Embedded Security as a Future Topic (Secure Boot, TrustZone)

• Resolution (bits): determines the LSB size (Vref/2^N).
• Sampling rate fs: determines how fast you measure (Nyquist: >= 2*fmax of the signal).
• Quantization/ENOB: real effective bits < datasheet bits; can be improved via oversampling & averaging (e.g. +1 bit at 4x, +2 bits at 16x). (Silicon Labs, Texas Instruments)

• The input source must charge the ADC's sample-and-hold capacitor quickly -- high source impedance leads to erroneous samples. Good practical explanations are available on this topic. (Embedded Related)
• RC filter close to the pin, star grounding, short traces, dedicated VDDA/VSSA (decoupling!). (EDN)

// Global buffers
#define ADC_BUF 1024
volatile uint16_t adc_buf[ADC_BUF];

// Init: ADC (Scan, Continuous off), external trigger e.g. TIM3_TRGO
// DMA: Peripheral->Memory, Half/Full Transfer IRQ, Circular
// Timer: Upcounter, Update Event as TRGO, Rate = desired fs

void start_adc_dma(void) {
    HAL_ADC_Start_DMA(&hadc1, (uint32_t*)adc_buf, ADC_BUF);
    HAL_TIM_Base_Start(&htim3); // provides TRGO to ADC
}

// Callbacks: deterministic processing in blocks
void HAL_ADC_ConvHalfCpltCallback(ADC_HandleTypeDef *hadc) {
    // process adc_buf[0 .. ADC_BUF/2-1]
}
void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef *hadc) {
    // process adc_buf[ADC_BUF/2 .. ADC_BUF-1]
}

const int PIN = A0;
const float VREF = 5.0; // or 3.3V -- depends on board/analogReference()

void setup() {
  Serial.begin(115200);
}

void loop() {
  const int N = 32;          // simple averaging
  long acc = 0;
  for (int i=0; i<N; ++i) acc += analogRead(PIN);
  float avg = acc / float(N);               // 10-bit -> 0..1023
  float volt = avg * (VREF / 1023.0);
  Serial.println(volt);
}

#include "driver/dac.h"
void app_main(void) {
    dac_output_enable(DAC_CHANNEL_1); // GPIO25
    for (;;) {
        for (int v=0; v<256; ++v) {
            dac_output_voltage(DAC_CHANNEL_1, v); // 0..255 -> 0..Vref
        }
    }
}

• Reference voltage (Vref): internal vs. external, temperature drift/noise; decouple separately, short return path to ADC ground. Poor Vref ruins any calibration; app notes on oversampling and practical articles show effects and countermeasures. (Silicon Labs, EDN)
• Layout: Separate AGND/DGND sensibly, star topology, no return currents crossing the measurement front; filter close to the pin. (EDN)
• Oversampling/Averaging: Sample N times, use averaged values; for high dynamics, use adaptive averaging. Formal bit gains as described above. (Silicon Labs)
• DMA double buffer: deterministic blocks; with caches: use non-cacheable regions. (STMicroelectronics)
• Calibration: 2-point (offset/gain), characterize across temperature.
• Anti-aliasing: Bessel for time-domain signals (clean phase), Butterworth for spectral separation; TI/ADI provide concrete dimensioning. (Texas Instruments, Analog Devices)

• MQTT (TCP/TLS, Pub/Sub): excellent for telemetry, "last will", QoS 0/1/2; ideal for gateway/cloud integration, v5 brings better error reporting and features. (OASIS Open Docs, mqtt.org)
• CoAP (UDP/DTLS, REST-like): lightweight, Observe/Blockwise; well-suited for constrained networks, IPv6/6LoWPAN. (IETF Datatracker)
• HTTP/2/3: updates/downloads, broad infrastructure -- with TLS/mTLS and resume mechanisms.

