Finally, we finished the design of FST-01.
Components of FST-01
- SPI Flash memory (32Mbit)
- USB Terminator
- LDO Regulator (3.3V)
- 12MHz XTAL
- SWD Port of GND/SWDIO/SWDCLK
- Extension port of VDD/GND/Port0/Port1
- USB plug A
MCU is one of Cortex-M3 based ARM by STMicroelectronics. It runs at 72MHz and has 128KB flash ROM and 20KB RAM.
SPI Flash memory
Secret data should be on MCU's flash, but non confidential data (such as public keyring) can be on SPI flash memory.
We use SST25VF032B (32Mbit = 4MB).
We use NUF2221W1 USB Upstream Terminator.
This is because it has the feature of electrostatic discharge protection It is also better for smaller footprint than two capacitors and two resistors.
We use CAT6217-330 for 3.3V Regulator (from 5V). Currency of 150mA is enough.
We use 12MHz XTAL.
This is because we can find smaller footprint XTAL for 12MHz (than 8MHz).
We use a LED to show status.
SWD port is to update MCU's flash ROM.
Extension port of VDD/GND/Port0/Port1
We decided to have two I/O pins. (One I/O pin is not enough even for minimalist.)
With two I/O pins, it can be:
- TX and RX of serial communication
- Drive and input of touch sensor
- controlling other MCU's SWD port
USB plug A
FST-01 usage is just connect it to USB port, like USB thumb drive.