Electronics Design #

Intro #

In this page, the process of designing a PCB board would be documented. This would be separated into three parts.

Requirements
Schematic Design
PCB Design

Requirements #

The first thing to do for designing a board is to do the requirements analysis. As the goal is to design a bluetooth extension board for a keyboard, the following requirements would be fulfilled:

Requirements	Possible Solutions
Bluetooth connectivity	XIAO nRF52840
Receive inputs & power for the keyboard	USB Type-C port
Inputs for the board	Switch
LED	NeoPixel THT
Display	SSD1306 OLED
Serial / Battery	2*2 PinHeader
QMK / ZMK firmware support	?

After the requirements and their possible solutions are listed, we can go over some part for more information.

XIAO nRF52840 #

There are two ICs that supports wireless communication in the library:

XIAO ESP32C3 which supports WiFi and Bluetooth 5 (LE)
XIAO nRF52840 which supports Bluetooth 5.0/BLE and NFC

Since we only concerned about bluetooth connectivity, the nRF52840 was chosen for this board because it’s lower energy consumption compared to the ESP32, which might beneficial since we are doing a battery powered project.

USB Type-C port #

To enable communication between the board and the keyboard, as well as providing the power for the keyboard to work, a Type-C port shall be added to the board.

This port might be deleted in the final version since I found that the keyboard that I wish to connect to the board have a daughter board, some this board could potentially replace it, thus no dedicated Type-C port would be needed.

Display #

There are multiple needs for a having a display:

Battery info
Keyboard status
Bluetooth connection info

Meanwhile, three displays are available according to the Fab Lab Inventory:

20X4 LCD Display LCD Screen I2C Serial
0.96 Inch OLED Display Module, I2C Serial 128 x 64 with SSD1306
1.3inch IPS 65K Full Color 3.3V 240x240 LCD Screen with SPI Interface ST7789 IC Driver

The OLED display with SSD1306 driver was chosen because of it’s higher resolution, better readability, and the most reasonable size.

Schematic Design #

This is the complete schematic design of the first version for the board. Modular design was implemented to make a cleaner layout.

schematic

IC #

The pin layout for the XIAO nRF52840 board is showed below.

As D4 to D10 all have dedicated functions, D0 to D3 was utilized for this board design.

USB Type-C #

To design the USB Type-C properly, we need to first know the functions of each pin in the port.

pins

This article provided an in-depth introduction for each of the pins and how to use them. Guide to USB-C Pinout and Features

Schematic of the USB-C port on the XIAO board

For the USB Type-C port, I just copied the design on the XIAO board, since both of them are using only the USB 2.0 protocol. In this case, only the d+ and d- pin would serve as the data transmitting port that need to connect to the IC. For the rest of the pins, I just followed the reference design with proper power connection.

PinHeaders and Switch #

There’s nothing much to say about the design for those components.

The TX and RX pins are connected to the corresponding pins on the IC board. The 2*2 PinHeaders would probably also serves as the power input, especially for the 5V input that was coming from the battery.

Noted that no pull-up resisters needed for the button since they are embedded inside the IC.

NeoPixel LED #

A NeoPixel LED was added to the board as the indicator. There’s no resistor since the NeoPixel is using 5V power.

The DIN and DOUT pins are used for controlling the LED, the DOUT would be especially useful when controlling a series of LEDs, but since here we only use one of them, it was just left empty.

OLED screen #

I did not find the nor the schematic or the footprint for the OLED display in the fab library, thus I used another library from github.

The connection are also fairly simple. The SCL and SDA pins were connected with the corresponded pin on the IC board, similar to the pin headers.

PCB design #

Multiple iterations for PCB design have been made in this process.

The biggest change within this process is that I gave up on the copper pour idea since I found that it was actually not necessary for this board design, and adding it was more trouble than it’s worth.

So, in this way, the PCB design can be separated into two different versions.

Version 1: with copper pour #

Despite being messy, there are multiple problems in this version.

Pins on the 2*2 headers are connected with the wrong pins on the IC board.
Copper fill was added in order to get the GND to different parts of the board, but in some areas it was too close or have overlays with the wires, which will cause unwanted connections.
The USB Type-C port was not placed on the edge of the PCB as it should, which might cause the USB plugs unable to be inserted.

Version 2: without copper pour #

After getting rid of the copper pour and adjusting the placement as well as the orientation of multiple components, the second version of the PCB was made. It was both cleaner and smaller than the original one, which is a good thing since we might have limited space in the real working scenarios.

3D render looks neat. Pretty satisfied with this board design.

References #