Table of contents

The RFzero is based on the SAMD21G18A micro controller that has six SERCOM interfaces available, each of which can be configured for one of the Serial UART, SPI or I2C protocols. In contrast to the smaller Arduino Uno compatible devices, the USB port is not part of the SERCOM subsystem and can thus be used independently no matter how the configuration is.

If you are just here to the find detailed pinout, you may want to jump directly to the pinout tables or the allocation configuration below.

If you need a more general description of the SERCOM interface of SAMD21 under the Arduino environment, see the Communications page under Tutorials. For detailed information and examples, check out the pages for the Serial UART, the I2C Wire or the SPI tutorials.

You can also keep reading to make yourself an overview of the flexible communication possibilities of the RFzero 🙂

Quick overview

On the RFzero board two of the SERCOM units have already been dedicated to the GPS module link and the local I2C bus for the EEPROM and Si5351A. The four remaining SERCOM ports can be configured for external connection in numerous way according to your communication needs.

The table below gives a quick overview of the default configuration. These are the communication ports that always are available and can be used without further configuration. (Unless, of course, you change the configuration manually as described below.)

Popular alias	Generic name	RFzero use	External pins
SerialGPS	Serial1	GPS serial link	-none-
WireLocal	Wire1	Local I2C bus	-none-
Serial	Serial2	Default UART	TX: A4, RX: A5
Wire	Wire4	Default I2C	SDA: D8, SCL: D9
SPI	SPI5	Default SPI	MISO: D16, MOSI: D18, SCK: D19

Each protocol function (Serial, Wire, SPI) has been given a default SERCOM device and pin allocation. However, this behavior is quite easily overwritten manually in the programming code, see below.

Arduino name	Signal	RFzero pin
Serial (UART)	TX	A4
Serial (UART)	RX	A5
Wire (I2C)	SDA	D8
Wire (I2C)	SCL	D9
SPI	MISO	D16
	MOSI	D18
	SCK	D19

Manual configuration

If you want a communication setup other than the default one in the above table, you need to configure it manually.

The table below shows an overview of the six SERCOM units and the possible configuration options for external communication. The default configuration is highlighted with boldface.

RFzero use	Name (I2C)	Name (UART)	Name (SPI)	SERCOM
GPS link	- not available -	SerialGPS (Serial1)	- not available -	4
Local I2C bus	Wire1 (WireLocal)	- not available -	- not available -	3
Default UART	Wire2	Serial (Serial2)	SPI2	0
(spare)	Wire3	Serial3	SPI3	2
Default I2C	Wire (Wire4)	Serial4	SPI4	1
Default SPI	- not available -	Serial5	SPI (SPI5)	5

These are the available RFzero SERCOM configurations which have been carefully chosen for optimal versatility. It is unlikely you should need to change it unless you are really pushing the limits. However, if you insist and you know how to do it, there is still a few pins left you can switch around. Taking a look at the variant.h and variant.cpp files — as well as the SAMD21 datasheet — will get you started.

See the individual sections on Serial, Wire or SPI for information on how to change the setup.

Serial (UART)

Up to four external serial links may be used at a time –that is, if you don’t need the remaining SERCOMs for other communication purposes, e.g. I2C or SPI. One is always present and the others will have to be configured manually.

The default Serial interface

The default serial port is called Serial and is configure and ready from startup. All you have to do to use it is initialize it with the Serial.begin() function call just like most other Arduino compatible boards. The signals are available on A4 (TXD) and A5 (RXD) as illustrated in the following table.

Serial signal	RFzero pin	RFzero MCU pin	Arduino Zero/M0 pins
TX	A4	PA6	A4
RX	A5	PA7	A5

Below is a simple “Hello World” example using the default Serial port.

