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Capstone: Embedded Hardware Platform

This capstone turns the embedded-hardware handbook into a practical design review. You will specify a small connected sensor-controller board and produce the engineering evidence needed before schematic release.

Learning Objectives

By the end of this exercise, you should be able to create a hardware architecture, justify power and MCU choices, define interfaces, plan protection and connectivity, estimate low-power behavior, and write a bring-up checklist.

Prerequisites

You should already understand:

  • requirements and system architecture;
  • power entry, protection, regulators, and sequencing;
  • MCU clocks, reset, boot, and programming;
  • digital interfaces and level shifting;
  • analog front ends;
  • sensors, actuators, and drivers;
  • memory and storage;
  • wired and wireless connectivity;
  • EMC, ESD, grounding, layout, low-power design, safety, and bring-up.

Concrete Task

Design a battery-backed environmental monitor with these functions:

  • 9 V to 24 V external input with reverse-polarity and surge protection;
  • single-cell backup battery for short outages;
  • MCU with ADC, I2C, SPI, UART, RTC wake, and SWD debug;
  • temperature and humidity sensor over I2C;
  • 0 V to 10 V analog input into ADC;
  • relay output for an external alarm;
  • SPI NOR flash for logs;
  • RS-485 wired port and BLE module;
  • low-power sleep mode below 100 uA when external power is absent;
  • safe reset state with relay off.

Produce a design review package with calculations, interface decisions, and test plan.

Required Implementation

Create a Markdown review document with these sections.

1. Architecture

Draw a block diagram:

flowchart LR IN[9 V to 24 V input] --> PROT[Protection] PROT --> BUCK[5 V buck] BUCK --> LDO[3.3 V LDO] BAT[Backup battery] --> PM[Power mux] PM --> LDO LDO --> MCU[MCU] MCU --> I2C[I2C sensor] MCU --> AFE[0 to 10 V AFE] MCU --> FLASH[SPI NOR flash] MCU --> RS485[RS-485 transceiver] MCU --> BLE[BLE module] MCU --> DRV[Relay driver] DRV --> RELAY[Alarm relay]

List every voltage domain and every connector-facing signal.

2. Power Budget

Create active and sleep budgets. Include regulator quiescent current, BLE current, sensor current, flash standby, divider leakage, and pull resistors.

Use:

$$
I_{avg} = \frac{\sum I_i t_i}{\sum t_i}
$$

For LDO heat:

$$
P = (V_{IN} - V_{OUT})I
$$

3. Interface Decisions

For each interface, specify:

  • voltage level;
  • direction;
  • pull resistors;
  • level shifting;
  • ESD or surge protection;
  • reset state;
  • test point or fixture access.

4. Analog Input

Design the 0 V to 10 V analog input:

  • divider ratio to keep ADC input below 3.3 V;
  • RC filter cutoff;
  • series resistance and clamp current;
  • calibration plan;
  • ADC source impedance check.

5. Relay Driver

Specify:

  • coil voltage and current;
  • MOSFET gate drive;
  • gate pulldown;
  • flyback clamp;
  • safe reset state;
  • fault behavior if relay coil shorts.

Use:

$$
P_{MOSFET} = I^2R_{DS(on)}
$$

6. EMC and Safety Review

List layout constraints:

  • high-current loops;
  • ESD location;
  • RS-485 termination and protection;
  • BLE antenna keepout;
  • analog input separation from switching node;
  • relay contact clearance if switching external energy.

7. Bring-Up Checklist

Write at least ten ordered bring-up steps, starting with visual inspection and current-limited first power.

Expected Behavior

A strong capstone submission should:

  • make every major hardware decision traceable to a requirement;
  • include calculations with units;
  • define safe reset and sleep states;
  • protect external connectors;
  • explain what will be measured during bring-up;
  • identify risks that remain before PCB layout.

Verification Steps

  1. Check every connector-facing pin has protection or a stated reason.
  2. Verify all voltage domains are compatible or level shifted.
  3. Recalculate the analog divider and confirm ADC headroom.
  4. Confirm relay is off during reset and MCU programming.
  5. Confirm sleep current budget is below 100 uA.
  6. Confirm flash and BLE are not back-powered when disabled.
  7. Review return paths for regulator, relay, analog input, RS-485, and antenna.
  8. Confirm bring-up steps can be executed with normal lab tools.

Common Failure Symptoms

  • Board draws high current on first power: short, wrong regulator feedback, reversed part, or back-powered IC.
  • ADC reads noisy values: poor filtering, bad reference, high source impedance, or switching-node coupling.
  • RS-485 fails on long cable: missing termination, no bias, wrong A/B polarity, or no common-mode protection.
  • BLE range is poor: antenna keepout violation, metal enclosure, weak supply during transmit, or wrong approved antenna.
  • Relay turns on at reset: missing gate pulldown or unsafe MCU default state.
  • Sleep current too high: pull resistors, divider leakage, regulator IQ, sensor standby, or debug circuit.

Debugging Guidance

  • Start with rail resistance and current-limited power.
  • Disable loads one at a time to isolate high current.
  • Use a known voltage source on the analog input before connecting field wiring.
  • Test relay output with a dummy load before external equipment.
  • Check RS-485 with short cable and known transceiver first, then add cable length.
  • Measure BLE supply droop during transmit.
  • Compare sleep current with each rail and load switched off.

Extension Challenge

Add a second board variant that removes BLE and uses only RS-485. Update the power budget, connector plan, compliance risks, and bring-up checklist. Explain which schematic blocks should be marked as do-not-populate and which firmware configuration pins identify the variant.

Concise Explained Solution

A good solution uses protected 9 V to 24 V input feeding a buck regulator, then a quiet 3.3 V rail for MCU and sensors. The 0 V to 10 V input uses a divider such as 22 kOhm over 10 kOhm, producing about 3.125 V at full scale, followed by an RC filter and ADC source-impedance check. The relay uses a low-side logic-level NMOS with gate pulldown and flyback clamp so it remains off during reset. RS-485 has termination strategy, ESD protection, and connector pinout review. BLE follows module antenna keepout and peak-current requirements. Sleep current is reduced by disabling the analog divider, sensor, flash, radio, and relay driver where possible. The bring-up plan starts with inspection, resistance checks, current-limited rails, reset/clock/debug, then peripheral tests with dummy loads.

Summary

The capstone is complete when the design package proves that requirements, power, interfaces, analog input, memory, connectivity, protection, low-power behavior, safety, and bring-up are connected. The review should expose risks before schematic release, not after prototypes fail.

Further Reading

  • Vendor hardware design guides for the selected MCU and BLE module.
  • RS-485 transceiver application notes from Texas Instruments or Analog Devices.
  • Analog Devices and Microchip ADC input design guides.
  • IPC-A-610 and selected PCB fabricator DFM checklists.

Mind Map

mindmap root((Embedded hardware capstone)) Core concept Complete platform review Link requirements to design Verify before PCB Applications Sensor controller Industrial monitor Battery backup node Connected product Formulas Iavg equals sum I t over total t LDO P equals Vin minus Vout times I Divider Vout equals Vin R2 over R1 plus R2 MOSFET P equals I squared Rds Design rules Safe reset state Protect connectors Budget sleep current Plan bring up Practical checks Rail current ADC headroom Relay dummy load BLE supply droop Common mistakes No protection Relay on at reset Sleep leakage missed Antenna keepout ignored