STM32 - Designing an STM32L0xx board

Oliv'

After the STM32L151, I am now designing a board for STM32L052. Following are some notes about ST application notes I read to design an board.

All documents can be found here.
Datasheet, Reference manual, Errata sheet.

Hardware development

AN4467 - Getting started with STM32L0xx hardware development

Hardware

  • Voltage range, p6
    • Full speed (Range 1) : 2.0 to 3.6 VDC
    • USB : 3.0 to 3.6 VDC
  • Power supply capacitors, p11
  • 100 nF by VDD pin
  • Min 4.7µf for the chip, 10µF recommended
  • 100 nF by VDDA pin
  • 1 µF for the chip, or 10 µF if high sampling rate ADC
  • Reset circuit, p16
  • No component mandatory
  • 10 nF to 100 nF pull-down can be added as EMS protection
  • Clock, p17
  • No external required
  • Better to provide a 32.768 kHz quartz for RTC accuracy
  • Recommended load capacitance for HSE is 2 to 7pF
  • SWD port, p25
  • Signals required are: NRST, SWDIO, SWCLK, GND

Debug

  • No JTAG port
  • SWD: SerialWire Debug

Boot

  • See AN2606
  • Boot configuration
    • BOOT1 bit in user option byte, set to 0 by default
    • BOOT0 pin, inline 10 kOhm resistor recommended
  • Boot selection (BOOT1 - BOOT0)
    • x-0: Flash memory
    • 0-1: System memory (ST bootloader)
    • 1-1: SRAM memory
  • ST bootloader on
    • USART1, USART2 (pulls-up required on TX and RX)
    • SPI1, SPI2 (pull-down required on CLK)
  • No DFU
  • All pins are in floating input state during reset => Do not forget a weak pull for components like MOSFET…

Oscillator design guide

AN2867 - Oscillator design guide

  • §4.2, p23 - Detailed steps to select an STM32-compatible crystal
  • §5, p26 - Some recommended resonators for STM32 microcontrollers
  • §7.1, p30 - PCB design guidelines

Touch sensing

  • AN3960 - ESD considerations
  • Most used method is to add an inline 50 Ohm
  • AN4312 - Guidelines for designing touch sensing applications
  • If a LED is placed closed to touch interface, use 10 nF bypass capacitor (p15)
  • If possible use FT I/O instead of TT as they are clamped to VDD, or use Shottky diode with capacitance <5 pf (p15)
  • Recommended to use the same shape for all electrodes (p18)
  • Recommended electrode size is 4 times panel thickness

USB

AN4879 - USB hardware and PCB guidelines

  • Signal lines
  • 90 ohm +/-15% differential
  • ESD protection circuits like ST USBLC6 are highly recommended

Let’s put it into practice

An open-source dev. board I designed can be found here.

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