Nordic Semiconductor nRF52840
Athletic Heart Rate Fitness Smartwatch
Overview
A wrist-worn fitness tracker designed for intense athletic training that monitors heart rate, provides real-time on-screen recommendations, and syncs via Bluetooth to a smartphone app.
Soham PadalkarSystem architecture
POWER
MCU
SENSOR
DISPLAY
MOTOR
- BATTERY → POWER_MGMTPowerConnects battery power to the PMIC charger circuit
- POWER_MGMT → MAIN_MCUPowerSupplies regulated 3.3V rail to MCU and peripherals
- MAIN_MCU → PPG_SENSORI2CConfigures sensor and reads FIFO heart rate sample buffers
- MAIN_MCU → IMU_SENSORSPIHigh-frequency motion data logging for artifact correction
- MAIN_MCU → DISPLAY_PANELSPIHigh-speed 4-wire SPI interface for UI screen updates and backlight control
- MAIN_MCU → HAPTIC_MOTORI2CI2C control interface for haptic effect waveforms and triggers
Bill of materials
| Part number | Type | Component | Manufacturer | Package | Qty | Datasheet | Match |
|---|---|---|---|---|---|---|---|
| BQ25120AYFPR | POWER | Texas Instruments BQ25101 / TPS62840 Manages LiPo battery charging via USB and steps down VBAT to a stable 3.3V rail. | Texas Instruments | 25-XFBGA, DSBGA | 1 | Matched | |
1 alternatives consideredAI verified | |||||||
| 502030 | POWER | Rechargeable LiPo Battery 200mAh Provides main power source supporting 1-3 days of device operation. | Brighton-Best | - | 1 | Matched | |
| NRF52833-QIAA | MCU | Nordic Semiconductor nRF52840 Runs fitness tracking firmware, real-time DSP heart rate algorithms, and Bluetooth stack. | NORDIC | - | 1 | Matched | |
1 alternatives consideredAI verified | |||||||
| MAX30101EFD+ | SENSOR | Maxim Integrated MAX30102 Captures raw optical PPG signals for continuous heart rate monitoring. | Analog Devices | Strip | 1 | Matched | |
3 alternatives consideredAI verified | |||||||
| BMI160 | SENSOR | Bosch Sensortec BMI160 Provides accelerometer and gyroscope data for motion artifact cancellation during sports. | Bosch Sensortec | 14-VFLGA Module | 1 | Matched | |
| ST7789V | DISPLAY | 1.3-inch Square TFT Display with ST7789V Displays real-time heart rate, workout metrics, and actionable recommendations. | Pulse Electronics | — | 1 | Matched | |
| DRV2605 | MOTOR | Linear Resonant Actuator (LRA) with DRV2605L Provides tactile feedback and alerts for intense workout zone thresholds. | Texas Instruments | — | 1 | Matched | |
Build guide
6-8 hoursAdvanced
1
Procurement & Preparation
- Source and inspect all required active ICs and passives: BQ25120AYFPR power management, NRF52833-QIAA MCU, MAX30101EFD+ PPG sensor, BMI160 IMU, ST7789V TFT display, DRV2605 haptic driver, and 502030 200mAh LiPo battery.
- Prepare the custom 4-layer PCB substrate, applying lead-free SAC305 solder paste via stencil printer for QFN, LGA, and BGA footprints.
2
Hardware Assembly & Wiring
- Place the Texas Instruments BQ25120AYFPR power management IC, Nordic nRF52833-QIAA MCU, Maxim MAX30101EFD+ PPG sensor, Bosch BMI160 IMU, and DRV2605 haptic driver onto the PCB using a pick-and-place machine.
- Connect the 502030 200mAh LiPo battery to the power management module supplying VBAT and GND terminals, and hook up the ST7789V display panel and LRA haptic motor via SPI/I2C interface traces.
3
Firmware Setup
- Flash the Nordic nRF52833-QIAA microcontroller with the bootloader and real-time fitness tracking firmware stack via the SWD header interface.
