Photon 2 from Photon migration guide

The Photon 2 is a development module with a microcontroller and Wi-Fi networking. The form-factor is similar to the Argon (Adafruit Feather), but the Photon 2 supports 2.4 GHz and 5 GHz Wi-Fi, BLE, and has much larger RAM and flash that can support larger applications.

It is intended to replace both the Photon and Argon modules. It contains the same module as the P2, making it easier to migrate from a pin-based development module to a SMD mass-production module if desired.

¹A small amount of the flash file system is used by Device OS, most is available for user data storage using the POSIX filesystem API. This is separate from the flash memory used for Device OS, user application, and OTA transfers.

² Total RAM; amount available to user applications is smaller. On the Photon 2, available RAM is approximately 3072 KB. On the Photon, it is 55 KB.

³ Total built-in flash; amount available to user applications is smaller. The Argon also has a 4 MB external flash, a portion of which is available to user applications as a flash file system.

⁴ Onboard or external antenna is selectable in software.

There are two Photon 2 migration guides, depending on what you are migrating from:

• From Photon
• From Argon

Hardware

Battery support

The Photon 2 has a connector for a Li-Po battery and built-in charger. The Photon does not include battery power functionality.

No 5V tolerance!

On Gen 2 devices (STM32F205), most pins are 5V tolerant. This is not the case for Gen 3 (nRF52840) and the Photon 2 (RTL872x). You must not exceed 3.3V on any GPIO pin, including ports such as serial, I2C, and SPI.

SPI

Both the Photon and Photon 2 have two SPI ports, however the pins are different for primary SPI, which is on the S pins on the Photon 2 instead of the A pins on the Photon.

SPI1 on the D pins is the same between the Photon and Photon 2.

If you are using SPI, Device OS 5.3.1 or later is recommended. Prior to that version, SPI ran at half of the set speed, and SPI1 ran at double the set speed. Timing has also been improved for large DMA transfers; prior to 5.3.1, there could be 1 µs gaps for every 16 bytes of data transferred.

SPI - Gen 2 devices (including Photon)

• Available clock divisors: 2, 4, 8, 16, 32, 64, 128, 256

SPI - Photon 2

I2C

Both the Photon and Photon 2 have a single I2C port on the same pins, D0/D1.

• The Photon 2 I2C port is not 5V tolerant
• The Photon 2 A3/A4 pins are physically in the same location as D0/D1.
• The Photon pins A3/A4 are D16/D15 on the Photon 2.
• On the P2 and Photon 2, the only valid I2C clock speeds are CLOCK_SPEED_100KHZ and CLOCK_SPEED_400KHZ. Other speeds are not supported at this time.

Serial (UART)

The primary UART serial (Serial1) is on the TX and RX pins on both the Photon and Photon 2. There is no hardware flow control on this port on the Photon or Photon 2.

The secondary UART serial (Serial2) is on different pins, however it does not conflict with the RGB LED, and also supports CTS/RTS hardware flow control.

¹CTS/RTS flow control only on Serial2. It is optional.

Supported Baud Rates:

Analog input (ADC)

For analog to digital conversion (ADC) using analogRead(), there are fewer ADC inputs on the Photon 2:

On the Photon 2, there are no pins A3 (hardware pin 21) and A4 (hardware pin 22); these are NC (no connection). However, Photon 2 pin D0 (hardware pin 36) can be used as an analog input and has the alias A3. The same is true for Photon 2 pin D1 (hardware pin 35), which has the alias A4.

The setADCSampleTime() function is not supported on the Photon 2 or P2.

PWM (Pulse-width modulation)

The pins that support PWM are different on the Photon and Photon 2.

All available PWM pins on the Photon 2 share a single timer. This means that they must all share a single frequency, but can have different duty cycles.

Digital to analog converter (DAC)

The Photon supports DAC one A3 and A6 (DAC). There is no DAC on the Photon 2 or Gen 3 devices.

If you need a DAC, it's easy to add one via I2C or SPI on your base board.

WKP (A7)

On Gen 2 devices (STM32), only the WKP pin can wake from HIBERNATE sleep mode.

This restriction does not exist on the Photon 2 and Gen 3 devices; any pin can be used to wake from all sleep modes.

CAN (controller area network)

The Photon supports CAN on pins D1 and D2. There is no CAN on the Photon 2 or Gen 3 devices (except the Tracker).

• The Tracker SoM includes CAN via a MCP25625 CAN interface with integrated transceiver.
• Both the MCP2515 and MCP25625 work with the library used on the Tracker and can be used to add CAN to the Photon 2.

I2S (Sound)

The Photon theoretically had I2S sound available on pins D1 and D2, however there has never been support for it in Device OS.

There is no software support for I2S on the Photon 2 either, and while the RTL872x hardware supports I2S, the pins that it requires are in use by other ports.

BLE (Bluetooth LE)

BLE Central Mode on the P2 and Photon 2 is only supported in Device OS 5.1.0 and later. Earlier versions only supported BLE Peripheral Mode.

Boot mode pins

These pins have a special function at boot. Beware when using these pins as input as they can trigger special modes in the MCU.

Interrupts

There are many limitations for interrupts on the STM32F205. All pins can be used for interrupts on Gen 3 devices and the Photon 2.

Sleep

The Photon 2 can wake from STOP or ULTRA_LOW_POWER sleep mode on any GPIO, RISING, FALLING, or CHANGE.

