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This technical documentation has been proofread by Claude-Sonnet-4 for readability, grammar, and formatting consistency. All technical content, project details, circuit designs, and conclusions have been verified and approved by the author.

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⚠️ Active Mission — Contribution & Confidentiality Notice

This is an active project under YJSP's Hardware-In-The-Loop (HITL) program. Although I am the sole Responsible Engineer (RE) for this project, it incorporates contributions from program leads and other team members. This page presents my contributions at a high level only.

Accordingly, this page does not display the latest revision that has passed any formal Design Review, nor does it include specific technical details.

Do not attempt to copy or replicate the designs shown here — this is NOT the final design and is NOT proven to work.

Program Overview

The Hardware-In-The-Loop (HITL) system aims to validate avionics hardware for YJSP rocket systems in a controlled laboratory environment, reducing the need for costly flight-ready sensors and complex integration processes during testing phases. The long-term goal is a fully-automatic CI/CD pipeline for avionics verification.

The very first project under the HITL program is the SAM Board Tester, which — as the name implies — tests and calibrates the functionality of a FSAM (Flight Systematic Avionics Module) or GSAM (Ground Systematic Avionics Module).

Design Goals for the SAM Board Tester

  • Provide fully protected 48 V / 24 V power supply for FSAM / GSAM, with enable control
  • Emulate the behavior of Thermal Couples, RTDs, Pressure Transducers, Differential Voltage Sensors, Valves, Detonators, and Remove-Before-Flight Tags
  • With precision DAC, ADC, source circuits, and a self-calibration algorithm, calibrate FSAM / GSAM
  • Allow disconnectivity check on all channels

Block Diagram

View the interactive block diagram (recommended):

🔗 Open Interactive Chart (Mermaid)

Or view the static render below:

System Architecture — 3 Stackable Boards

The HITL SAM Board Tester consists of 3 stackable boards that connnects thru 40+8pin conectors.

1 · Controller Board

The Controller Board is the brain of the system:

  • Hosts a Raspberry Pi CM5 / CM4, providing 3× SPI lanes, 2× I2C lanes, and direct control over power converters
  • Provides primary power conversion and distribution:
    • 24 V @ 3 A for GSAM.
    • 48 V @ 1.5 A for FSAM. Uses random spread spreatrum to lower EMI and noise.
    • 5V0-D @ 10 A for digital rails. Uses D-CAP3 control mode to provide fast transient response.
    • 6V0-A @ 1.5 A for analog pre-regulation. Uses random spread spreatrum to lower noise.
  • Hot-swap Control, Comprehensive Protection, Power and Temperature sensing on all power rails.

2 & 3 · Peripheral Boards (×2)

Each peripheral board supports half of the emulation channels. Key specs per board:

  • Independent 5V0-A and 3V3 LDOs (from the 6V0-A rail) for clean analog power
  • 2× 8-ch DAC, 1× 10-ch ADC, 1× 2-ch DigiPOT, 2× 16-ch GPIO Expander
    • 24 buffered output channels + 8 unbuffered I/O channels
  • Emulation channels per board:
    • 4× Valve
    • 4× Thermal Couple
    • 4× Current Loop
    • 4× RTD (2 from DAC, 2 from DigiPOT)
    • 1× Differential Sensor
    • 1× Detonator
    • 1× Remove-Before-Flight Tag

Interface & Communication

The boards communicate over SPI and I2C:

Device Count Bus Role
AD5676R DAC 4× (2 per peripheral) SPI0 32 output channels for analog emulation
ADS1261 ADC 2× (1 per peripheral) SPI1 20 input channels for valve / detonator V&I sensing
AD8403 DigiPOT 2× (1 per peripheral) SPI5 4 resistance channels for low-cost RTD emulation
PCF8575 GPIO Expander + ULN2803 4× (2 per peripheral) I2C0 Disconnectivity check relays & RBF control
TMP112 Temperature Sensor 3× (1 per board) I2C2 Temperature compensation
INA228 Power Meter 4× (controller) I2C2 48 V / 24 V / 6V0-A / 5V0-D V, I, P sensing

Power Architecture

  • LM5069 two-stage load switches — comprehensive UV/OV, over-power, short circuit, constant-current startup, and hotswap-ready protection. The 24 V / 48 V output channels include shutdown control and power-good indicator.
  • TPS56A37 — 10 A, 24 V → 5V0-D step-down for digital rails (CM5 needs ~2 A max; ~40 relays need ~6 A)
  • LM51571 — 1.5 A, 24 V → 48 V low-EMI step-up for FSAM; random-spread-spectrum supported
  • LMR36015 — 1.5 A, 24 V → 6V0-A low-EMI step-down for analog pre-regulation; random-spread-spectrum supported
  • TPS7A20xx — 300 mA, 6V0-A → 5V0-A / 3V3 LDO for each board's independent analog rail

Analog Backends

Function Implementation
Differential voltage output (Diff. Sensors, TCs) FDA — THP210 Fully Differential Amplifier
Low-side current sink (Valves, Detonators) Op-amp (OPA2210) + MOSFET gate driver
Current source (Pressure Transducers) Integrated V/I converter — XTR111
RTD emulation Single-end amplifiers + differential V / current sink, or direct impedance from DigiPOT
Remove-Before-Flight Tag MOSFET switch

Board Interconnects

The three boards connect via two connector types:

40-pin 1.27 mm IDC — carries 3× SPI and 2× I2C lanes, with ground pins adjacent to clock lines for signal integrity.

8-pin Molex Flex3 — carries 24 V, 6V0-A, 5V0-D, PGND, and AGND.

Layout Status

Layout Rev 1 is currently in progress. The three boards together sum to a total of 254 + 476 × 2 = 1,206 components. Magnificent work!