Default Pin/VPin Allocations and Recommendations

On this page, we aim to outline and summarise the various default and recommended pins (and vpins for I/O expanders) for use with various input and output devices for our primary supported microcontrollers and I/O devices and expanders. We will also include the default I²C addresses where appropriate.

Microcontrollers 

This section outlines pins to avoid, pins that are free to use, and pins that are to be used with caution for each of our primary supported microcontrollers. The pins are grouped as such:

Arduino Mega2560 

Table 4 Mega2560 Pins to avoid
Pin(s)	Details
A0 - A1	Analogue I/O pins used for main and program track current sensing
3 - 4	Digital I/O pins commonly used by motor shields or Ethernet shields
10 - 13	Digial I/O pins commonly used by motor shields
50 - 53	Digital I/O pins reserved for special system utilities

Table 5 Mega2560 Pins to use
Pin(s)	Details
SDA/SCL (20/21)	I²C connection for use with I²C displays, I/O expanders, etc., not for general I/O
SDA/SCL (near USB)	I²C connection for use with I²C displays, I/O expanders, etc., not for general I/O
Tx1/Rx1/Tx2/Rx2/Tx3/Rx3	Additional serial ports available for use, not for general I/O
A8 - A15	Analogue I/O pins available for use
2	Digital I/O pin available for use
22 - 49	Digital I/O pins available for use

Table 6 Mega2560 Pins to use with caution
Pin(s)	Details
A2 - A7	Analogue I/O pins available by default, but may be in use for additional current sensing
Tx0/Rx0	Can be used for WiFi or Bluetooth adapters but coexists with onboard USB and will need to be disconnected for software uploads
5 - 9	Digital I/O pins available by default, but some motor shields or other peripherals use these

Arduino Uno 

Table 7 Uno Pins to avoid
Pin(s)	Details
A0 - A1	Analogue I/O pins used for main and program track current sensing
3 - 4	Digital I/O pins commonly used by motor shields or Ethernet shields
10 - 13	Digital I/O pins commonly used by motor shields

Table 8 Uno Pins to use
Pin(s)	Details
SDA/SCL (A4/A5)	I²C connection for use with I²C displays, I/O expanders, etc., not for general I/O
SDA/SCL (near USB)	I²C connection for use with I²C displays, I/O expanders, etc., not for general I/O
2	Digital I/O pin available for use

Table 9 Uno Pins to use with caution
Pin(s)	Details
A2 - A3	Analogue I/O pins available by default, but may be in use for additional current sensing
Tx0/Rx0	Can be used for WiFi or Bluetooth adapters but coexists with onboard USB and will need to be disconnected for software uploads
5 - 9	Digital I/O pins available by default, but some motor shields or other peripherals use these

I/O expanders 

Note

If using an I²C LCD display with the PCF8574 backpack, it is possible that it resides at I²C address 0x27. If this is the case, you need to either not use any other devices at this address, change the PCF8574 address to 0x3F (if possible), or replace it. OLEDs typically use address 0x3C, so should not cause issues.

Refer to I2C Displays for further information.

MCP23017 digital I/O expander 

MCP23017 I/O expanders have 16 digital I/O pins and 8 available I²C addresses from 0x20 to 0x27.

The device driver name is “IO_MCP23017.h”, however this is included by default.

Two devices are defined by default.

Refer to MCP23017 Modules and HAL Programming Interface for further information.

Table 10 Default defined MCP23017s
I²C address	VPin range	Comments
0x20	164 - 179	Defined by default, default I²C address on some breakout boards Some breakout boards (e.g. Waveshare or CQRobot Ocean) have 0x27 as default and will need to be changed
0x21	180 - 195	Defined by default, address typically needs to be configured on breakout boards

When adding more MCP23017s, select an unused VPin that allows a total of 16 consecutive numbers for all I/O pins.

Table 11 Additional MCP23017s
I²C address range	VPin count	Comments
0x22 - 0x27	16	Need to add to myAutomation.h or myHal.cpp, and I²C address typically needs to be configured on breakout boards

MCP23008 digital I/O expander 

MCP23008 I/O expanders have 8 digital I/O pins and 8 available I²C addresses from 0x20 to 0x27.

The device driver name is “IO_MCP23008.h” and will need to be included in “myAutomation.h” or “myHal.cpp”.

Note that devices at 0x20/0x21 will conflict with the default defined MCP23017 devices at these addresses, but this conflict should be resolved automatically at startup.

Note also the potential conflict with an LCD at I²C address 0x27 as per MCP23017 digital I/O expander.

Refer to HAL Programming Interface for further information.