• Monorepo vs. multirepo: Monorepo simplifies cross-cutting changes, multirepo isolates components/products. There is no dogma -- tooling and team size decide. Martin Fowler examines the trade-offs and branching patterns. (martinfowler.com)
• Vendor drops (HAL/SDK): pull in as submodules/mirrors with immutable tags; reference changelogs; no manual patch mixing in the project root.
• Release traces: build SBOMs (e.g. CycloneDX) and provenance (SLSA) directly into the pipeline -- artifacts can later be traced back to sources/builds unambiguously. Invaluable for IoT fleets (forensics, CVE matching). (CycloneDX, GitHub, SLSA)

• Unit tests (host): e.g. Unity/Ceedling -- a lean ANSI C framework, ideal for driver logic (with CMock for HAL stubs). (Throw The Switch, GitHub) Mini example (scaling):
  C

  // scales raw ADC value (0..4095) to volts (Vref=3.3)
  float adc_to_volt(uint16_t raw) { return (3.3f * raw) / 4095.0f; }
  
  TEST(AdcScale, ConvertsCorrectly) {
    TEST_ASSERT_FLOAT_WITHIN(0.005f, 1.650f, adc_to_volt(2048));
  }
  

• Unit tests (target): small test suite on-target (e.g. via semihosting/serial) to catch compiler/ABI/linker effects.
• HIL: real controller hardware ↔ simulated environment (ESP32 DAC feeds STM32 ADC; digital I/Os drive simulated sensors). Critical for timing/I/O paths and safety functions (e.g. secure update recovery). (Ansys)
• Release metrics: code coverage (host), on-target smoke tests, latency/jitter measurements of the DMA chain, ENOB/noise floor, update MTTR (recovery time per SP 800-193). (NIST Publications)

1. Threat model: Which attackers? (Physical/remote). Which assets? (Keys, IP, cloud credentials, sensor integrity).
2. Harden the boot chain: ROM root, mandatory signatures, rollback protection, secure storage. SBSFU/TF-M/MCUboot are battle-tested. (STMicroelectronics, mcuboot)
3. Isolation: TrustZone-M for crypto/keys/update agent. (Arm Developer)
4. Lifecycle updates: consider standardized PSA Firmware Update API concepts; clean, repeatable process -- before series production. (arm-software.github.io)
5. Transports: MQTT/CoAP with (m)TLS/DTLS, device identity via mTLS, robust retry/backoff, clock sync. (OASIS Open Docs, IETF Datatracker)
6. Supply chain: SBOM + SLSA provenance per release. (CycloneDX, SLSA)

• Anti-Aliasing & Oversampling TI: Antialiasing Filter Circuit Design...; Designing an anti-aliasing filter... (Texas Instruments) Analog Devices: Guide to Anti-Aliasing Filter Basics; Anti-aliasing filtering considerations... (Analog Devices) Silicon Labs: Improving ADC Resolution by Oversampling and Averaging (Silicon Labs)
• STM32 ADC/DMA ST Wiki: Getting started with ADC (DMA, TIM); ST App Notes: ADC modes; DMA introduction (STMicroelectronics, STMicroelectronics)
• Arduino/ESP32 Arduino Docs: analogRead() & examples (docs.arduino.cc, Arduino) Espressif Docs: ESP32 DAC (API & TRM) (Espressif Docs, Espressif)
• Secure Boot / TrustZone / Lifecycle ST SBSFU (product page, AN5056), TrustZone-based solution (AN5447) (STMicroelectronics) ARM/TF-M & TrustZone-M fundamentals (Arm Developer, trustedfirmware-m.readthedocs.io) MCUboot docs & signing tool (mcuboot) NIST SP 800-193: Platform Firmware Resiliency (NIST Publications) PSA Firmware Update API (design goals/model) (arm-software.github.io)
• Edge-to-Cloud Protocols MQTT (OASIS v3.1.1/v5) (OASIS Open Docs) CoAP (RFC 7252) & extensions (IETF Datatracker)
• Workflow & Traces CycloneDX SBOM (spec/guide) (CycloneDX) SLSA Provenance/Levels (SLSA) Monorepo trade-offs (Fowler) (martinfowler.com)