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:
- USB device hardware initialization
- USB interrupt handling
- Management of the serial interface engine SIE
- Management of endpoints
- Fragmentation and reassembly of control transfers
- Handling and sending standard setup requests
The following device classes are supported:
- Vendor Defined (no-class) devices, suitable for use with libusb, WinUSB, or with custom drivers
- HID - Human Interface Device
- CDC-ACM - Communication Device Class - Abstract Control Model
- Mass Storage Class (MSC) with MMC/SD card support
Other features include:
- Clean, well-documented examples (device, and PC-host (libusb))
- Fast PIC24 bootloader
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.
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
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:
- PIC32MX460F512L - PIC32 USB Starter Board
- PIC18F46J50 - PIC18F Starter Kit
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.
The USB stack is supported by the following software:
- Microchip XC8 compiler
- Microchip XC16 compiler
- Microchip XC32 compiler
- Microchip MPLAB X IDE
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
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
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
Running the Test Software
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.
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.
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.
Set the Endpoint halt feature on Endpoint 1 IN. Passing the
clear parameter clears endpoint halt.
Source Tree Structure
(root) | +- 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
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
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:
- Create a new project with MPLAB X.
- Copy and add the
usb/directory as a subdirectory of your project.
- Add the
usb/includedirectory 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).
.to the include path of your project (same note from #3 applies).
- Copy a
usb_descriptors.cfile from one of the example projects into your main project directory.
usb_config.hto match your desired device configuration.
usb_descriptors.cto match your device configuration.
- If you're using a PIC16F/18F platform, add an interrupt handler similar to one of the examples.
main.cin one of the examples, and the Doxygen-generated documentation to add your application logic.
- Make sure to configure the MCU for your board (
Nothing's perfect. Here are the known limitations:
- Control transfers are supported on endpoint 0 only.
- Isochronous transfers are not supported.
- Remote wake-up is not supported.
The following features are on the horizon:
- Support for more specific MCUs
- dsPIC33E and PIC24E support
- Isochronous transfers
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.
- USB Implementers Forum
- USB Specification, 2.0
- Jan Axelson's "USB Complete," 5th Edition
- Jan Axelson's online USB resources
Please contact Signal 11 Software directly for support or consulting help. Sometime in the near future there will be a mailing list.
Signal 11 Software