Photon 2 from Argon 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 Argon, it is 80 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

Antennas

The Argon requires an external Wi-Fi antenna, and has a built-in chip antenna for BLE. It can optionally use an external chip antenna.

The Photon 2 has a built-in trace antenna that is shared by Wi-Fi and BLE. It can optionally use an external 2.4 GHz antenna for both Wi-Fi and BLE.

Pin names

Some pins in the same positions are named differently between the Argon and Photon 2:

Additionally, D pin aliases D8 and higher are different, however these names are rarely used.

SPI

Pins for SPI are unchanged between the Argon and Photon 2.

The the pin functions for SPI1 on the D pins are different between the Argon and Photon 2! While they both use D2 - D4, the order of SCK, MOSI, and MISO are different so you cannot plug a Photon 2 into an Argon base board that uses SPI1.

Most boards, including Ethernet, use primary SPI, which works the same between the Argon and Photon 2.

• Any available GPIO can be used for SS/CS (chip select) pins.
• Each SPI device must have a unique CS pin.
• The Argon supports SPI slave mode only on SPI1 (D pins).

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 3 devices (including Argon)

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

SPI - Photon 2

I2C

• 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 2 and Argon. There is no hardware flow control on this port on the Photon 2, but there is on the Argon.

The secondary UART serial (Serial2) exists on the Photon 2 but not the Argon, and also supports CTS/RTS hardware flow control. This is recommended if you need serial with hardware flow control on the Photon 2.

There is a third UART serial (Serial3) on the Photon 2 that also supports optional CTS/RTS hardware flow control.

¹CTS/RTS flow control only on Serial2 and Serial3. It is optional.

²On the Argon, the buffer be resized larger in Device OS 3.2.0 and later.

Supported Baud Rates:

Analog input (ADC)

For analog to digital conversion (ADC) using analogRead().

• Pin A0, A1, A2, and A5 are analog inputs on both the Argon and Photon 2.
• Pins A3 and A4 are only analog inputs on the Argon.
• Pins D0 and D1 can also be used as analog inputs on the Photon 2.
• 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 Argon 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.

CAN (controller area network)

Neither the Argon nor the Photon 2 support CAN.

• 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.

PDM

Pulse density modulation digital microphones can be used with the Microphone_PDM library and the P2, but only on specific pins. The Argon and Boron can use any pins for PDM (with the same library).

I2S (Sound)

The Argon supports I2S (sound) input and output with a third-party library.

There is no software support for I2S on the Photon 2, 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.

Battery and charge pins

The Photon 2 does not have a fuel gauge chip, however you can determine the voltage of the LiPo battery, if present. The P2 does not include a LiPo battery connector, but if you connect your battery to VBAT_MEAS, this technique also works with the P2.

float voltage = analogRead(A6) / 819.2;

The constant is from the ADC range (0 - 4095) mapped to the voltage from 0 - 5 VDC (the maximum supported on VBAT_MEAS).

The current Photon 2 hardware is unable to reliably read the CHG pin. In particular, if you have a Photon 2 powered by both USB and battery and is charging, then unplug the USB power, the charge LED will turn off, but the CHG pin will not change state.

Interrupts

All pins can be used for interrupts on Gen 3 devices and the Photon 2.

There is a limit of 8 pin interrupts on the Argon; this limitation does not exist on 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 certain pins, RISING, FALLING, or CHANGE.

Pin D10 WKP is the same module pin location as the Argon pin D8, which is also the WKP pin.

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 Argon (Gen 3), the internal pull is approximately 13K.

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 on RTL872x devices in the following cases:

Calling System.backupRamSync() will manually save the contents of retained memory to a dedicated flash page on the RTL872x processor and will be restored after the device is reset. You should avoid saving the data extremely frequently as it is slower than RAM and will cause flash wear and is relatively slow to execute.

Prior to Device OS 5.3.1, retained memory is not supported on RTL872x devices. 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.

NFC tag

The Photon 2 does not have NFC Tag support. The Argon does.

Full module pin comparison

RST

3V3

MODE

GND

A0

A1

A2

A3 / A5

A4 / S4

A5 / S3

SCK

MOSI

MISO

RX

TX

D0

D1

D2

D3

D4

D5

D6

D7

D8 / D10

VUSB

EN

LI+

Software

Wi-Fi Configuration

The Photon 2 and Argon utilize BLE for configuration of Wi-Fi. Using BLE allow mobile apps to more easily set up the device Wi-Fi without having to modify the mobile device's network configuration. A React Native reference will be provided to simplify Wi-Fi setup.

Neither the Photon 2 nor Argon use the Wi-Fi based setup (SoftAP) that is used on the Photon and P1.

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

BLE (Bluetooth LE)

• BLE long-range (coded PHY) is not supported on the Photon 2. It is on the Argon with Device OS 3.1 or later.

Platform ID

The Platform ID of the Photon 2 will different from that of the Argon (12) because of the vastly different hardware.

If you have a product based on the Argon, 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