Minimum PCB Design for the Particle P1

The Particle P1 is an electronics development platform consisting of two major parts: a Broadcom WiFi chip and a 32-bit ARM Cortex M3 microcontroller. The module is designed to speed and simplify the transition from prototype to production consumer product. The module communicates with the outside world using the Particle Cloud platform, which allows events and functions to be linked remotely between the Particle P1 and other sensors, online services, or smartphone apps.

On the prototyping side, Particle provides several development boards that house both their microcontroller modules and all of the various peripheral components necessary to get the modules up and running; the most well-known of Particle’s development solutions is the Particle Photon. This board is powered by the Particle P0 and it also includes all the peripheral components, power regulation systems, a USB connector, and a breadboard-friendly form-factor with all of the microcontroller’s GPIO pins broken out. The Particle Photon is a powerful solution for prototyping all kinds of WiFi-connected products.

When it comes time to move from breadboard prototypes to a bespoke PCB for your project, you will need to design your own power system, connections with sensors, the mandatory setup, reset, and status components, and other peripherals needed to make your project function. It can be a lot of work.

In order to make the transition smoother for you and your team, this post provides the minimum PCB design for the Particle P1, along with some information about how it all works. The idea is that you can download the design files from this post (provided in Autodesk Eagle .sch format) and then modify or add to the design as needed. The Particle P1 datasheet provides all the information you will need to start designing around the P1, but this post provides design files that incorporate all the information in the datasheet into a single, clean package.

Particle P1 Minimum PCB Design

The minimum schematic design linked below incorporates all the various parts that will be required to make the Particle P1 operate, along with some optional systems that may or may not be useful depending upon the nature of your project. Below, more information is provided about the various subsystems on the schematic.

Schematic: User I/O

The first major subsystem on the schematic is for user input/output. This subsystem consists of several components: a setup (or mode) button, a reset button, a status LED, and, optionally, a power LED. The first of these, the setup button, allows the user to place the Particle P1 into one of several operational modes including the mode in which the Particle P1 can be connected to WiFi via the Particle app or changed to a different WiFi network than the one to which the module was initially connected, this is called Listening Mode. The schematic for the setup button is as simple as it gets, just a button connected to GPIO pin 8 and ground.

The minimum schematic design for the Particle P1 module includes a second button, the reset button. The reset button can be used to restart the firmware running on the Particle P1 in the event that the module experiences some kind of issue. The reset button and setup button can also be used together to place the Particle P1 into some of its more advanced modes like safe mode and DFU mode.

The schematic design for the reset button is slightly more complex than the one for the setup button. The reset button connects between the MICRO_RST pin on the Particle P1 and ground. A pull-up resistor is used to pull the signal to HIGH while the button is not pressed. Finally, a capacitor across the button reduces “bouncing.”

Aside from the buttons which allow user input, two LED systems provide visual feedback to the user. The first, the Status LED, is a mandatory part of the minimum Particle P1 schematic, the other, a power LED, is optional.

The Status LED is an RGB LED used to indicate the mode the P1 is currently in. The LED turns different colors, and flashes with different patterns, depending upon the current mode. Particle provides a reference for all the device modes as part of their documentation. The type of LED used in the schematic shown on this page is the same as the one used on the Particle Photon board, a Cree, Inc. CLMVB-FKA-CFHEHLCBB7a363 RGB LED. The LED connects to three pins on the Particle P, pins 29, 31, and 32. On each pin, a 1k resistor is used for limiting the current to the RGB LED.

The final part of the user I/O system is a power LED. This LED is optional. Depending upon the nature of your project, you may or may not need to indicate that power is being supplied to the system. If you choose to incorporate this LED into your design, the schematic is simple. The LED connects to GPIO pin 7 with a current-limiting resistor.

Schematic: USB

The next subsystem in the minimum design for the P1 is the USB connection. This entire subsystem is optional. Including a USB connection in the schematic gives the user an easy way to upload firmware to the P1 without using the Particle Cloud. The USB connection can also be used as a way to power the system.

If you are working on a consumer product, the USB connection might be useful for prototyping. The USB connection can serve an extremely useful troubleshooting and diagnostics function because with it you can interface your P1-powered device with a computer and communicate with the P1 using the Particle CLI tool. However, you will likely not want to include the USB connection in the final version of your product, otherwise end users could reprogram your board.

Schematic: JTAG

There are several different ways to program the Particle P1. As discussed in the previous section, firmware can be flashed to the board over USB. One of the most powerful features of the Particle platform in general is the ability to flash firmware over the air via Particle’s online IDE. Yet another way to program the Particle P1, or even to modify the bootloader, is by using an interface called JTAG.

JTAG is especially important if you are working on a product that will be mass produced. JTAG is a common interface used by electronics manufactures to flash firmware on the assembly line and to test PCBs. Therefore, for consumer products, it is good practice to include a JTAG interface, although it is technically not necessary as there are other ways to program your boards.

