Project Specification
Overview
Design a flexible PCB sensor interface board that connects to an Adafruit Feather M0 Adalogger. The flex board includes two Texas Instruments ADS1232 24-bit ADCs, one Analog Devices/Maxim DS3231M RTC, one ST H3LIS331 accelerometer, and a CR2020 coin cell holder for RTC keep-alive power.
Key Components
- 2 × ADS1232IPW 24-bit bridge/load-cell ADCs, TSSOP-24
- 1 × DS3231M RTC, preferred compact DS3231MZ 8-SOIC variant unless a 16-SOIC DS3231M package is required
- 1 × H3LIS331DLTR high-g accelerometer, LGA-16
- 1 × CR2020-compatible coin-cell holder for RTC VBAT backup
- Direct plug-in interface to the Adafruit Feather M0 Adalogger headers, using the Feather header footprint/pin locations rather than an FPC or board-to-board connector
Assumed Architecture
- Main logic rail: 3.3 V supplied by the Adafruit Feather M0 Adalogger.
- The Adafruit Feather M0 Adalogger onboard battery connector receptacle is removed. The remaining Feather battery solder pads are wired to a separate main LiPo battery holder.
- Main battery wiring: two 6 inch, 22 AWG wires leave from the two battery contacts under the removed JST battery connector footprint/pads, route over the center of the Adalogger M0, and exit near the Micro USB connector just to the right of the pin 13 LED. Red wire = battery positive; black wire = ground. These wires connect to a battery holder that is separate from the sensor and Adalogger assembly. No strain relief is required.
- DS3231M connects over the Feather M0 Adalogger I2C bus using the dedicated SDA and SCL header pins, and uses the CR2020 only on VBAT for timekeeping backup.
- H3LIS331 connects over the same Feather M0 Adalogger I2C bus using the dedicated SDA and SCL header pins.
- ADS1232 devices use separate digital control/data lines to the Feather. Their analog bridge/sensor inputs require external sensor connector definitions before final wiring.
- Common ground between flex board and Feather.
- The separate main LiPo battery holder feeding the Feather battery pads is distinct from the CR2020 coin cell, which is only for DS3231M RTC keep-alive backup.
- Mechanical/electrical interface: flex board plugs directly into the Feather headers. The layout should include appropriate stiffener/support around the header contact area because flex material alone is not mechanically robust for repeated plug/unplug cycles.
Open Requirements Before Schematic Capture
- Confirm the Feather M0 Adalogger pin assignments available for ADS1232 DOUT/SCLK/PDWN/SPEED/GAIN/A0/TEMP. The RTC and H3LIS331 are assigned to the dedicated SDA/SCL I2C pins.
- Confirm whether each ADS1232 connects to one bridge sensor or two differential inputs, and define sensor connector type/pinout.
- Confirm ADS1232 excitation/reference strategy: use 3.3 V directly, a filtered 3.3 V analog rail, or a dedicated reference/excitation supply.
- Confirm flex-board mechanical constraints: outline, bend zones, stiffener areas, minimum bend radius, and whether coin cell/ICs must sit on stiffened rigid areas.
- Confirm whether the flex board should mate to Feather female headers via male pins, plug into Feather stacking headers, or be soldered directly to the Feather header pads.
- Battery wiring details confirmed: removed connector is the JST battery connector footprint/pads; use 22 AWG red/black 6 inch leads; red = battery positive, black = ground; no strain relief required.
Flex PCB Notes
- Place the coin cell holder, TSSOP ADCs, SOIC RTC, LGA accelerometer, and Feather header plug-in region on stiffened non-bending zones.
- Avoid traces crossing bend regions where possible; where unavoidable, route perpendicular/straight with generous width and no vias in bend areas.
- Keep analog bridge input routing away from digital clocks and connector edges.
Initial Library Findings
- ADS1232IPW is available in the Flux library with datasheet.
- DS3231M/DS3231MZ variants are available; DS3231MZ+TRL 8-SOIC is likely preferred for compact layout.
- H3LIS331DLTR is available in the Flux library.
- CR2020-compatible/generic coin-cell holders are available.
- Connector decision updated: use direct Feather-header plug-in. FPC/FFC and mezzanine connectors are no longer the baseline architecture.