Quick reference for the i.MXRT family

Teensy 4.1 board

The Teensy 4.1 board.

Below is a quick reference for i.MXRT-based boards. If it is your first time working with this board it may be useful to get an overview of the microcontroller:

Installing MicroPython

See the corresponding section of tutorial: Getting started with MicroPython on the i.MXRT. It also includes a troubleshooting subsection.

General board control

The MicroPython REPL is on the USB port, configured in VCP mode. Tab-completion is useful to find out what methods an object has. Paste mode (ctrl-E) is useful to paste a large slab of Python code into the REPL.

The machine module:

import machine

machine.freq()          # get the current frequency of the CPU

Delay and timing

Use the time module:

import time

time.sleep(1)           # sleep for 1 second
time.sleep_ms(500)      # sleep for 500 milliseconds
time.sleep_us(10)       # sleep for 10 microseconds
start = time.ticks_ms() # get millisecond counter
delta = time.ticks_diff(time.ticks_ms(), start) # compute time difference


The i.MXRT port has three hardware timers. Use the machine.Timer class with a timer ID from 0 to 2 (inclusive):

from machine import Timer

tim0 = Timer(0)
tim0.init(period=5000, mode=Timer.ONE_SHOT, callback=lambda t:print(0))

tim1 = Timer(1)
tim1.init(period=2000, mode=Timer.PERIODIC, callback=lambda t:print(1))

The period is in milliseconds.

Virtual timers are not currently supported on this port.

Pins and GPIO

Use the machine.Pin class:

from machine import Pin

p0 = Pin('D0', Pin.OUT) # create output pin on GPIO0
p0.on()                 # set pin to "on" (high) level
p0.off()                # set pin to "off" (low) level
p0.value(1)             # set pin to on/high

p2 = Pin('D2', Pin.IN)  # create input pin on GPIO2
print(p2.value())       # get value, 0 or 1

p4 = Pin('D4', Pin.IN, Pin.PULL_UP) # enable internal pull-up resistor
p5 = Pin('D5', Pin.OUT, value=1) # set pin high on creation

p6 = Pin(pin.cpu.GPIO_B1_15, Pin.OUT) # Use the cpu pin name.

Available Pins follow the ranges and labelling of the respective board, like:

  • 0-33 for Teensy 4.0,

  • 0-21 for the MIMXRT10xx-EVK board, or ‘D0-Dxx’, or ‘A0-Ann’,

  • 0-14 for the Olimex RT1010Py board, or ‘D0’-‘Dxx’ and ‘A0’-‘Ann’

  • ‘J3_xx’, ‘J4_xx’, ‘J5_xx’ for the Seeed ARCH MIX board,

or the pin names of the Pin.board or Pin.cpu classes.


  • The MIMXRT1xxx-EVK boards may have other on-board devices connected to these pins, limiting it’s use for input or output.

  • At the MIMXRT1010_EVK, pins D4, D5 and D9 of the Arduino connector are by default not connected to the MCU. For details refer to the schematics.

  • At the MIMXRT1170_EVK board, the inner rows of the Arduino connectors are assigned as follows:
    • D16 - D23: J9, odd pin numbers; D17 is by default not connected.

    • D24 - D27: J26, odd pin numbers; J63-J66 have to be closed to enable these pins.

    • D29 - D36: J25, odd pin numbers; D29 and D30 are by default not connected.

There’s a higher-level abstraction machine.Signal which can be used to invert a pin. Useful for illuminating active-low LEDs using on() or value(1).

UART (serial bus)

See machine.UART.

from machine import UART

uart1 = UART(1, baudrate=115200)
uart1.write('hello')  # write 5 bytes
uart1.read(5)         # read up to 5 bytes

The i.MXRT has up to eight hardware UARTs, but not every board exposes all TX and RX pins for users. For the assignment of Pins to UART signals, refer to the UART pinout.

PWM (pulse width modulation)

The i.MXRT has up to four dedicated PWM modules with four FLEXPWM submodules each and up to four QTMR modules with four channels, which can be used to generate a PWM signal or signal pair.

The PWM functions are provided by the machine.PWM class. It supports all basic methods listed for that class and a few additional methods for handling signal groups.