The JTAG interface itself consists of an industry-standard 2×10 shrouded connector. The interface connects to GPIO pins 3-7 on the P1, and also to the reset pin.

Schematic:  P1 Passive Components

There are a number of passive components and other nets needed to make the Particle P1 operate correctly. First, there are several bypass capacitors used to reduce electrical noise in the system. In order to maximize their effectiveness, these bypass capacitors should be placed as close as possible to the power input pins on the Particle P1. Next, the Particle P1 module has a number of ground connections that must all be connected, ultimately, to the ground from the power supply for your board. In the schematic image below, these connections are on the top of the P1 symbol.

Finally, there a number of GPIO pins on the board that are not yet connected to any of the existing systems. These pins have been labeled in the schematic so you can use them for other systems, sensors, or outputs.

Schematic:  Power

The last part of the minimum schematic design for the Particle P1 is the power input. The Particle P1 is powered by a 3.3V DC input. There are, of course, numerous potential power options for the P1. The minimum schematic simply shows a 3.3V and GND external connection. A second sheet in the schematic shows a number of potential power input options. These are far from exhaustive, but they should provide a starting point for most power sources you might wish to use.

Power from two AA or AAA Batteries

One way to power the Particle P1 is with a pair of AA or AAA batteries. When they are fully charged, AA or AAA batteries each output 1.5V. Therefore, a pair will output 3V. In order to bring this voltage up to the required 3.3V, a boost regulator is necessary. The boost regulator will also need to increase the voltage from the batteries as they get closer to being depleted. When they are close to being fully dead, AA or AAA batteries output closer to 1.2V.

The example schematic uses an NCP1402 DC/DC converter by ON Semiconductor. This DC/DC converter is specifically designed for portable systems powered by batteries. The DC/DC converter requires only a few external components to operate. The NCP1402 will output 3.3V to power the Particle P1.

Power from a 5V DC Supply

It would certainly be possible to power your Particle P1 project directly with a 3.3V wall adapter, however adapters in this voltage are uncommon. Wall adapters with 5V outputs are readily available though because so many other electronics development platforms operate on 5V, along with USB devices of all kinds.

Converting a 5V input from a wall adapter to 3.3V can be done with a simple linear voltage regulator. The Particle P1 operates with a typical 80mA current consumption. WIth a voltage step-down of 1.7V, a linear voltage regulator will only dissipate around 0.136 watts. This power will not produce any damaging heat for the regulator.

Power from 12V or Higher Supply

If you plan to use a power supply of 12V or greater, a linear voltage regulator will probably not be your best option. Linear regulators convert voltage by dumping the excess voltage as waste heat. Therefore, the bigger the difference between the input voltage and 3.3V, the more heat the regulator will generate.

With an input voltage of 12V, a linear regulator will be dissipating around 0.7 watts. This is enough to make smaller regulator packages dangerously hot. Higher supply voltages could either burn out a linear voltage regulator, or cause its output to become unstable.

The solution is to use a switching voltage regulator. Switching regulators are significantly more expensive than linear voltage regulators, but they are also highly efficient. This means that switching regulators are capable of dropping the supply voltage much farther without damaging heat buildups. Furthermore, switching regulators are available in plug-and-play modules meaning they are just as easy to use as linear regulators.

In this example, a RECOM power R-78E-1.0 is used to convert high(ish) voltages down to 3.3V while minimizing waste heat. The R-78E-1.0 can source up to 1A which is sufficient to supply the Particle P1 along with a host of sensors or outputs. The schematic looks quite a bit like the one above with a linear voltage regulator because this type of switching regulator module can be used as a drop in replacement to most linear regulators.

Power from a Rechargeable 3.7V LiPo Battery

Many consumer products today are powered by internal, rechargeable batteries. The example schematic in this section incorporates a circuit that consists of two power sources. The Particle P1 will be powered by a 3.7V LiPo battery while the device is not connected to a power source (LiPo batteries are available in a vast range of sizes and capacities). When the device is connected to a 3.75 to 6V power source (most commonly a 5V USB connection), the Particle P1 will run off this power source while, at the same time, the LiPo battery charges. I other words, your device can run and charge the battery simultaneously. This design is more complex than the other power input designs listed above.

The component that determines which power source the Particle P1 will use is Q1, a P-channel MOSFET. When USB power is applied to Vin, the MOSFET turns off, effectively disconnecting the battery from the P1. The P1 is then powered directly from the power input.

The MCP7381 is a LiPo charge management controller. The chip is powered by Vin and, while power is connected to the system, the controller handles charging the battery. An LED is used to provide visual feedback to the user. So, when your device is connected to USB, or another power source, the Particle P1 will be powered by that source, while the MCP7381 charges the battery from that source at the same time.

When power is not connected to Vin, the MOSFET turns on, which connects the battery to the Particle P1 in order to provide it with power.

Regardless of whether the P1 is powered from Vin or from the battery, a linear voltage regulator is used to bring the voltage down to 3.3V. This part of the circuit is the same as the design used for the 5V DC input above.