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Part 2 - Sourcing

Once you have your system requirements, you will need to select which components to use in your system.

Selection Order

  1. Choose the main parts first, all the Main ICs
    • Processor
    • Motor driver
    • Motors
    • Screen
    • Wireless Chip
    • Sensors
    • Any other important IC
  2. Calculate the power requirements (voltages and currents) of all previous ICs
  3. Choose power supply based on calculated power requirements
    • Take in account continuous and peak currents at given input and output voltages
  4. Choose passive components (usually given in datasheets and application notes of ICs)
    • Resistors
    • Capacitors
    • Op-Amps
    • Any other circuitry that surrounds the main ICs
  5. Choose specific connectors

Selection Criteria

  1. Parts that are already in stock
    • Low lead times
    • Sufficient stocks
  2. Parts available at your PCB manufacturer (for example: JLCPCB)
  3. Parts you are familiar with
  4. Parts Distributor search (for example: Mouser or Digikey)
  5. Easy to assemble packages
    • Don’t choose very small packages like 0201 or BGA, this will make debugging and soldering harder
  6. Right performance
  7. Least amount of additional circuitry needed
  8. Cost
  9. Part Lifetime
    • Don’t take NRND (Not recommended for new design) parts

To seach for parts in a distributor like mouser.com:

  1. Select the product category (example: Products > Semiconductors > Embedded Processors & Controllers)
  2. Select the product type (example: Microcontrollers - MCU)
  3. Set first filters (example: Manufacturer > STMicroelectronics, Mounting Style > SMD/SMT)
  4. Apply (this reduce the possibilities of filters for each step)
  5. Set second filters (example: Package/Case > LQFP-100 to LQFP-62, Data Bus Width > 32bit, Maximum Clock Frequency > 48 to 72Mhz, Program Memory Size > 64kB - 128kB, Data RAM Size > 20kB - 768kB)
  6. Apply (repeat as many times as necessary)
  7. Sort by price. It is cost effective
  8. Tick In Stock and Normally Stocked. Ensure the parts are in stock.
  9. Make sure the price is reasonable for the part you are searching for. (example: 2.55 USD per unit is reasonable for an inexpensive MCU)
  10. Check the Datasheet of the component and whether a footprint (PCB and 3D) is provided

Part Selection Examples

Microcontroller

Important Microcontroller (MCU) parameters:

  • Programming tools, development environment
  • Bus width (8 bit, 32 bit)
  • Maximum clock speed (8Mhz to 550Mhz)
  • Flash and RAM sizes
  • Number of cores (typically 1), DMA, supply voltage
  • Peripherals and I/O count (I2C, SPI, USB, CAN, Timers, GPIO)
  • Packaging (BGA, QFN, LQFP) and package size
  • Cost

You can have a preferred MCU type, for example STM32 are cheap, easy and have affordable programming tools, free IDE, good app notes and datasheets. They exist om various flavors:

  • High-performance: F2, F4, F7, H7
  • Mainstream: F0, F1, F3, G0, G4
  • Low-power: L*
  • Wireless: WL, WB

// TODO: change to another STM32 example that matches the macromouse example Example of a choice of MCU: STM32F103CBT6

  • Very common MCU, LQFP-48 package, +3V3 supply
  • Supports USB 2.0 FS natively and SPI
  • Flash: 128 kBytes, RAM: 20 kBytes
  • 72 MHz clock frequency

Power Supply

Check the Voltage and Current requirements

Many different types of DC/DC converters

  • Switching (buck, boost, buck-boost, flyback)
    • High efficiency, higher powers, switching noise, higher part count, harder layout
  • Linear, low-dropout
    • low efficiency, typically lower noise

General rule:

  • Use switchers for digital circuitry
    • Digital has high noise margins, higher current requirements Use linear regulators for analogue circuitry
    • Analogue circuitry far more sensitive, lower current requirements

Switching Regulator

For the digital circuitry power supply

  • Require step-down/buck switching converter
  • Input voltage (+7.5V) is greater than output voltage (+3V3)
  • Current requirements: TODO

Remember: higher switching frequency means smaller components around regulator, which is good (filters components can have a smaller value and thus a smaller size)

You can select parts with extra features: over-current protection, soft start-up, thermal shutdown, small SOT package, …

Passive components