// RFzero include and object creation
#include <RFzero.h>               // MUST ALWAYS be included for RFzero

void setup()
{
    // Initialize Serial, also the same as Serial2
    Serial.begin(9600);          // Set the Serial port to 9600 Baud and 8N1 (default)
    Serial.println("Hello World - here is RFzero");
}

void loop()
{
}

// RFzero include and object creation

#include <RFzero.h> // MUST ALWAYS be included for RFzero

void setup()

{

// Initialize Serial, also the same as Serial2

Serial.begin(9600); // Set the Serial port to 9600 Baud and 8N1 (default)

Serial.println("Hello World - here is RFzero");

}

void loop()

{

}

Setting up additional Serial ports

The ports are named from Serial1 to Serial5 as shown in the following table of the port pinouts of which Serial1 is already dedicated to the GPS link. The table below sums up the ports.

Interface name	TX	RX	Device	RFzero note
Serial1 = SerialGPS	N/A	N/A	Sercom4	Dedicated serial connection to the GPS module. Not available outside PCB
Serial2 = Serial	A4	A5	Sercom0	Default
Serial3	D4	D5	Sercom2	Must be initialized manually
Serial4	D8	D9	Sercom1	Must be initialized manually
Serial5	D18	D19	Sercom5	Must be initialized manually (see below)

Please note that Serial1 (the GPS module link) and Serial2 (the default serial port) have been given the aliases SerialGPS and Serial for convenient referencing.

In order to use Serial ports 3, 4 or 5, they will have to be enabled first by declaring the UART in the top of your sketch (.ino) file. This is preferably done right after the inclusion of the RFzero header file. In brief, to enable Serial5, include the following line at the top of your sketch.

// enable UART no 5 (Serial5)
Uart Serial5(&PERIPH_SERIAL5, PIN_SERIAL5_RX, PIN_SERIAL5_TX, PAD_SERIAL5_RX, PAD_SERIAL5_TX) ;
void SERIAL5_IT_HANDLER() { Serial5.IrqHandler(); }

// enable UART no 5 (Serial5)

Uart Serial5(&PERIPH_SERIAL5, PIN_SERIAL5_RX, PIN_SERIAL5_TX, PAD_SERIAL5_RX, PAD_SERIAL5_TX) ;

void SERIAL5_IT_HANDLER() { Serial5.IrqHandler(); }

The above code lines contain a number of macros that have been predefined in the RFzero environment to make it convenient to enable Serial5. Getting the pin numbers and pad assignments of the serial port configurations correct is a little tricky which is why a similar set of macros exists for each serial port.

So, in short, to enable either Serial3, Serial4 or Serial5 instead, simply include the above line, taking care to change all the macros to represent the correct port.

// ** replace # with the desired number 3-5 **
Uart Serial#(&PERIPH_SERIAL#, PIN_SERIAL#_RX, PIN_SERIAL#_TX, PAD_SERIAL#_RX, PAD_SERIAL#_TX) ;
void SERIAL#_IT_HANDLER() { Serial#.IrqHandler(); }

// ** replace # with the desired number 3-5 **

Uart Serial#(&PERIPH_SERIAL#, PIN_SERIAL#_RX, PIN_SERIAL#_TX, PAD_SERIAL#_RX, PAD_SERIAL#_TX) ;

void SERIAL#_IT_HANDLER() { Serial#.IrqHandler(); }

As it is also the case with I2C and SPI, you cannot arbitrarily choose the pins and define your own macros since the SAMD SERCOMs only support a certain set of pin positions. The pinouts chosen above have been chosen for optimal versatility and you really should not need to change it. However, if you insist on meddling and you know how to do it, taking a look at variant.h and variant.cpp –as well as the SAMD21 datasheet– will get you started.

Adding an extra serial port to the an example programs

If you want to add an extra serial port to one of the example programs you must apply the above steps the program. Below is how this is done in the PI4 + CW + carrier example program for Serial5.

In the beginning of the .ino file.

...
// RFzero include and object creation
#include   // MUST ALWAYS be included for RFzero
#include   // Only relevant if CW or a MGM is needed
#include   // Adds some general purpose functions

// Add Serial5
Uart Serial5(&PERIPH_SERIAL5, PIN_SERIAL5_RX, PIN_SERIAL5_TX, PAD_SERIAL5_RX, PAD_SERIAL5_TX) ;
void SERIAL5_IT_HANDLER() {
    Serial5.IrqHandler();
}

// Program includes. Located in the same directory as this .ino file
#include "global.h"
#include "config.h"
#include "commands.h"

...