# Samples for Teensy

from machine import Pin, PWM

pwm2 = PWM(Pin(2))      # create PWM object from a pin
pwm2.freq()             # get current frequency
pwm2.freq(1000)         # set frequency
pwm2.duty_u16()         # get current duty cycle, range 0-65535
pwm2.duty_u16(200)      # set duty cycle, range 0-65535
pwm2.deinit()           # turn off PWM on the pin
# create a complementary signal pair on Pin 2 and 3
pwm2 = PWM((2, 3), freq=2000, duty_ns=20000)

# Create a group of four synchronized signals.
# Start with Pin(4) at submodule 0, which creates the sync pulse.
pwm4 = PWM(Pin(4), freq=1000, align=PWM.HEAD)
# Pins 5, 6, and 9 are pins at the same module
pwm5 = PWM(Pin(5), freq=1000, duty_u16=10000, align=PWM.HEAD, sync=True)
pwm6 = PWM(Pin(6), freq=1000, duty_u16=20000, align=PWM.HEAD, sync=True)
pwm9 = PWM(Pin(9), freq=1000, duty_u16=30000, align=PWM.HEAD, sync=True)

pwm3                    # show the PWM objects properties

PWM Constructor

class PWM(dest, freq, duty_u16, duty_ns, *, center, align, invert, sync, xor, deadtime)

Construct and return a new PWM object using the following parameters:

  • dest is the entity on which the PWM is output, which is usually a machine.Pin object, but a port may allow other values, like integers or strings, which designate a Pin in the machine.PIN class. dest is either a single object or a two element object tuple. If the object tuple is specified, the two pins act in complementary mode. These two pins must be the A/B channels of the same submodule.

PWM objects are either provided by a FLEXPWM module or a QTMR module. The i.MXRT devices have either two or four FLEXPWM and QTMR modules. Each FLEXPWM module has four submodules with three channels, each, called A, B and X. Each QTMR module has four channels. Each FLEXPWM submodule or QTMR channel may be set to different parameters. Not every channel is routed to a board pin. Details are listed below.

Setting freq affects the three channels of the same FLEXPWM submodule. Only one of duty_u16 and duty_ns should be specified at a time.

Keyword arguments:

  • freq should be an integer which sets the frequency in Hz for the PWM cycle. The valid frequency range is 15 Hz resp. 18Hz resp. 24Hz up to > 1 MHz.

  • duty_u16 sets the duty cycle as a ratio duty_u16 / 65536. The duty cycle of a X channel can only be changed, if the A and B channel of the respective submodule is not used. Otherwise the duty_16 value of the X channel is 32768 (50%).

  • duty_ns sets the pulse width in nanoseconds. The limitation for X channels apply as well.

  • center=value. An integer sets the center of the pulse within the pulse period. The range is 0-65535. The resulting pulse will last from center - duty_u16/2 to center + duty_u16/2.

  • align=value. Shortcuts for the pulse center setting, causing the pulse either at the center of the frame (value=0), the leading edge at the begin (value=1) or the trailing edge at the end of a pulse period (value=2).

  • invert=True|False channel_mask. Setting a bit in the mask inverts the respective channel. Bit 0 inverts the first specified channel, bit 2 the second. The default is 0.

  • sync=True|False. If a channel of a module’s submodule 0 is already active, other submodules of the same module can be forced to be synchronous to submodule 0. Their pulse period start then at at same clock cycle. The default is False.

  • xor=0|1|2. If set to 1 or 2, the channel will output the XOR’d signal from channels A or B. If set to 1 on channel A or B, both A and B will show the same signal. If set to 2, A and B will show alternating signals. For details and an illustration, please refer to the MCU’s reference manual, chapter “Double Switching PWMs”.

  • deadtime=time_ns. This setting affects complementary channels and defines a deadtime between an edge of a first channel and the edge of the next channel, in which both channels are set to low. That allows connected H-bridges to switch off one side of a push-pull driver before switching on the other side.

PWM Methods

The methods are identical to the generic machine.PWM class, with additional keyword arguments to the init() method, matchings those of the constructor.

Each FLEX submodule or QTMR module may run at different frequencies. The PWM signal is created by dividing the pwm_clk signal by an integral factor, according to the formula:

f = pwm_clk / (2**n * m)

with n being in the range of 0..7, and m in the range of 2..65536. pmw_clk is 125Mhz for MIMXRT1010/1015/1020, 150 MHz for MIMXRT1050/1060/1064 and 160MHz for MIMXRT1170. The lowest frequency is pwm_clk/2**23 (15, 18, 20Hz). The highest frequency with U16 resolution is pwm_clk/2**16 (1907, 2288, 2441 Hz), the highest frequency with 1 percent resolution is pwm_clk/100 (1.25, 1.5, 1.6 MHz). The highest achievable frequency is pwm_clk/3 for the A/B channels, and pwm_clk/2 for the X channels and QTMR signal.

PWM Pin Assignment

Pins are specified in the same way as for the Pin class. For the assignment of Pins to PWM signals, refer to the PWM pinout.

ADC (analog to digital conversion)

On the i.MXRT ADC functionality is available on Pins labeled ‘Ann’.