The Photon 2 can only wake from HIBERNATE sleep mode on pin D10, RISING, FALLING, or CHANGE. The Photon can only wake from HIBERNATE on WKP RISING so this should not be an issue, other than making sure the pins are mapped appropriately.

Internal pull-up or pull-down

Internal (MCU) pull-up and pull-down can be enabled using the pinMode() function and INPUT_PULLUP or INPUT_PULLDOWN.

• On the Photon 2, the internal pull is approximately 2.1K.
• On the Photon (Gen 2), the internal pull is approximately 40K.

Retained memory

The P2 and Photon 2 have limited support for retained memory, also referred to as Backup RAM or SRAM, in Device OS 5.3.1 and later.

Retained memory is preserved with the following limitations:

• When entering HIBERNATE sleep mode.
• Under programmatic reset, such as System.reset() and OTA firmware upgrades.
• In limited cases when using pin reset (RESET button or externally triggered reset).

By default, the retained memory is saved every 10 seconds, so changes made to retained variables between the last save and an unplanned system reset will be lost. Calling System.backupRamSync on the P2 and Photon 2 can make sure the data is saved. The data is saved to a dedicated flash page in the RTL827x MCU however you should avoid saving the data extremely frequently as it is slower than RAM and will cause flash wear.

Prior to Device OS 5.3.1, retained memory is not supported. The flash file system can be used, or you can use an external chip such as an I2C or SPI FRAM.

Retained memory is 3068 bytes.

Photon Bottom

The Photon 2 has components on both sides of the board, like the Argon. It is not available without the mounted headers, and cannot be reflowed directly to a base board like the Photon without headers.

The Photon 2 does not have the solder pads for the RGB LED and SETUP/MODE button on the bottom. The RGB LED can be directed in software to other pins on the Photon 2. The SETUP/MODE button is available on the header pins on the Photon 2.

Classic adapter

The Particle classic adapter can be used to plug a Photon 2 into a socket that is intended to support an Electron. It can also fit in a Photon socket, however pins will hang past the socket, so there must not be anything in the way, or anything that would short the overhanging pins.

There are many pin limitations, and in particular the classic adapter does not work if you need to use SPI.

Pins B0 - B5 and C0 - C5 are not available if plugging into a Photon socket, as these pins are the pins that hang over the edge and exist only on the Electron, not the Photon

Full module pin comparison

3V3

A0

A1

A2

A3

A4

A5

D0

D1

D2

D3

D4

D5

D6

D7

DAC

EN

GND

LI+

MISO

MODE

MOSI

RGBB

RGBG

RGBR

RST

RX

S3

S4

SCK

SETUP

TX

USBDATA-

USBDATA+

VBAT

VIN

VUSB

WKP

Software

Wi-Fi Configuration

The Photon 2 and Argon utilize BLE for configuration of Wi-Fi rather than the SoftAP approach taken with the P1. Using BLE allow mobile apps to more easily set up the device Wi-Fi without having to modify the mobile device's network configuration.

Sample applications for React Native, iOS, and Android will be provided in the future.

User firmware binary size

One major advantage of the Photon 2 is that user firmware binaries can be up to 2048 Kbytes, instead of 128 Kbytes on Gen 2 devices including the Photon.

Flash file system

On the Photon 2, P2, and Gen 3 devices, there is a flash file system (2 MB) for storing user data. This is not available on Gen 2 devices including the Photon.

Combined and resumable OTA

On the Photon 2, P2, and Gen 3 devices, over-the-air (OTA) updates have two features that can improve the speed and reliability of OTA updates:

• Combined OTA can combine Device OS and user firmware updates into a single binary that requires only one download and one reboot to install.
• Resumable OTA allows an update to resume from the point it stopped, instead of starting over from the beginning if interrupted.

Increased API field limits

The maximum size of a variable, function parameter, or publish is 1024 bytes on the Photon 2 vs. 864 bytes on Photon.

Platform ID

The Platform ID of the Photon 2 will different from that of the Photon (6) because of the vastly different hardware.

If you have a product based on the Photon, you will need to create a separate product for devices using the Photon 2. While you may be able to use the same source code to build your application, the firmware binaries uploaded to the console will be different, so they need to be separate products. This generally does not affect billing as only the number of devices, not the number of products, is counted toward your plan limits.

Third-party libraries

Most third-party libraries are believed to be compatible. The exceptions include:

• Libraries for MCU-specific features (such as ADC DMA)
• Libraries that are hardcoded to support only certain platforms by their PLATFORM_ID
• Libraries that manipulate GPIO at high speeds or are timing-dependent

DS18B20 (1-Wire temperature sensor)

• Not compatible
• OneWire library requires high-speed GPIO support
• Can use DS2482 I2C to 1-Wire bridge chip instead
• SHT30 sensors (I2C) may be an alternative in some applications

FastLED

• Not compatible.
• In theory the library could be modified to use the same technique as the NeoPixel library.

NeoPixel (WS2812, WS2812B, and WS2813)

• Requires Device OS 5.3.2 or later and Particle-NeoPixel version 1.0.3.

OneWire

• Not compatible
• OneWire library requires high-speed GPIO support
• Can use DS2482 I2C to OneWire bridge instead

DHT22 and DHT11 (temperature and humidity sensor)

• Not compatible, requires high-speed GPIO support
• Using an I2C temperature and humidity sensor like the SHT3x is recommended instead

SHT1x (temperature and humidity sensor)

• Not compatible, requires high-speed GPIO support
• SHT3x using I2C is recommended

SparkIntervalTimer

• Not compatible at this time
• Requires hardware timer support from user firmware

Version history