Node rev.2

RP2350 ADXL355 node hardware notes

Working notes for the first custom RP2350A board with ADXL355, USB-C development power/data, 12 V field input, RS485, and a power architecture biased toward a clean accelerometer supply.

Design goals

• Keep firmware changes small by staying close to Raspberry Pi Pico 2/RP2350A.
• Give ADXL355 the quietest practical supply and layout environment.
• Allow development from USB-C without a 12 V supply.
• Allow normal field operation from 12 V.
• Prevent backfeeding between USB VBUS and the 12 V/buck input.
• Keep the schematic easy to manufacture externally, even if components are small.

Proposed power tree

12V_IN
  -> input fuse/PTC
  -> TVS to GND
  -> reverse polarity protection
  -> TPS62175 buck, 12 V to 5V_BUCK

USB-C VBUS
  -> optional fuse/filter
  -> TPS2121 IN2

5V_BUCK
  -> TPS2121 IN1

TPS2121 OUT = 5V_SYS
  -> TLV75733 -> 3V3_DIG
  -> low-noise LDO -> 3V3_ADXL_PRE
      -> ferrite / 0R option -> 3V3_ADXL

Recommended source priority:

• IN1: 5V_BUCK from 12 V, main/field source.
• IN2: USB_5V, fallback/development source.
• OUT: 5V_SYS.

This means that when both 12 V and USB-C are connected, the board should run from 12 V while USB is used for data/debug. Exact TPS2121 priority and threshold resistors must be set from the datasheet during schematic entry.

Candidate BOM and KiCad mapping

Local downloaded libraries already found:

ADXL355 symbol:
/Users/anone/Downloads/ul_ADXL355BEZ/KiCADv6/2026-05-26_12-26-55.kicad_sym

ADXL355 footprints:
/Users/anone/Downloads/ul_ADXL355BEZ/KiCADv6/footprints.pretty/E-14-1_ADI.kicad_mod
/Users/anone/Downloads/ul_ADXL355BEZ/KiCADv6/footprints.pretty/E-14-1_ADI-L.kicad_mod
/Users/anone/Downloads/ul_ADXL355BEZ/KiCADv6/footprints.pretty/E-14-1_ADI-M.kicad_mod

TPS2121 symbol:
/Users/anone/Downloads/ul_TPS2121RUXR/KiCADv6/2026-05-26_13-55-57.kicad_sym

TPS2121 footprint:
/Users/anone/Downloads/ul_TPS2121RUXR/KiCADv6/footprints.pretty/RUX0012A.kicad_mod

Input power schematic notes

Initial 12 V input chain:

J_PWR.1 = +12V_IN_RAW
J_PWR.2 = GND

+12V_IN_RAW -> F1 PTC -> +12V_PROT
+12V_PROT -> D_TVS -> GND
+12V_PROT -> D_REV Schottky series -> +12V_SAFE
+12V_SAFE -> TPS62175 VIN

Notes:

• PTS181212V150 is acceptable for a controlled 12 V lab input, but consider a higher voltage-rated fuse/PTC for field or industrial-style inputs.
• SM6T12A is aggressive for nominal 12 V. It is fine for a well-controlled 12 V adapter, but for systems that can sit above 12 V, consider a 15 V or 18 V working-voltage TVS while keeping the TPS62175 28 V input limit safe.
• SS14 as a series reverse-protection diode is simple and acceptable for V1. A P-MOS/ideal-diode style reverse protector would reduce voltage drop and heat.

Buck notes: TPS62175DQCR

Target output: 5V_BUCK.

Required schematic items:

• VIN input ceramic capacitor close to VIN/GND.
• Additional optional bulk capacitor footprint near the protected 12 V input.
• Inductor from SW to 5V_BUCK, selected from TPS62175 datasheet guidance.
• Output capacitor close to inductor/output return.
• Feedback divider for 5 V output.
• EN tied to +12V_SAFE or configured with UVLO if desired.
• PG routed to a testpoint or left according to datasheet recommendation.
• Testpoint on 5V_BUCK.

