A USB device stack is the software necessary to drive the USB device peripheral hardware on a microcontroller (MCU) or other device. Typically, USB peripheral hardware only supports the transaction level and below of the USB protocol. Enumeration and transfers are left to the firmware or software to implement. The Microchip PIC line of microcontrollers work exactly this way.

M-Stack is a functional, well-documented, free and open source implementation of a USB stack for Microchip PIC platforms. It performs the following operations:

The following device classes are supported:

Other features include:

While having a working USB stack is of great benefit when starting a USB project, know that there is no substitute for actually knowing the details of how USB works. The USB specification should be consulted frequently when creating a USB device.


The software is dual-licensed under the LGPL version 3 and the Apache License version 2.0. It may be used without royalty payment in both commercial and open hardware projects so long as the conditions of either the LGPL or the Apache License can be met.

Commercial licenses are available for purchase for companies and projects which cannot or wish to not comply with the terms of either the LGPL or the Apache License.


Code contributions to the project are welcome, but copyright on code submitted will need to be assigned to Signal 11 Software. Shared copyright is permissible.


Free support for this product is somewhat limited. A mailing list will be setup soon.

Paid support is available through Signal 11 Software.

Getting Started

Downloading the Software

M-Stack's source code is currently hosted on Github. To get the latest version, run:

git clone https://github.com/signal11/m-stack.git


View the Doxygen-generated Documentation.

For an overview of how to use M-Stack, see the Unit Test Example

Supported hardware

M-Stack has currently been tested on PIC16F, PIC18F, PIC24F, and PIC32MX devices. Microchip has obviously made a conscious effort to make the register-level interfaces to their USB peripherals as similar as possible across MCUs and even across MCU families. While many devices should be able to be easily supported with this software, there are often times small differences which need to be worked out, the biggest of which being buffer descriptor and data buffer locations with respect to the DMA capabilities of the microcontroller being used.

The following MCU's and configurations have been tested:

If your hardware is not supported, and it's in the PIC16F/18F/24F/32MX family, I can probably easily make you a port without very much trouble. The easiest way is for you to send me a development board. If your hardware is in another MCU family which is not currently supported, I can also make you a port, but it will be more effort. In either case, I'd be happy to talk with you about it.

Supported Software

The USB stack is supported by the following software:

Note that the C18 compiler is not currently supported. There are some #defines in the code for C18 because this project came from code that was originally done on a PIC18F4550 using C18. It has not yet been determined whether a port to C18 will be made, as C18 has been deprecated by Microchip. Further, C18 has some properties which make a port somewhat more difficult than other compilers.

Building the Unit Test Firmware

Open the MPLAB.X project in apps/unit_test in the MPLAB X IDE. Select a configuration and build. Make sure a supported compiler is installed.

Building the Test Software

The host_test/ directory contains Libusb-based test programs for testing the functionality of a USB device running the unit test firmware. Currently the Libusb test software has only been tested Linux, but since its only dependency is the cross-platform Libusb library, it is easily portable to other operating systems on which Libusb is supported.

With Libusb installed, run make in the host_test/ directory to build the test software.

Running the Test Software

./control_transfer_in [number_of_bytes]

Execute an IN control transfer requesting number_of_bytes bytes from the device. The data returned will be printed. The unit test firmware supports control transfers up to 512 bytes.

./control_transfer_out [number_of_bytes]

Execute an OUT control transfer sending number_of_bytes bytes to the device. A message will be printed. The unit test firmware supports control transfers up to 512 bytes.

./test [number_of_bytes]

Send and then ask for number_of_bytes bytes on EP 1 OUT and EP 1 IN, respectively. The data is printed out. The unit test firmware will support up to 128-bytes of this kind of operation.

./feature <clear>

Set the Endpoint halt feature on Endpoint 1 IN. Passing the clear parameter clears endpoint halt.

Source Tree Structure

 +- usb/                   <- USB stack software
 |   +- include/           <- API include file directory
 |   +- src/               <- Source files
 +- storage/               <- MMC/SD card implementation
     +- include/           <- API include files
     +- src/               <- Source files
 +- apps/                  <- Firmware USB device applications,
     |                        examples, and tests
     +- unit_test/         <- Unit test firmware
     +- hid_mouse/         <- HID Mouse example
     +- cdc_acm/           <- CDC/ACM virtual COM port example
     +- msc_test/          <- Mass Storage Class example
     +- bootloader/        <- USB Bootloader firmware and software
 +- host_test/             <- Software applications to run from a PC Host

USB Stack Source Files

usb/src/usb.c - The implementation of the USB stack.

usb/src/usb_hal.h - Hardware abstraction layer (HAL) containing differences specific to each platform.

usb/src/usb_hid.c - Implementation of the HID class.

usb/include/usb.h - The API header for the USB stack. Applications should #include this file.

usb/include/usb_ch9.h - Enums and structs from Chapter 9 of the USB specification which deals with control transfers and enumeration. An application should #include this file from their usb_descriptors.c and from any file which deals with control transfers.

usb/src/usb_hid.h - Enums, structs, and callbacks for the HID class

Application Source Files (Unit Test Example)

apps/unit_test/main.c - Main program file

apps/unit_test/usb_config.h - M-Stack configuration file. The USB stack will include this file and use it for configuration. The application should set the #defines in this file to suit the application's needs.

apps/unit_test/usb_descriptors.c - The application's descriptors. The application should set the descriptors in this file as desired to suit the application's needs.

Making Your Own Project

The easiest way to create a project using M-Stack is to simply copy one of the examples and modify it accordingly. Sometimes it's better though to do things the hard way in order to understand better.

To create a new project, perform the following steps:

  1. Create a new project with MPLAB X.
  2. Copy and add the usb/ directory as a subdirectory of your project.
  3. Add the usb/include directory to the include path of your project. (Note that the include paths are relative to the Makefile. If you set up your project like the examples, with an MPLAB.X/ subdirectory, you'll need to add an additional ../ to the beginning of the include path).
  4. Add . to the include path of your project (same note from #3 applies).
  5. Copy a usb_config.h and a usb_descriptors.c file from one of the example projects into your main project directory.
  6. Modify usb_config.h to match your desired device configuration.
  7. Modify usb_descriptors.c to match your device configuration.
  8. If you're using a PIC16F/18F platform, add an interrupt handler similar to one of the examples.
  9. Reference main.c in one of the examples, and the Doxygen-generated documentation to add your application logic.
  10. Make sure to configure the MCU for your board (__CONFIG registers, etc.).


Nothing's perfect. Here are the known limitations:

Future Plans

The following features are on the horizon:

Consulting Services

USB is hard. There's no getting around it. There's a good chance if you're reading this that USB is not the main focus of your project. USB is a means to an end, and your specialization is likely more related to the end than to the means. Given that, doesn't it makes sense to hire someone to help you with the USB aspect of your project? Many have found that hiring specialized consultants for specialized jobs can drastically reduce the total cost of a project. Signal 11 can help you. This software testifies to the expertise Signal 11 Software has with respect to USB and software development. See the website for more information.



Microchip PIC


Please contact Signal 11 Software directly for support or consulting help. Sometime in the near future there will be a mailing list.

Alan Ott
Signal 11 Software
+1 407-222-6975