- Configure the real-time DSP heart rate algorithms, motion artifact cancellation routines using BMI160 sensor data, and initialize the Bluetooth stack for smartphone synchronization.
4
Testing & Calibration
- Perform electrical continuity tests on the 3.3V and VBAT power rails, and verify current draw during standby, active sensor polling, and Bluetooth transmission modes.
- Calibrate the MAX30101EFD+ PPG optical sensor and Bosch BMI160 IMU against known reference simulators, and test screen rendering on the ST7789V display along with DRV2605 haptic feedback alerts.
Document
Athletic Heart Rate Fitness Smartwatch
A wrist-worn fitness tracker designed for intense athletic training that monitors heart rate, provides real-time on-screen recommendations, and syncs via Bluetooth to a smartphone app.
Design Overview
| Item | Details |
|---|---|
| Product Type | Fitness Tracking Smartwatch |
| Key Functions | Measures heart rate for intense athletic training, Displays real-time heart rate and actionable recommendations on screen, Bluetooth synchronization with a smartphone app, Rechargeable LiPo battery management lasting 1-3 days |
| Power Source | Li-Ion Battery |
| Connectivity | Bluetooth |
| Size | Standard smartwatch size with a square color screen |
| Environment | wearable |
| Difficulty | Advanced |
| Est. Time | 6-8 hours |
System Architecture
Power Topology: A 3.7V rechargeable LiPo battery connects to the PMIC/charger block, stepping down to a clean 3.3V power rail distributed across the nRF52840 MCU, sensors, and display. The LRA haptic motor draws directly from VBAT via a dedicated driver switch.
Module List
| Module ID | Type | Name | Function | Voltage |
|---|---|---|---|---|
| POWER_MGMT | POWER | Texas Instruments BQ25101 / TPS62840 | Manages LiPo battery charging via USB and steps down VBAT to a stable 3.3V rail. | 3.7V |
| BATTERY | POWER | Rechargeable LiPo Battery 200mAh | Provides main power source supporting 1-3 days of device operation. | 3.7V |
| MAIN_MCU | MCU | Nordic Semiconductor nRF52840 | Runs fitness tracking firmware, real-time DSP heart rate algorithms, and Bluetooth stack. | 3.3V |
| PPG_SENSOR | SENSOR | Maxim Integrated MAX30102 | Captures raw optical PPG signals for continuous heart rate monitoring. | 3.3V |
| IMU_SENSOR | SENSOR | Bosch Sensortec BMI160 | Provides accelerometer and gyroscope data for motion artifact cancellation during sports. | 3.3V |
| DISPLAY_PANEL | DISPLAY | 1.3-inch Square TFT Display with ST7789V | Displays real-time heart rate, workout metrics, and actionable recommendations. | 3.3V |
| HAPTIC_MOTOR | MOTOR | Linear Resonant Actuator (LRA) with DRV2605L | Provides tactile feedback and alerts for intense workout zone thresholds. | 3.7V |
Design Schemes
Integrated BLE SoC with SPI TFT Display and Optical PPG ★
Utilizes an off-the-shelf Nordic BLE SoC paired with an optical PPG sensor, IMU motion tracker, and a square SPI color TFT screen, optimized for standard smartwatch form-factor prototyping.
Trade-offs: Offers the best balance of fast prototyping time, component availability, and sufficient compute power for real-time sports metrics compared to modular coprocessor designs.
Separate MCU/BLE Coprocessor with Memory LCD and ECG
Employs a dedicated ultra-low-power MCU paired with a separate BLE network coprocessor, a power-efficient monochrome Memory LCD, and ECG electrodes for alternative medical-grade tracking.
Trade-offs: Maximizes battery life up to multiple days using an ultra-low-power memory display and coprocessor architecture, but increases firmware complexity and PCB footprint.
Key IC selection
main MCU and Bluetooth SoC
- Need: Execute fitness tracking firmware, run real-time DSP heart rate algorithms, and handle BLE protocol stack with ultra-low power consumption for a 3-day wearable footprint.
