PCB-STEVE

STM32F4 based Flight Computer Development PCB

STEVE, the name of this flight computer, is an acronym: Starter Testbed for Evolving the Vehicle Electronics

As the name implies, the intention of STEVE is to be a testbed circuitboard for proving out avionics functionality. STEVE is very much a development board as this is the first STM32 PCB that I have designed.

Future versions of STEVE will have a focus on exapnding a flight vehicle's envelope (Starter Testbed for Expanding the Vehicle Envelope)

STEVE is intended to be used on a flight vehicle such as a model rocket but can also be used in static environments such as a ground test stand.

STEVE Version 1.1.0 Hardware Features

• 70 x 70 mm, 6 layer board with all components on one side for ease of manufacturing
• 1 STM32F407 Microcontroller
  • (STM32F407VGT6 is the specific SKU being used)
  • 1024 KB (1MB) of internal flash memory
  • 192 (112+16+64) KB of system RAM
• Reset and Boot buttons
• USB-C input for programming the microcontroller
  • the GCT USB4105-GF-A connector is allegedly rated for 5A
  • also usable for power when not connected to the PSM board
• Terminal block for 5V regulated input
• Micro SD card
• Winbond W25Q 128 Mbit (16 MB) external flash storage
• 1 BMI270 Inertial Measurement Unit (IMU) connected over SPI
• 1 BMP390 Barometric Pressure Sensor connected over SPI
• A 2x5 1.27mm pitch debug header
• A 32.768 kHz Low Speed External Crystal Oscillator
• A 8 MHz High Speed External Crystal Oscillator
• A TI TLV1117 1A 3.3V Output Linear Regulator
• A dedicated telemetry modem connection
  • uses screwless terminal blocks or a JST-GH connector
    • (each JST-GH connection is rated for 1A and the RFD900 has a max draw of ~1A at 5V)
  • GND, 5V, and UART signals (RX and TX) are present
  • placed close to both the USB-C and Regulated 5V power inputs
• 2 JST-SH 4 pin connectors for I2C
  • allows for easy hardware development for I2C devices (load cells, GNSS, etc)
• 1 passive buzzer (passive buzzers allow for different beep noises)
• 4 standard LEDs
• 2 WS2812 (NeoPixel) Addressable LEDs
• Additional GPIO and bus pins broken out
  • SPI2 (also used for BMP390) pins
  • UART1 pins
  • 2 ADC (12-bit) capable pins
  • Uses screwless terminal blocks
• Connectors for PSM board signals (3 x 6 pin connectors)
  • Uses JST-GH connectors
  • Servo PWM signals (4)
  • Mosfet gate signals (5)
  • Mosfet continuity signals (5)
  • Battery #1 Voltage sense line

Future Updates

Future STEVE Updates

• use a 3.3V plane on layer 5 (some signals ok) if doing a 6 layer board with components on 1 side
• move the 5V components to the same area
• make the board smaller (60 x 60 mm)
• consider moving the complicated components (BMI, BMP, WS2812) to a separate board
  • probably a bad idea since creating a simple daughter board with Standard PCBA would cost at least (10 PCB + 30 PCBA + 25 shipping) = $65
  • using Economic PCBA only saves ~$25
• give serious consideration to creating a STEVE revision with the PSM circuitry on the same board
  • keep in mind that any board orders also have tarrifs, taxes, and shipping applied, which can easily double the initial PCB/PCBA cost
• use a PMOS mosfet for reverse polarity protection
• add an RJ45 connector to utilize the ethernet capabilities of the STM32F407 (this might be part of a completely different board design)

Using/opening this project on a different machine

• open the project file in KiCad

On the top bar, the following thing needs to be updated based on the machine you are using:

• Preferences > Configure Paths Add an enironment variable with a name of: CURRENT_PROJ_DIR The value of the enirnment variable should be: your-repo-dir-path/PCB-STEVE/hardware

On the top bar, the following things should not need to be updated but both use the CURRENT_PROJ_DIR variable

• Preferences > Manage Symbol Libraries > Project Specific Libraries (tab)
• Preferences > Manage Footprint Libraries > Project Specific Libraries (tab)

PCB Ordering Notes

STEVE V1.1.0

STEVE V1.1.0 is a 6 layer PCB that uses vias as small as 0.3mm (hole size) / 0.45mm (overall via diameter)

On JLC's website, 6 layer boards use the "Advanced PCB/PCBA service". This has different default options from the standard service

Nonstandard options chosen for the advanced PCB service:

• (optional) Specify Stackup > No requirement
• Mark on PCB = 2D barcode (Serial Number)
  • 2D barcode Only | QR Code | STEVE_V1-1-0_SN | Remove Unique Number | 0001
    • 8*8mm (5x5 also ok) | Specify Position (No Requirement is ok too)

Advanced Options

• $0.71 Blank Box

PCB Assembly Options A board with 6 or fewer layers and single sided assembly qualifies for JLC's Economic PCBA https://jlcpcb.com/capabilities/pcb-assembly-capabilities See the above link for Economic PCBA constraints such as

• only supporting green silkscreens
• only supporting the ENIG finish

STEVE 1.1.0 is a 70 x 70mm board; using Standard PCBA auotmatically sets the board size to at least 70x70mm for adding edge rails/fiducials. "The board size would be modified to be 80*70mm for Standard PCBA due to adding two 5mm edge rails on the shorter sides"

Nonstandard PCBA options chosen:

• PCBA Qty = 2 (can choose 2+ based on component availibility)
• Edge Rails/Fiducials = Added by JLCPCB (default)
• Confirm Parts Placement = Yes

Advanced Options

• Photo Confirmation = Yes (Not available for Economic)
• Conformal Coating (+cleaning) = No (can choose yes for future boards)
  • Not available for Economic PCBA
• Packaging = ESD+Cardboard (Cardboard is ok too)
• Solder Paste = Sn96.5/Ag3.0/Cu0.5 (Do NOT use Bismuth/Bi solder)
• The "Bake Components" option is not available for Economic PCBA
  • Will need to use the Standard PCBA service or omit the WS2812 LEDs (C2761795) from being assembled since they are moisture sensitive
• The BMI270 and BMP390 also require Standard PCBA

"Note: The setup fee is $25 per assembly side for Standard PCBA."

Stencil Order Options

• No stencil being ordered
  • If boards with many small pitch components (ex: STM32) are going to be assembled, a reflow oven is strongly recommended
  • if a stencil is ordered, make sure to get it cut to size to save on shipping costs

Other Notes

• Don't use special characters such as "µ" for part designators, footprints, or values as this can cause issues with processing the bom and positions (CPL) files.