ESP32-S2-Breakout

May 2021: New v2 boards assembled with USB-C connector, tested and working well over USB CDC-ACM and DFU programming.

Feb 2021: Updated to v2, switched from micro usb to usb-c connector, minor routing changes. PCB ordered, updates to come.

July 2020: Board has been assembled, but not fully tested. Currently waiting for ESP-IDF release with full s2 support.

This board is a minimal breakout board for the new ESP32-S2-WROOM module from Espressif. It was designed with a few criteria in mind,

• Breakout as many pins as possible, if not all, from the module
• Be relatively easy for a hobbyist to order & assemble
• Use a minimal amount of components

To that end, a few choices were made,

• Designed in KiCad to be accessible to most people
• Use as little active components as possible
  • Note, it is definitely possible to use less than is currently laid out and the design will (slowly) be iterated on towards this.
• Using 0603 (imperial) sized passives
  • Ideally the board would be constructed using solder paste, a stencil and either some form of reflow oven or hot plate (even a skillet on a stove is fine)
• To have the boards cheaply manufactured by most turn-key pcb fab houses,
  • Power traces should be at least 12 mil
  • Signal traces should be at least 8 mil
  • Vias should be 0.3mm drill size with a 0.6mm pad size
• No LGA parts, and ideally no QFN parts

Initial Design

The initial design was taken from the ESP32-S2-WROOM & ESP32-S2-WROOM-I Datasheet (v0.5) section 6. Peripheral Schematics Figure 5.

A few changes were made from this schematic.

• The USB data lines were directed to a USB-C connector
• 22 ohm termination resistors are used on the USB data lines, along with including the 20pF caps
• No 0 resistor was used on the EN reset switch, just tied directly to the EN pin
• A switch was also used on IO0 for the boot mode, instead of a connector/header
• No separate connectors for UART/JTAG, all pins were broken out to 2.54mm/0.1" headers

Component Selection

1. 3v3 Regulator

A regulator which has high efficiencies (ie, low ground/quiescent current) is needed to get the best out of powering the device from higher voltages. The XC6220 has a hysterisis mode called "Green Operation" where the supply current current is much lower when the load current is below 2mA. The v1 of this board will also be a bit of a test for this regulator to see how it handles higher loads for wifi, and lower loads for sleep modes.

2. 32.768 KHz Crystal

A few resources were useful when researching crystal and component choices.

1. https://en.wikipedia.org/wiki/Pierce_oscillator
2. http://ww1.microchip.com/downloads/en/AppNotes/00943A.pdf
3. http://www.crystek.com/documents/appnotes/Pierce-GateIntroduction.pdf
4. http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/CD00221665.pdf

A cheap crystal FC-135 with a max ESR of 70kOhms was used which fits on the KiCad 3215 pad. A few passive values will need to be tested for v1 to determine if they're even necessary.

3. Switches / Connectors

The switches and connectors were chosen because they had footprints already in the KiCad library and are relatively accessible from most vendors.

4. USB ESD Protection

A USBLC6-2SC6 was chosen for ESD protection of the USB power and data lines. This part was already available in KiCad and at a few distributors. An input capacitor was used fort this part, along with a ferrite bead before the 5v bus.

Another ferrite bead and capacitor was used between the shield and the ground. Probably not strictly necessary.