SLAAE30 July   2021 MSP430FR2355

 

  1.   Abstract
  2.   Trademarks
  3. 1PIR Design Description
    1. 1.1 PIR Sensor
    2. 1.2 PIR Signal Chain
      1. 1.2.1 Traditional Motion Detection Signal Chain Design
      2. 1.2.2 Capacitor-Free Signal Chain Design
  4. 2Software
    1. 2.1 Software Architecture
    2. 2.2 Software Flow Chart
    3. 2.3 Power Profile
    4. 2.4 Data Processing
      1. 2.4.1 Digital Signal Conditioning
      2. 2.4.2 Low-Pass Filter for Temperature Drift
      3. 2.4.3 Spikes and Noise
      4. 2.4.4 Motion Detection Function
  5. 3Hardware and Schematic
    1. 3.1 MSP430FR2355
    2. 3.2 Schematic
  6. 4Results
    1. 4.1 Distance: 15 ft (4.5 m)
    2. 4.2 Distance: 22.5 ft (6.9 m)
    3. 4.3 Distance: 36.5 ft (11.1 m)
  7. 5Summary
  8. 6References

1.2.2 Capacitor-Free Signal Chain Design

Figure 1-6 shows a simplified version of our capacitor-less signal chain. The signal chain now is mostly purely buffering and gain stages. To remove DC blocking capacitors in the traditional circuit that were adding noise to the system, the stages are being biased using DACs. Instead of using comparators, the signals are fed into an analog-to-digital converter (ADC) which enables a feedback loop for the DAC for tracking changes to environment and ambient temperature and allows digital filtering of the signal.
GUID-5A7D9B5C-238A-4500-B217-047EFDB35D6D-low.pngFigure 1-6 Capacitor Free Signal-Chain

The biggest benefit of the MSP430FR2355 is that this entire signal chain is integrated into the MCU and is software configurable. The MSP430FR2355 includes four smart analog combos (SACs). Each of these SACs include an general purpose op-amp with configurable gain from 1-32x and a 12-bit reference DAC. Figure 1-7 illustrates the simplified interface to the PIR sensor leveraging the full signal chain inside the MSP430FR2355. The SACs can be chained together inside the chip, connected directly to the internal ADC, and can be shutdown for lowest possible sleep currents.

GUID-FDC01CC6-0087-4CD1-BE9A-1391175EA105-low.pngFigure 1-7 PIR Signal-Chain Integrated Into MSP430FR2355