspected by a person or agent.
- Every rule in this file checked off in the review record.