Layout priority:

• Keep the buck hot loop tiny.
• Keep SW copper small and away from ADXL355.
• Place buck, inductor, and high-current input/output caps away from the accelerometer side of the board.

USB-C and power mux notes

USB-C development connector:

VBUS -> USB_5V
GND  -> GND
CC1  -> 5.1k -> GND
CC2  -> 5.1k -> GND
D+   -> USB ESD -> RP2350 USB_DP
D-   -> USB ESD -> RP2350 USB_DM

Power mux:

TPS2121 IN1 <- 5V_BUCK
TPS2121 IN2 <- USB_5V
TPS2121 OUT -> 5V_SYS
TPS2121 GND -> GND

Do not leave TPS2121 control pins floating. Configure:

• priority / mode selection,
• input overvoltage thresholds,
• current limit,
• status output if useful.

Exact resistor values should be calculated directly from the TPS2121 datasheet.

3.3 V rails

Digital rail:

5V_SYS -> TLV75733 -> 3V3_DIG

3V3_DIG powers:

• RP2350A I/O and support circuitry,
• W25Q128JVS flash,
• RS485 transceiver logic supply,
• status LED,
• digital pull-ups and test circuitry.

Analog/accelerometer rail:

5V_SYS -> low-noise LDO -> 3V3_ADXL_PRE
3V3_ADXL_PRE -> MPZ2012S601AT000 / 0R option -> 3V3_ADXL
3V3_ADXL -> ADXL355 VDD and VDDIO

Recommendation:

• Use TLV75733 for 3V3_DIG.
• Use a low-noise LDO such as TPS7A2033 or ADP150-3.3 for 3V3_ADXL.
• Keep the ferrite footprint, but make it replaceable by 0R in case the filter resonates with ceramic capacitors or is unnecessary.

Unreleased custom-board pin plan

Despite the historical directory name, this in-progress schematic is not the source of truth for the V2 boards installed in Sanok. The installed V2 pin map is maintained in node/config/board_profile.h.

Keep the first custom board close to the current firmware pinout.

ADXL355 SPI:

SCK  -> GPIO10
MOSI -> GPIO11
MISO -> GPIO12
CS   -> GPIO13
DRDY -> GPIO14
INT1 -> GPIO15

RS485:

UART TX -> GPIO0
UART RX -> GPIO1
DE      -> GPIO2

This minimizes firmware changes during bring-up.

ADXL355 layout notes

• Place ADXL355 away from the buck inductor, SW node, USB connector, and RS485 cable connector.
• Do not route fast digital traces under the sensor.
• Keep SPI traces short and calm; avoid unnecessarily high drive strength.
• Put local decoupling very close to ADXL355 supply pins.
• Keep a continuous ground plane under and around the sensor area.
• Consider mechanical placement early: the accelerometer measures the board and mounting structure, not just the electrical design.

RS485 notes

Basic RS485 block:

3V3_DIG -> THVD1450DR VCC
RP2350 UART_TX -> D
RP2350 UART_RX <- R
RP2350 GPIO_DE -> DE and /RE control as desired
A/B -> connector
A/B -> CDSOT23-SM712 -> GND
Optional 120R termination across A/B via jumper or DNP resistor
Optional failsafe bias resistor footprints if needed

Use 3V3_DIG, not 3V3_ADXL, for RS485.

Items to verify before PCB release

• RP2350A footprint and exposed pad dimensions against the current Raspberry Pi hardware design files.
• ADXL355 land pattern against Analog Devices package drawing.
• Exact USB-C vertical connector part and matching footprint.
• TPS62175 DQC footprint and feedback/inductor/capacitor values.
• TPS2121 resistor configuration for priority, input thresholds, and current limit.
• Flash footprint/package for the selected W25Q128JVS ordering suffix.
• TVS working voltage against real 12 V input tolerance.
• Whether the field input should tolerate 24 V mistakes or only regulated 12 V.