When adding MCP23008s, select an unused VPin that allows a total of 8 consecutive numbers for all I/O pins.

Table 12 Adding MCP23008s
I²C address range	VPin count	Comments
0x20 - 0x27	8	Need to add to myAutomation.h or myHal.cpp, and I²C address typically needs to be configured on breakout boards May need to disable default MCP23017s if 0x20/0x21 are used

PCF8574 digital I/O expander 

PCF8574 I/O expanders have 8 digital I/O pins and 8 available I²C addresses from 0x20 to 0x27.

The device driver name is “IO_PCF8574.h” and will need to be included in “myAutomation.h” or “myHal.cpp”.

Note that devices at 0x20/0x21 will conflict with the default defined MCP23017 devices at these addresses, but this conflict should be resolved automatically at startup.

Refer to HAL Programming Interface for further information.

When adding PCF8574s, select an unused VPin that allows a total of 8 consecutive numbers for all I/O pins.

Table 13 Adding PCF8574s
I²C address range	VPin count	Comments
0x20 - 0x27	8	Need to add to myAutomation.h or myHal.cpp, and I²C address typically needs to be configured on breakout boards May need to disable default MCP23017s if 0x20/0x21 are used

PCA9685 PWM servo module 

PCA9685 servo modules (or I/O expanders) have 16 PWM output pins and 62 available I²C addresses from 0x40 to 0x7D.

The device driver name is “IO_PCA9685.cpp”, however this is loaded by default.

Two devices are defined by default.

Refer to PCA9685 Modules and HAL Programming Interface for further information.

Table 14 Default defined PCA9685s
I²C address	VPin range	Comments
0x40	100 - 115	Defined by default, typically default I²C address on breakout boards
0x41	116 - 131	Defined by default, address typically needs to be configured on breakout boards

When adding more PCA9685s, select an unused VPin that allows a total of 16 consecutive numbers for all I/O pins.

Table 15 Additional PCA9685s
I²C address range	VPin count	Comments
0x42 - 0x7D	16	Need to add to myAutomation.h or myHal.cpp, and I²C address typically needs to be configured on breakout boards

EX-IOExpander - digital and analogue I/O expander 

EX-IOExpander is designed to support up to 256 pins and is currently in testing. The default I²C address is 0x65, however any valid and available address can be used.

The device driver name is “IO_EXIOExpander.h” and is included by default.

Refer to EX-IOExpander for further information.

When adding EX-IOExpander devices, select an unused VPin that allows for the appropriate number of consecutive vpins for all I/O pins.

Table 16 Additional MCP23017s
I²C address range	VPin count	Comments
Any valid/available (default 0x65)	16 - 256	I²C address needs to be configured in software

Sensors 

VL53L0X time-of-flight sensor 

VL53L0X time-of-flight sensors have a default I²C address of 0x29, however this address is programmable by software.

The device driver name is “IO_VL53L0X.h” and will need to be included in “myAutomation.h” or “myHal.cpp”.

Refer to VL53L0X Time of Flight Sensor for further information.

Table 17 Adding VL53L0Xs
I²C address range	Suggested first VPin	VPin count	Comments
0x29 - 0x7F	4000	3	Need to add to myAutomation.h or myHal.cpp, do not use I²C address 0x29 if using multiple devices

Other devices 

EX-Turntable 

Note

EX‑Turntable is in Beta testing, however the device driver is unlikely to change much if at all.

EX‑Turntable is configured with a default I²C address of 0x60, however this address is configurable by the software.

The device driver name is “IO_EXTurntable.h” and will need to be included in “myAutomation.h” or “myHal.cpp”.

Refer to EX-Turntable for further information.

Table 18 Adding EX-Turntable
I²C address and range	Suggested VPin	VPin count	Comments
0x60 default, any valid and available I²C address	600	1	Need to add to myAutomation.h or myHal.cpp, and use the specified version of EX‑CommandStation

DCC-EX rotary encoder 

Note

The rotary encoder device driver is in early development and is likely to change.

The DCC-EX rotary encoder is configured with a default I²C address of 0x80, however this address is configurable by the software.

The device driver name is “IO_RotaryEncoder.h” and will need to be included in “myAutomation.h” or “myHal.cpp”.

Refer to Controlling EX-Turntable with a rotary encoder and the project page for further information.

Table 19 Adding DCC-EX rotary encoder
I²C address and range	Suggested VPin	VPin count	Comments
0x80 default, any valid and available I²C address	800	1 - 3	Need to add to myAutomation.h or myHal.cpp, and use the specified version of EX‑CommandStation