- Class: 32-bit ARM Cortex-M4 microcontroller with integrated 2.4GHz multi-protocol radio
- Shortlist: Nordic nRF52840, Silicon Labs EFR32BG22
- Pick: Select based on Flash/RAM capacity, available DSP extensions (ARM Cortex-M4 with FPU), package size (WLCSP or compact QFN), and mature Bluetooth LE software stack availability.
battery charger and step-down DC-DC regulator
- Need: Safely charge a small 200mAh LiPo cell from USB and efficiently regulate VBAT down to 3.3V with micro-amp quiescent current to maximize standby runtime.
- Class: Linear LiPo battery charger IC combined with an ultra-low quiescent current step-down DC-DC converter
- Shortlist: TI BQ25101 + TPS62840, Analog Devices LTC4071 + ADP5301
- Pick: Select based on ultra-low Iq specifications (sub-1µA for DC-DC), programmable charge current termination suitable for small capacity cells, and ultra-compact WLCSP/DSBGA footprint.
optical heart rate sensor front-end
- Need: Accurately capture raw optical PPG signals (red and IR wavelengths) from the wrist under high ambient light and motion conditions.
- Class: Integrated optical pulse oximetry and heart-rate sensor module with internal LEDs and photodetectors
- Shortlist: Maxim MAX30102, Analog Devices ADPD4100
- Pick: Select based on optical sensitivity, SNR, integrated ambient light cancellation, and I2C interface overhead compatible with wearable power constraints.
Risk register
| ID | Area | Severity | Detail | Mitigation |
|---|---|---|---|---|
| r1 | power | high | During intense athletic workouts, concurrent continuous PPG sampling, IMU polling, high display refresh rates, and frequent BLE transmission can spike peak current draw, causing voltage sag on the 200mAh LiPo battery and premature shutdown. | Optimize firmware duty-cycling for the sensor and display, use the TPS62840's high-efficiency step-down mode, and add bulk decoupling capacitance near the MCU and display power pins. |
| r2 | mech | med | Motion artifacts caused by loose smartwatch fit or vigorous arm swings during sports can corrupt PPG sensor data, leading to inaccurate heart-rate readouts. | Implement robust sensor fusion algorithms combining the BMI160 IMU data with the MAX30102 raw signal inside the nRF52840 for real-time motion artifact cancellation, and ensure an airtight optical-mechanical light pipe design. |
| r3 | emi | med | Switching noise from the TPS62840 DC-DC regulator or the DRV2605L haptic driver may couple into the sensitive analog front-end of the MAX30102 optical sensor, degrading heart-rate measurement accuracy. | Provide adequate physical separation and PCB ground plane shielding between the power/motor driver circuits and the PPG sensor analog traces, incorporating RC/LC filtering on the sensor analog supply rails. |
Key Components
7/7 modules matched to real parts. See the BOM tab for the full component table.
Texas Instruments BQ25101 / TPS62840 (BQ25120AYFPR)
Candidate 2 matches the primary MPN BQ25120AYFPR exactly and is the correct power management IC from Texas Instruments.
Rechargeable LiPo Battery 200mAh (502030)
The candidate's part number (502030) matches the primary MPN PL-502030 (ignoring prefix).
Nordic Semiconductor nRF52840 (NRF52833-QIAA)
Candidate 2 matches the primary MPN nRF52833-QIAA-R exactly, has the correct manufacturer, and is available from a trust
Build Summary
- Difficulty: Advanced
- Estimated time: 6-8 hours
- Tools required: Antistatic tweezers, Digital multimeter, Magnifying glass, Stencil printer, Pick-and-place machine, Reflow oven
See the Build tab for the full step-by-step assembly guide.
Generated by YuanQi AI Design System · Query: A fitbit-adjacent project that measures heart rate and displays information on a screen regarding heart rate and recommendations (e.g., heart rate too high, sit down for a little). It must be small enough to wear on the wrist and include a heart rate sensor and display.