Resound — Carrier PCB Design & BOM
Rev A (design-for-layout). Target fab JLCPCB, 4-layer 1.6 mm. Module: ESP32-WROOM-32E (SMD castellated). Schematic-level netlist + layout plan + BOM — not Gerbers.
0. Intent — ONE PCB, two assembly variants (SINK vs BROADCASTER)
Same copper/BOM skeleton; differences: (a) which 3 GPIOs route to the I2S 0R jumper field, (b) address-strap resistors, (c) I2C pull-ups + LED parts populated on one board only.
I2S pin map: SINK BCK=5/WS=25/DATA=23; BROADCASTER BCK=19/WS=18/DATA=22. Because the roles use different GPIOs for the same bus nets, route I2S to a 3-position 0R jumper field (SINK column vs BROADCASTER column) — the key "one PCB, two roles" trick.
1. Netlist highlights (WROOM-32E)
| Net | GPIO | Notes |
|---|---|---|
| I2S (sink) | 5/25/23 | strapping pin 5: pull-up, driven post-boot |
| I2S (broadcaster) | 19/18/22 | |
| I2C SDA/SCL | 32/33 | RTC-domain, safe |
| UART2 TX/RX | 17/16 | inter-module aux |
| LED | (APA102: DATA+CLK over SPI — see LED page; revised from WS2812) | |
| Address straps | 13/14/27/26 | safe non-strapping pins; 0R/10k to GND/3V3 |
| Prog UART0 | TX=1/RX=3 | 6-pin header |
| GPIO12 (MTDI) | leave floating | must NOT be high at boot |
| GPIO6-11 | RESERVED | internal flash — do not route |
- Power/decoupling: rails arrive on stacking connector (+5V/+3V3/GND). 22 uF on 3V3 + 10 uF on 5V at entry; 2x 100 nF at the WROOM 3V3 pad + 10 uF local (BT TX peaks ~500 mA -> avoid brownout reset). EN: 10k pull-up + 1 uF (RC reset). IO0: 10k + 100 nF + button.
- I2C pull-ups: 4.7k to 3V3 footprints on every board but DNP except one (the SINK). 2.2k fallback footprint.
- Reset/boot buttons (tactile, board edge). Programming: 6-pin 2.54 header (GND/5V/RX/TX/DTR/RTS) + 2-transistor auto-reset/boot circuit (MMBT3904 x2) so a CP2102/CH340 cable flashes hands-free; + test pads fallback.
- Stacking connector: use the 26-pin 2.54 mm pass-through from the connector page (this PCB's earlier 2x10/2.0mm was a placeholder). Top + bottom mirrored footprints = straight vertical bus column. Grounds interleaved beside I2S/LED.
2. HUD integration (ESP32-S3-Touch-LCD-1.28)
HUD is a self-contained S3 board + round LCD + onboard touch/IMU; it is an I2C peer running the UI firmware, not a dumb display. Carrier feeds it power + shared I2C via a 6-pin SH1.0 socket (J_HUD, populate on top carrier only):
| J_HUD | HUD signal | Carrier net |
|---|---|---|
| 1 | VSYS (5V) | +5V |
| 2 | GND | GND |
| 3 | GPIO15 | I2C_SDA |
| 4 | GPIO16 | I2C_SCL |
| 5 | GPIO17 | SPARE (opt IRQ/sync) |
| 6 | GPIO18 | n/c |
Power HUD from 5V (its onboard LDO makes 3V3 — don't also feed 3V3). Verify SH1.0 pin order vs Waveshare silk before fab.
3. Layout plan
- 45 x 45 mm, 4-layer (Sig / solid GND / power / Sig). 4-layer needed for clean 2.4 GHz BT, I2S edges, ground reference.
- WROOM at one edge, antenna overhanging the board edge; 15 mm copper keep-out all layers under/around antenna.
- 4x M3 corner standoffs (38x38 mm) carry the stack load; stacking connector centered; buttons/prog header on the opposite edge for access while stacked.
- Antenna coupling is the #1 risk: three stacked WROOM antennas in ~45 mm will interfere -> overhang + rotate each board's orientation, or use WROOM-32UE (external IPEX) for inner boards.
4. BOM — one carrier (~$7 assembled, JLCPCB-style)
| Ref | Part | Qty | ~$ |
|---|---|---|---|
| U1 | ESP32-WROOM-32E-N8 | 1 | 3.50 |
| J_STACK | 26-pin 2.54 pass-through pair | 1 | ~0.8 |
| J_PROG | 1x6 2.54 header | 1 | 0.05 |
| J_HUD | SH1.0 6-pin (top board) | 1 | 0.10 |
| J_LED | APA102 out (DATA+CLK+5V+GND) | 1 | 0.05 |
| SW1/2 | tactile RST/BOOT | 2 | 0.06 |
| Q1/Q2 | MMBT3904 auto-prog | 2 | 0.04 |
| Caps | 22u/10u bulk + 4x100n + 1u + couplers | ~9 | ~0.1 |
| Res | 2x10k (EN/IO0), 2x4.7k I2C (DNP), LED series, 3x0R I2S jumper, 4 addr straps, 2 auto-prog | ~15 | ~0.05 |
| PCB | 45x45 4-layer ENIG | 1 | ~1.20 |
Full stack (3 WROOM carriers + HUD) ~ $36 parts (excl. power/battery/speakers). Assembly variant table: SINK populates left I2S jumpers + I2C pull-ups + auto-prog; BROADCASTER populates right I2S jumpers + unique address straps; top broadcaster also populates J_HUD.
5. Open items for EE
- Confirm HUD SH1.0 pin sequence from Waveshare schematic.
- Match the 26-pin stacking connector footprint (per connector page).
- Antenna coupling in the stack (biggest risk) — overhang/rotate or WROOM-32UE inner boards.
- GPIO12 must stay low at boot.
- Recompute I2C pull-up for total bus capacitance (4.7k assumed, 2.2k fallback).
Sources
ESP32-WROOM-32E/32UE datasheet; ESP32 Hardware Design Guidelines (antenna keep-out); ESP32 strapping pins; esptool boot-mode; Waveshare ESP32-S3-Touch-LCD-1.28 wiki + Spotpear user guide.