...

// RFzero include and object creation

#include // MUST ALWAYS be included for RFzero

#include // Only relevant if CW or a MGM is needed

#include // Adds some general purpose functions

// Add Serial5

Uart Serial5(&PERIPH_SERIAL5, PIN_SERIAL5_RX, PIN_SERIAL5_TX, PAD_SERIAL5_RX, PAD_SERIAL5_TX) ;

void SERIAL5_IT_HANDLER() {

Serial5.IrqHandler();

}

// Program includes. Located in the same directory as this .ino file

#include "global.h"

#include "config.h"

#include "commands.h"

...

In the setup() function in the .ino file.

...
    // Setup USB
    SerialUSB.begin(9600);
    delay(2000);  // Delay used instead of "while (!(SerialUSB));" because it halts if not connected to USB. Checks show up to 1 s so 2 s to be on the safe side

    // Setup Serial5
    Serial5.begin(9600);

    // Load configuration
    if (RFzero.eeprom_isUnconfig())
...

...

// Setup USB

SerialUSB.begin(9600);

delay(2000); // Delay used instead of "while (!(SerialUSB));" because it halts if not connected to USB. Checks show up to 1 s so 2 s to be on the safe side

// Setup Serial5

Serial5.begin(9600);

// Load configuration

if (RFzero.eeprom_isUnconfig())

...

In the beginning of all the other .cpp files where the Serial5 port is used.

...
// Program includes. Located in the same directory as the .ino file
#include "global.h"
#include "config.h"
#include "commands.h"

uint8_t configMode = 0;     // Indicates if program is in config mode
uint8_t configChanged = 0;  // Indicates if the configuration has changed

extern Uart Serial5;

void ParseCommand(char *str)
...

...

// Program includes. Located in the same directory as the .ino file

#include "global.h"

#include "config.h"

#include "commands.h"

uint8_t configMode = 0; // Indicates if program is in config mode

uint8_t configChanged = 0; // Indicates if the configuration has changed

extern Uart Serial5;

void ParseCommand(char *str)

...

I2C (Wire)

Default I2C interfaces is called Wire in Arduino and is configured and ready to go from startup. Don’t forget to connect the pull-up resistors from SDA and SDC to the 3,3 V supply. 4,7 kΩ to 10 kΩ will work in most cases.

I2C signal	RFzero Pin/Port	Arduino Zero or M0 pins
SDA	D8/PA16	SDA/PA22
SCL	D9/PA17	SCL/PA23

Please note that if your I2C device(s) use 5 V you will need a 3,3 V to/from 5 V level converter that can be made either by discrete components, ready made board or an IC. Please see the Third party accessories section on the Shop page.

Make sure to check out the examples on the I2C tutorial page. For more information about the I2C bus please see the application report from Texas Instruments: Understanding the I2C Bus.

Setting up additional I2C ports

In total four wire interfaces (I2C) are available of which Wire1 is occupied for the internal bus (WireLocal) and Wire4 already is configured as the default (Wire). However, Wire2 and Wire3 have been prepared for use and can easily be enabled. See the following table for an overview of the RFzero pinouts for each Wire interface. Keep in mind that you can only configure a new I2C port if the corresponding SERCOM unit is not already in use.

Interface name	SDA	SCL	Device	RFzero note
WireLocal (Wire1)	N/A	N/A	SERCOM3	Internal I2C bus on board. Not available outside PCB
Wire2	D0	D1	SERCOM0	Must be initialized manually (see below)
Wire3	D4	D5	SERCOM2	Must be initialized manually
Wire (Wire4)	D8	D9	SERCOM1	Default

For instance, to make Wire2 available, all you have to do is include the following lines at the top of your sketch (preferably right after including the RFzero library).