Use the machine.ADC class:

from machine import ADC

adc = ADC(Pin(32))          # create ADC object on ADC pin
adc.read_u16()              # read value, 0-65536 across voltage range 0.0v - 3.3v

The resolution of the ADC is 12 bit with 10 to 11 bit accuracy, irrespective of the value returned by read_u16(). If you need a higher resolution or better accuracy, use an external ADC.

Software SPI bus

Software SPI (using bit-banging) works on all pins, and is accessed via the machine.SoftSPI class.

from machine import Pin, SoftSPI

# construct a SoftSPI bus on the given pins
# polarity is the idle state of SCK
# phase=0 means sample on the first edge of SCK, phase=1 means the second
spi = SoftSPI(baudrate=100000, polarity=1, phase=0, sck=Pin(0), mosi=Pin(2), miso=Pin(4))

spi.init(baudrate=200000) # set the baudrate

spi.read(10)            # read 10 bytes on MISO
spi.read(10, 0xff)      # read 10 bytes while outputting 0xff on MOSI

buf = bytearray(50)     # create a buffer
spi.readinto(buf)       # read into the given buffer (reads 50 bytes in this case)
spi.readinto(buf, 0xff) # read into the given buffer and output 0xff on MOSI

spi.write(b'12345')     # write 5 bytes on MOSI

buf = bytearray(4)      # create a buffer
spi.write_readinto(b'1234', buf) # write to MOSI and read from MISO into the buffer
spi.write_readinto(buf, buf) # write buf to MOSI and read MISO back into buf

The highest supported baud rate is 500000.

Hardware SPI bus

There are up to four hardware SPI channels that allow faster transmission rates (up to 30Mhz). Hardware SPI is accessed via the machine.SPI class and has the same methods as software SPI above:

from machine import SPI

spi = SPI(0, 10000000)
spi.write('Hello World')

For the assignment of Pins to SPI signals, refer to Hardware SPI pinout.


  1. Even if the highest reliable baud rate at the moment is about 30 Mhz, setting a baud rate will not always result in exactly that frequency, especially at high baud rates.

  2. Sending at higher baud rate is possible. In the tests receiving worked up to 60 MHz, sending up to 90 MHz.

Software I2C bus

Software I2C (using bit-banging) works on all output-capable pins, and is accessed via the machine.SoftI2C class:

from machine import Pin, SoftI2C

i2c = SoftI2C(scl=Pin(5), sda=Pin(4), freq=100000)

i2c.scan()              # scan for devices

i2c.readfrom(0x3a, 4)   # read 4 bytes from device with address 0x3a
i2c.writeto(0x3a, '12') # write '12' to device with address 0x3a

buf = bytearray(10)     # create a buffer with 10 bytes
i2c.writeto(0x3a, buf)  # write the given buffer to the slave

The highest supported freq is 400000.

Hardware I2C bus

There are up to four hardware I2C channels that allow faster transmission rates and support the full I2C protocol. The I2C signals have fixed assignments to GPIO pins. For the assignment of Pins to I2C signals, refer to Hardware I2C pinout.

Hardware I2C is accessed via the machine.I2C class and has the same methods as software SPI above:

from machine import I2C

i2c = I2C(0, 400_000)
i2c.writeto(0x76, b"Hello World")

I2S bus

See machine.I2S. Example using a Teensy 4.1 board with a simple external Codec like UDA1334.:

from machine import I2S, Pin
i2s = I2S(2, sck=Pin(26), ws=Pin(27), sd=Pin(7),
    mode=I2S.TX, bts=16,format=I2S.STEREO,
i2s.write(buf)             # write buffer of audio samples to I2S device

Example for using I2S with a MIMXRT10xx_DEV board:

from machine import I2S, I2C, Pin
import wm8960


wm=wm8960.WM8960(i2c, sample_rate=SAMPLE_RATE_IN_HZ,

i2s = I2S(1, sck=Pin("SCK_TX"), ws=Pin("WS_TX"), sd=Pin("SD_RX"),
    mck=Pin("MCK),mode=I2S.RX, bts=16,format=I2S.MONO,
i2s.readinto(buf)          # fill buffer with audio samples from I2S device

In this example, the input channels are swapped in the WM8960 driver, since the on-board microphone is connected to the right channel, but mono audio is taken from the left channel. Note, that the sck and ws pins are connected to the TX signals of the I2S bus. That is intentional, since at the MW8960 codec these signals are shared for RX and TX.

