Getting Started with MicroMod

2021-01-21 | By SparkFun Electronics

License: See Original Project

Courtesy of SparkFun

Guide by NATE

Introduction

MicroMod is a compact interface to connect a microcontroller to various peripherals. You can generally think of the MicroMod system as a ‘brain’ plugging into a ‘carrier board’.

A MicroMod processor board is approximately 22x22mm and can insert into any MicroMod carrier board. A small screw holds the processor board in place. Whereas the original M.2 standard was designed for swapping out peripherals (user could change one solid state hard drive to a larger one) the MicroMod standard is designed for swapping out controllers (user can start with a powerful processor and then change to a low power controller to extend battery life).

Suggested Reading

If you aren't familiar with the MicroMod ecosystem, we recommend reading here for an overview.

MicroMod_1

MicroMod Ecosystem

If you aren’t familiar with the following concepts, we recommend checking out these tutorials before continuing.

Serial Peripheral Interface (SPI): SPI is commonly used to connect microcontrollers to peripherals such as sensors, shift registers, and SD cards.

Pulse Width Modulation: An introduction to the concept of Pulse Width Modulation.

Logic Levels: Learn the difference between 3.3V and 5V devices and logic levels.

I2C: An introduction to I2C, one of the main embedded communications protocols in use today.

How Does It Work?

The MicroMod standard leverages the M.2 connector and specification to increase the availability of parts and reduce the cost of the connector. All MicroMod ‘brains’ share a common pinout. For example, the I²C pins for the MicroMod ESP32 are in the same position as the I²C pins on the MicroMod Artemis.

A varient of MicroMod carrier boards give the user access to different technologies. Because the MicroMod connector is standardized, the controller can be easily and quickly swapped out as processing power, power consumption, and wireless connectivity. For example, a user may start with the MicroMod Artemis and a RFID carrier board. They then might decide they need Wi-Fi for their project. Swapping to the MicroMod ESP32 allows the user to instantly add Wi-Fi capabilities without changing the underlying hardware.

The MicroMod interface is defined as follows:

Hardware Overview

What Connector and Key Does MicroMod Use?

MicroMod uses the common M.2 connector. This is the same connector found on modern motherboards and laptops. We recommend the connector with 4.2mm height.

Connector_2

TE makes the 2199230-4 that is widely available and for reasonable cost (1k budgetary pricing is $0.56). You can also order the MicroMod DIY Carrier Kit that includes 5 of the connector, screw, and reflow-able standoff.

There are various locations for the plastic ‘key’ on the M.2 connector to prevent a user from inserting an incompatible device. The MicroMod standard uses the ‘E’ key but diverges from the M.2 standard by moving the mounting screw 4mm to the side. The ‘E’ key is fairly common, so a user could insert a M.2 compatible Wi-Fi module but because the screw mount doesn’t align, the user would not be able to secure an incompatible device into a MicroMod carrier board.

What is a Processor Board?

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Each processor board follows the M.2 standard of '2222' or 22x22mm overall size.

Each processor board is approximately 22x22mm and has a microcontroller or processor on it. The pins on the processor are brought to the card edge to match the MicroMod pinout specification.

Every processor board is expected to need only USB D+/- to be programmed. This means that a processor that does not have built-in USB Support must have it added. For example: the Artemis Processor board has the CH340E added to provide serial programming support.

Every processor board is expected to have one on-board status LED that is not routed to the board edge.

Note: The MicroMod spec moves the screw position from the board's center line to 4mm right-of-center. This is meant to prevent incorrect mixing of a growing number of devices that use the M.2 connector (such as Wi-Fi cards, SSDs, cellular modems, etc) and MicroMod devices. While a user could insert a Wi-Fi card into a SparkFun data logging carrier board the screw holes would not line up making it obvious the devices don't work together.

The MicroMod spec may incorporate larger sizes in the future, and users are welcome to create their own processor boards but note that the standoff hole on most carrier boards will be located to fit the 2222 MicroMod key.

What is the MicroMod Pinout?

The MicroMod interface is defined as follows:

Below is the general MicroMod interface pinout for v1.0 processor and carrier boards.

Not all of the pins are guaranteed to be connected when using the MicroMod form factor. Please see the documentation specific to your processor board for more information.

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Table_2

Each pin on the M.2 connector is specified to have a given function. There are additional rules to the MicroMod specification to ensure cross platform compatibility. At the extreme case, a maximum of 49x GPIOs are supported. In general, MicroMod focuses on interface types and locations. For example, if a carrier board requires PWM capabilities then the carrier board should leverage pins 32 (aka PWM0) and 47 (aka PWM1) as these are most likely to support PWM.

Supported Interfaces:

USB for programming and serial debug
2x Analog Dedicated
2x PWM Dedicated
2x Digital I/O Dedicated
12x GPIO
2x I²C
2x SPI
2x UART
SDIO
USB-HOST
CAN
SWD
PDM / PCM / I2S
Differential ADC

12x GPIOs may not sound like much but once all the other interfaces have been connected (UART, SPI, I2C, PWM, ADC) 12x GPIOs should cover most remaining applications.

Hardware Hookup

To get started with MicroMod, you'll need a processor board as well as a carrier board. Here we are using the Artemis MicroMod Processor Board with the Machine Learning Carrier Board. Align the top key of the MicroMod Artemis Processor Board to the screw terminal of the Machine Learning Carrier Board and angle the board into the socket. Insert the board at an angle into the M.2 connector.