TwoWire Wire2(&PERIPH_WIRE2, PIN_WIRE2_SDA, PIN_WIRE2_SCL) ;
void WIRE2_IT_HANDLER(void) { Wire2.onService(); }

1 2	TwoWire Wire2(&PERIPH_WIRE2, PIN_WIRE2_SDA, PIN_WIRE2_SCL) ; void WIRE2_IT_HANDLER(void) { Wire2.onService(); }

Here, PERIPH_WIRE2 and the other upper case names are macros that have been predefined in the RFzero variant configuration to make it convenient to automatically assign the correct pins and SERCOM device. Equivalent to the above example, you can initialize Wire3 using the predefined macros:

TwoWire Wire3(&PERIPH_WIRE3, PIN_WIRE3_SDA, PIN_WIRE3_SCL) ;
void WIRE3_IT_HANDLER(void) { Wire3.onService(); }

1 2	TwoWire Wire3(&PERIPH_WIRE3, PIN_WIRE3_SDA, PIN_WIRE3_SCL) ; void WIRE3_IT_HANDLER(void) { Wire3.onService(); }

You cannot arbitrarily choose the pins and define your own macros since the SAMD SERCOMs only support a certain set of pin positions.

SPI

The SPI is a four wire full duplex interface that allows multiple devices, named master and slave(s), to use the same four wires to send and receive data to and from the master or slave. The bus clock can vary from circuit to circuit.

SPI signal	Pin/Port	Arduino Zero/M0 pinout
MISO	D16	4 (SPI connector)
SS	D17	Not assigned
MOSI	D18	1 (SPI connector)
SCK	D19	3 (SPI connector)

The four wires are called Master Out Slave In (MOSI), Master In Slave Out (MISO), Serial ClocK (SCK) and Slave Select (SS). The SS is seldom used if there is only one slave connected to the master.

Setting up additional SPI ports

While SPI5 is the default SPI port, the other ports may be enabled if you are not using the corresponding SERCOM for other purposes. Below is shown the possible pinout allocations.

Interface name	MISO	MOSI	SCK	Device	RFzero note
SPI2	A4	A5	A3	SERCOM0	Must be initialized manually (see below)
SPI3	D6	D4	D5	SERCOM2	Must be initialized manually (see below)
SPI4	D10	D8	D9	SERCOM1	Must be initialized manually
SPI5	D16	D18	D19	SERCOM5	Default

To enable, for instance, SPI2 include the first two lines in the below code box. The lines should by in the top of your sketch file, preferably right under the RFzero.h inclusion.

// include to enable SPI2 on pins A3, A4, A5
SPIClass SPI2 (&PERIPH_SPI2, PIN_SPI2_MISO, PIN_SPI2_SCK, PIN_SPI2_MOSI, PAD_SPI2_TX, PAD_SPI2_RX);

// include to enable SPI3 on pins D4,D5,D6
SPIClass SPI3 (&PERIPH_SPI3, PIN_SPI3_MISO, PIN_SPI3_SCK, PIN_SPI3_MOSI, PAD_SPI3_TX, PAD_SPI3_RX);

// include to enable SPI4 on pins D8,D9,D10
SPIClass SPI4 (&PERIPH_SPI4, PIN_SPI4_MISO, PIN_SPI4_SCK, PIN_SPI4_MOSI, PAD_SPI4_TX, PAD_SPI4_RX);

// include to enable SPI2 on pins A3, A4, A5

SPIClass SPI2 (&PERIPH_SPI2, PIN_SPI2_MISO, PIN_SPI2_SCK, PIN_SPI2_MOSI, PAD_SPI2_TX, PAD_SPI2_RX);

// include to enable SPI3 on pins D4,D5,D6

SPIClass SPI3 (&PERIPH_SPI3, PIN_SPI3_MISO, PIN_SPI3_SCK, PIN_SPI3_MOSI, PAD_SPI3_TX, PAD_SPI3_RX);

// include to enable SPI4 on pins D8,D9,D10

SPIClass SPI4 (&PERIPH_SPI4, PIN_SPI4_MISO, PIN_SPI4_SCK, PIN_SPI4_MOSI, PAD_SPI4_TX, PAD_SPI4_RX);

Similarly, you can enable SPI3 or SPI4 by including the relevant lines from the above list.

While the line to include do have macros that refer to the relevant pin numbers, you cannot arbitrarily choose the pins and define your own macros since the SAMD SERCOMs only support a certain set of pin positions.