Example using the Teensy audio shield:

from machine import I2C, I2S, Pin
from sgtl5000 import CODEC
i2s = I2S(1, sck=Pin(21), ws=Pin(20), sd=Pin(7), mck=Pin(23),
    mode=I2S.TX, bits=16,rate=44100,format=I2S.STEREO,

# configure the SGTL5000 codec
i2c = I2C(0, freq=400000)
codec = CODEC(0x0A, i2c)
codec.dac_volume(0.9, 0.9)
codec.volume(0.7, 0.7)

i2s.write(buf)             # write buffer of audio samples to I2S device

The SGTL5000 codec used by the Teensy Audio shield uses the RX signals for both RX and TX. Note that the codec is initialized after the I2S device. That is essential since MCK is needed for its I2C operation and is provided by the I2S controller.

MIMXRT boards may have 1 or 2 I2S buses available at the board connectors. On MIMXRT1010 devices the bus numbers are 1 and 3. The I2S signals have fixed assignments to GPIO pins. For the assignment of Pins to I2S signals, refer to I2S pinout.

Real time clock (RTC)

See machine.RTC:

from machine import RTC

rtc = RTC()
rtc.datetime((2017, 8, 23, 1, 12, 48, 0, 0)) # set a specific date and time
rtc.datetime() # get date and time
rtc.now() # return date and time in CPython format.

The i.MXRT MCU supports battery backup of the RTC. By connecting a battery of 1.5-3.6V, time and date are maintained in the absence of the main power. The current drawn from the battery is ~20µA, which is rather high. A CR2032 coin cell will last for about one year.

SD card

See machine.SDCard:

import machine, os

sd = machine.SDCard()
fs = os.VfsFat(sd)
os.mount(fs, "/sd")  # mount
os.listdir('/sd')    # list directory contents
os.umount('/sd')     # eject

Note: The i.mx-rt 1011 and 1015 based boards do not support the machine.SDCard class. For these, the SPI based driver sdcard.py from the MicroPython drivers can be used. When using it, you have to overdrive the CS pin of the SPI hardware module. Example:

import os, sdcard, machine

cs_pin = "D10"
spi = machine.SPI(0) # SPI0 with cs at Pin "D10" used for SDCARD
cs = machine.Pin(cs_pin, machine.Pin.OUT, value=1)
sd = sdcard.SDCard(spi, cs)
vfs = os.VfsFat(sd)
os.mount(vfs, "/sdcard")

OneWire driver

The OneWire driver is implemented in software and works on all pins:

from machine import Pin
import onewire

ow = onewire.OneWire(Pin(12)) # create a OneWire bus on GPIO12
ow.scan()                     # return a list of devices on the bus
ow.reset()                    # reset the bus
ow.readbyte()                 # read a byte
ow.writebyte(0x12)            # write a byte on the bus
ow.write('123')               # write bytes on the bus
ow.select_rom(b'12345678')    # select a specific device by its ROM code

There is a specific driver for DS18S20 and DS18B20 devices:

import time, ds18x20
ds = ds18x20.DS18X20(ow)
roms = ds.scan()
for rom in roms:

Be sure to put a 4.7k pull-up resistor on the data line. Note that the convert_temp() method must be called each time you want to sample the temperature.

DHT driver

The DHT driver is implemented in software and works on all pins:

import dht
import machine

d = dht.DHT11(machine.Pin(4))
d.temperature() # eg. 23 (°C)
d.humidity()    # eg. 41 (% RH)

d = dht.DHT22(machine.Pin(4))
d.temperature() # eg. 23.6 (°C)
d.humidity()    # eg. 41.3 (% RH)

Be sure to have a 4.7k pull-up resistor on the data line. Some DHT modules may already have one.

Ethernet driver

All MIMXRT boards except the MIMXRT1011 based boards and Teensy 4.0 support Ethernet. Example usage:

import network

lan = network.LAN(0)

If there is a DHCP server in the LAN, the IP address is supplied by that server. Otherwise, the IP address can be set with lan.ifconfig(). The default address is

Teensy 4.1 does not have an Ethernet jack on the board, but PJRC offers an adapter for self-assembly. The Seeed ARCH MIX board has no PHY hardware on the board, however you can attach external PHY interfaces. By default, the firmware for Seeed Arch Mix uses the driver for a LAN8720 PHY. The MIMXRT1170_EVK is equipped with two Ethernet ports, which are addressed as LAN(0) for the 100M port and LAN(1) for the 1G port.

For details of the network interface refer to the class network.LAN.

Transferring files

Files can be transferred to the i.MXRT devices for instance with the mpremote tool or using an SD card. If Ethernet is available, you can also use ftp. See the MicroPython forum for the FTP server or other community-supported alternatives to transfer files to an i.MXRT board, like rshell or Thonny.