Raspberry Pi breakout board with ESD, PoE and Cortex M0

This is Raspberry Pi GPIO connector breakout board with some additional features:

  • “Real” or “passive” PoE with 12V output
  • 5V DC/DC module
  • Additional 3,3V regulator
  • ESD protection on all GPIO pins
  • two additional pins for each pin on R.Pi 40 pin GPIO connector
  • Separate UART and I2C headers
  • 3,3V Supply for periphery is selectable: from R.Pi or from external regulator
  • STM32F070 or similar in TSSOP-20 housing for controlling additional sensor or any other hardware
  • Jumper for Cortex-M0 / R.Pi UART connection

Top view of the PCB with R.Pi contour

This is revision 1.0 – work in progress and the PCB boards are not yet produced (the order is out). For now I will briefly explain the schematics.

Power supply

Power supply section

The power supply is based on Silvertel Ag9700 PoE module. It is IEEE 802.3af compliant module with 12, 5 or 3V isolated output (1500V barrier) delivering 12W of power when proper PoE injector is used. Input voltage has wide operating range. I bought samples via distribution and the price was about 8 EUR per module.

Ag9700 Block diagram (drawing from Silvertel web page)

If this price is too high, there is “Ghetto” option for “passive” PoE. It simply connects unused twisted pairs to the input of the 5V DC/DC. Unfortunately with such solution there is no galvanic isolation. Two 0 ohm jumpers are provided for this: R3 and R4. They can be replaced with fuses as well.

Ethernet

Few words about ethernet connection:

The cable from the outside is connected directly to the heaqder without the RJ45 cable connector. I did this way for simpler installation in the IP65 plastic or metal housings. I put one RJ45 female plug to connect the R.Pi ethernet through short (5-10cm) patch cable.

Cortex M0

Next section is microcontroller. It has SWD debug header and all pins connected to 100 mils headers. The UART PA2/PA3 can be connected to R.Pi UART GPIO14/GPIO15 via two 0 ohm jumpers R1 and R2:

Microcontroller section

Headers

And finally, there are headers for I2C0, I2C1, 1 wire, UART0 and selecting header for 3,3V supply:

IO port headers and power supply selection

 

The PCB is 56x68mm and slightly covers the display connector of the R.Pi. Other connectors are free.

There are additional pads with +3,3V, +5V and +12V as well. Please check the complete schematic for further details.

Stay tuned for further details about progress (when PCBs arrive).

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