Simplify Rev-A hardware to reduce BOM cost and routing complexity

[Twotoz]

Goal

Simplify OpenPocket Rev-A so the PCB is cheaper, easier to place and route, and focused only on the core product requirements:

• power on/off
• USB-C charging
• 1S battery protection
• ESP32-S3 firmware
• ELRS UART/power
• gimbals, buttons, switches and trims
• RX5808 analog video reception
• AT7456E OSD
• AMT630A + 5" TFT output
• basic buzzer feedback

Anything that does not materially contribute to those functions should be removed, consolidated, or deferred to a later revision.

Why

The current Rev-A has grown into a feature-heavy board with many power domains, optional peripherals and support ICs. Even on an 8-layer stackup with six usable routing layers, routing remains difficult and the safe autorouting milestone still leaves a large number of unconnected items. This suggests that continuing to tune the router alone is not the best path.

Before more routing work, perform a deliberate hardware-diet pass and rebuild placement from the simplified netlist.

Proposed changes

1. Consolidate the GPIO expanders

Current:

• U18 TCA9535
• U19 TCA9535

Replace with a single >=24-bit I2C GPIO expander if practical.

The control set is approximately:

• 4 menu buttons
• 3 encoder signals
• 1 ARM input
• 6 AUX inputs
• 8 trim inputs

Total: ~22 digital inputs.

Target: one GPIO-expander IC instead of two.

2. Remove optional speaker/audio hardware

Remove unless there is a strong product requirement:

• U21 I2S speaker amplifier
• J4 speaker connector
• J13 speaker solder pads
• associated passives/routing

Keep the existing buzzer path for feedback.

3. Remove the dedicated fuel gauge if not essential

Evaluate removing:

• U4 MAX17048

For Rev-A, battery voltage from an ADC/charger telemetry may be sufficient. Accurate percentage/SOC is useful but not a core requirement if it materially increases complexity.

4. Remove microSD from Rev-A

Unless DVR/storage is a launch requirement, remove:

• J12 microSD socket
• U22 SD load switch
• ESD2 / ESD3
• SD pull-ups / series resistors
• SD test points
• all SDMMC routing

This removes an entire high-speed routing island and several support parts.

5. Remove OLED and general-purpose developer headers

Remove:

• J14 optional OLED pads
• J15 developer GPIO pads

Rev-A should be treated as a product PCB, not a general development board. Keep only the minimum factory/debug pads required for bring-up and programming.

6. Reduce the number of switched 5 V domains

Current:

• U7 5V_VIDEO load switch
• U8 5V_DISPLAY load switch
• U9 5V_ELRS load switch

Evaluate removing these and feeding the relevant loads directly from the master switched 5 V rail, with local filtering where required.

Desired behavior for a simple product:

radio on = ESP + ELRS + video + display on

Avoid separate software-controlled power islands unless they solve a real requirement.

7. Consolidate OSD level translation

Current:

• U12 3V3 -> 5V translation
• U13 5V -> 3V3 translation

Replace with one suitable multi-bit dual-rail translator if the required directions/timing allow it.

8. Evaluate removing the AMT flash mux

Review whether U16 is required in the shipping product.

If the AMT flash can be programmed during manufacturing and does not require runtime MCU multiplexing, remove the mux and associated control/routing.

Do not remove it until the AMT630A programming/update workflow is proven without it.

9. Reduce test pads

Keep only essential bring-up/factory points, for example:

• GND
• BAT
• 3V3
• 5V
• USB/ESP programming/debug as required
• ELRS UART
• a small number of critical power/video signals

Remove redundant per-net test pads that create placement/routing congestion.

Do not simplify away required safety/reliability

Keep unless a replacement architecture is explicitly validated:

• USB ESD / VBUS protection
• BQ25895 charger/power-path
• battery protection U3 + Q1
• master load switch
• required regulators
• required decoupling/filtering
• RF/video integrity components

Do not remove protection hardware purely to reduce BOM count.

Proposed simplified architecture

18650
  |
protection
  |
BQ25895 / USB-C
  |
master power
  +---- 3V3_LOGIC
  |       +-- ESP32-S3
  |       +-- one GPIO expander
  |
  +---- 5V
          +-- ELRS
          +-- RX5808 + OSD
          +-- display 3V3 regulator
          +-- backlight boost

Use local filtering instead of unnecessary independently switched rails.

Placement strategy after simplification

Do not try to preserve the existing component placement blindly.

Keep only mechanically constrained items fixed first:

• USB-C
• TFT FFC
• U.FL
• battery pads
• gimbal/control pads
• master switch

Then place by functional island:

1. battery / charger / protection
2. 5 V and 3.3 V power conversion
3. RX5808 + OSD + level translation
4. AMT630A + flash + crystal + decoupling
5. backlight
6. ESP32-S3 on the rear/central area
7. GPIO expander close to the control-pad region

Critical nets should be placed so they can be routed directly before autorouting begins.

Routing workflow

Hand/deterministically route first:

• switching loops
• battery/high-current paths
• USB
• crystals
• analog video
• RF
• critical display/clock nets

Only after those are clean should Freerouting be used for low-risk digital/control nets.

Acceptance criteria

• Every U1-Uxx device is classified as KEEP, CONSOLIDATE, or REMOVE
• Optional launch features are removed or justified
• One GPIO expander replaces two if electrically practical
• Speaker path is removed unless explicitly required
• microSD is removed unless explicitly required for Rev-A launch
• OLED/dev headers are removed
• OSD translation is reduced to one IC where practical
• 5 V load-switch count is reduced where practical
• New BOM/component-count comparison is documented
• New net-count comparison is documented
• Placement is rebuilt from the simplified netlist rather than patched incrementally
• Critical nets have direct, reviewable escape paths before autorouting
• Run a fresh routing benchmark after simplification
• Evaluate whether 6 layers are sufficient after the hardware diet

Product principle

Rev-A should contain only what is required to:

turn on, charge safely, read controls, transmit ELRS, receive analog FPV video, overlay OSD and show it on the 5-inch display.

Everything else should justify its BOM cost, routing cost and failure surface before remaining on the board.