SERCOM allocation planning

Since each SERCOM unit can only serve one protocol (Serial, SPI or I2C) at a time, great care has been taken to arrange the RFzero pinout for maximum flexibility. The table below shows possible pin allocations for each SERCOM unit and what other RFzero functions that may overlap in pin assignment.

RFzero name	RFzero number	UART	SPI	I2C	Connector	Other use that may conflict
A3	A3	Serial or Serial2	SPI2		JP4	Analog input, Rotary input
A4	A4	Serial or Serial2	SPI2		JP4	Analog input, Rotary input
A5	A5		SPI2		JP4	Analog input, Rotary input
D0	0			Wire2	JP5/JP6	Open collector driver
D1	1			Wire2	JP5/JP6	Open collector driver

D4	4	Serial3	SPI3	Wire3	JP5/JP6	Open collector driver
D5	5	Serial3	SPI3	Wire3	JP5/JP6	Open collector driver
D6	6		SPI3		JP5/JP6	Open collector driver

D8	8	Serial4	SPI4	Wire4	JP11
D9	9	Serial4	SPI4	Wire4	JP11
D10	10		SPI4		JP11/JP12	HD44780 LCD header

D17	17		SPI5		JP10
D18	18	Serial5	SPI5		JP10
D19	19	Serial5	SPI5		JP10

Note that the RFzero board still contains another 3 analog pins and 9 digital I/O pins that are not shown in the above table which may be used for other purposes without conflicting with your allocation of SERCOM devices.

Detailed pinout reference tables

Below is a compact table that sums up the RFzero pin assignments for each SERCOM configuration option.

SERCOM	Reserved use	I2C name	Serial name	SPI name
0		Wire2 SDA: D0, SCL: D1	Serial (Serial2) TX: A4, RX: A5	SPI SCK: A3, MISO: A4, MOSI: A5
1		Wire (Wire4) SDA: D8, SCL: D9	Serial4 TX: D8, RX: D9	SPI4 MOSI: D8, SCK: D9, MISO: D10
2		Wire3 SDA: D4, SCL: D5	Serial3 TX: D4, RX: D5	SPI3 MOSI: D4, SCK: D5, MISO: D6
3	EEPROM and SI5351A	WireLocal (Wire1) SDA: D30, SCL: D31
4	GPS		SerialGPS (Serial1) TX: D32, RX: D33
5		Does not exists	Serial5 TX: D18, RX: D19	SPI (SPI5) MISO: D16, MOSI: D18, SCK: D19

The below tables detail the pinout for each device configuration along with the intrinsic processor pins. Note that it is not possible to use the same device for different protocols even though the pins do not overlap.

Pinout for Serial2, Wire2 or SPI2

RFzero pin	MCU name/pin	Serial2	SPI2	Wire2
A3	PA05 / 10		SCK
A4	PA06 / 11	TX	MISO
A5	PA07 / 12	RX	MOSI
D0	PA08 / 13			SDA
D1	PA09 / 14			SCL

Pinout for Serial3, Wire3 or SPI3

RFzero pin	MCU name/pin	Serial3	SPI3	Wire3
D4	PA12 / 21	TX	MOSI	SDA
D5	PA13 / 22	RX	SCK	SCL
D6	PA14 / 23		MISO

Pinout for Serial4, Wire4 or SPI4

RFzero pin	MCU name / pin	Serial4	SPI4	Wire4
D8	PA16 / 25	TX	MOSI	SDA
D9	PA17 / 26	RX	SCK	SCL
D10	PA18 / 27		MISO

Pinout for Serial5 or SPI5

RFzero pin	MCU name/pin	Serial5	SPI5
D17	PB03 / 48		SCK
D18	PB22 / 37	TX	MISO
D19	PB23 / 38	RX	MOSI

The USB interface

The Universal Serial Bus (USB) is a two wire interface bus that allows many devices to communicate with each other. The communication speed is typically in the same range as the UART but can go up to 12 Mb/s.

The USB interface is called SerialUSB and with DP on D28/pin 33 and DM on D29/pin 34.

RFzero™ – where it all starts

UART, I2C and SPI