Note: There is no way to insert the processor backward since the key prevents it from mating with the M.2 connector and as an extra safeguard to prevent inserting a processor that matches the key, the mounting screw is offset so you will not be able to secure an improperly connected processor board.

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The Processor Board will stick up at an angle (at around 25°), as seen here:

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Once the board in the socket, gently hold the MicroMod Processor Board down and tighten the screw with a Phillip's head. We recommend the classic SparkFun reversible mini-screw driver or the fancier pocket screw driver set but any #00, #0, or #1 Phillip's head driver will work.

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Once the board is secure, your assembled MicroMod system should look similar to the image below!

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Note: If you've never connected an CH340 device to your computer before, you may need to install drivers for the USB-to-serial converter. Check out our section on How to Install CH340 Drivers" for help with the installation.

Designing with MicroMod

Can I Make My Own MicroMod Processor Board?

Absolutely. SparkFun is an open source hardware company and is not patenting this interface. All we ask is that you don’t fork the spec, follow the rules, and try not to muddy the community by introducing competing or partially compatible similar interfaces.

We recommend starting from one of our open source processor board designs. Currently all these files are EAGLE PCB format. If you have a different PCB package and you'd like to add your design to the list as a reference design please let us know!

Additionally, we've written Designing with MicroMod that goes into depth about how to create a good processor and carrier board.

Can I Make My Own MicroMod Carrier Board?

Absolutely! This where things get really exciting. We’ve got a variety of resources including a connector footprint and symbol for Eagle PCB. We had multiple carrier boards already designed and open source, so you can use them as a reference design and starting point. We can’t wait to see what you make.

logging_8

We recommend starting from one of our open source carrier board designs. Currently all these files are EAGLE PCB format. If you have a different PCB package and you'd like to add your design to the list as a reference design please let us know!

Additionally, we've written Designing with MicroMod that goes into depth about how to create a good processor and carrier board.

When designing your own carrier board keep these rules of thumb in mind:

All carrier boards must provide a regulated 3.3V supply capable of 1A.
All carrier boards must provide a USB D+/- connection for programming.
Not all processor boards have connections to every pin.
The A0/1, PWM0/1, and D0/1 should be supported by every processor board so you can trust that those pins will be available.
UART1, SPI, and I2C ports are super common and on nearly every processor board, but peripherals beyond those varies between processor boards. For example: support for a 2nd I2C port varies a lot so if your carrier board requires the 2nd I2C port be aware of what processor boards will be supported.

To help get you started with your own custom carrier board we've put together the MicroMod DIY Carrier Kit that includes 5 of the connector, screw, and standoff so that you can get all the ‘special’ parts you may need to make your own carrier board.

SparkFun MicroMod DIY Carrier Kit (5 pack)

The M.2 connector has a 0.5mm pitch and alignment pegs. Hand stenciling and reflow-at-home is possible but we recommend using a stainless steel stencil (do not use mylar) and a higher quality reflow oven (sorry hot plate!) to help prevent jumpers.

Tell Me about Heat Sinking!

One of the benefits to the M.2 standard is the ability to put components under the module. Using this we can now add heatsinks to our microcontrollers!

For this reason, we recommend the connector with 4.2mm height. TE makes the 2199230-4 that is widely available and for reasonable cost (1k budgetary pricing is $0.56).

What if I Need A LOT of GPIO?

There are applications where a user will need more than 12 GPIO. The MicroMod specification is flexible. If you would like to design a MicroMod that has only a few peripherals connected (for example, just UART and I2C) and leaving the rest for GPIO (45 available for GPIO in this example) that’s fine. Your carrier board would utilize the UART and I2C pins in the normal location and GPIOs in non-standard locations. This would prevent other MicroMods from being absolutely compatible (perhaps one or two of the MicroMod Artemis would not be able to drive your carrier board’s relays) but it’s allowed. You, the designer, just need to think about the tradeoffs.

We’ve written a guide for creating a MicroMod Processor Board but here are the guiding principles:

Connect dedicated hardware of the microcontroller to the available I2C, SPI, UART, USB, USB_HOST, CAN, SDIO, and JTAG pins exposed on the MicroMod connector edge
Next, A0/A1 on the MicroMod connector edge should be assigned to pins on the microcontroller that are exclusively ADC (no PWM capability)
PWM0/PWM1 should be assigned to pins that are exclusively PWM (no ADC capability)
D0/D1 should be assigned to pins that are exclusively GPIO (no ADC or PWM capability)
Remaining pins should be assigned to Gx with ADC + PWM capable pins given priority (0, 1, 2, etc) positions
The intent is to guarantee PWM, ADC, and Digital Pin functionality on those specific pins. Whereas Gx pins do not guarantee ADC/PWM function
If the microcontroller lacks a specific pin function, and has left over GPIO, they can be over-ruled with GPIO. For example, CTS/RTS can be overwritten with a GPIO if the microcontroller does not have flow control

For more information, check out the Designing with MicroMod tutorial.

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Designing with MicroMod

This tutorial will walk you through the specs of the MicroMod processor and carrier board as well as the basics of incorporating the MicroMod form factor into your own PCB designs!

Resources and Going Further

Valuable MicroMod documents: