Peter's electronic projects

USB digital I/O extender

description

Add general purpose input/output lines to your computer based projects. This project is a 12-bit digital I/O extender using the Microchip PIC18f14k50 microcontroller which connects to an USB host port. The microcontroller is available in through-hole DIP20 and SMD packages, too.

NOTE for beginners: PIC18F14k50 is a general purpose microcontroller which has to be programmed before you can use it in the actual circuit! Please check out this link to learn more.

Circuit diagram

The device is powered by the USB bus. The ports RB4-7, RC0-7 can be set on a bit-by-bit basis to input or output direction. All LEDs on the schematic are optional, and are only shown for testing the device. You can find the USB connector pinouts at http://pinouts.ws/usb-pinout.html

USB digital I/O extender schematic

Controlling the I/O extender

A computer or an embedded RISC board with an USB host port can be used to control the I/O extender. The controlio example code uses libusb. This means that you don't need a kernel driver to interface with it.
The controlio example code can be invoked with these parameters:

command line	description
controlio getport	read the digital I/O port states
controlio setdir <n>	set the digital I/O port directions. Use bit 1 to set direction to input, bit 0 to set direction to output <n> is a 16-bit decimal number between 0-65535
controlio setport <n>	set the digital I/O port output states. Sets all output I/O pins. Use bit 1 to set output to HIGH, 0 to set output to LOW <n> is a 16-bit decimal number between 0-65535
controlio setbit <n>	set the selected digital I/O port output pins to HIGH. Use bit 1 to select I/O pins to set <n> is a 16-bit decimal number between 0-65535
controlio clearbit <n>	set the selected digital I/O port output pins to LOW. Use bit 1 to select I/O pins to clear

Mapping of I/O port pins, directions to parameter <n> and buffer output:

controlio getport output

buffer[0]

buffer[1]

I/O pin

RC7

RC6

RC5

RC4

RC3

RC2

RC1

RC0

RB7

RB6

RB5

RB4

controlio setport <n>
controlio setdir <n>
controlio setbit <n>
controlio clearbit <n>

bit#7

bit#6

bit#5

bit#4

bit#3

bit#2

bit#1

bit#0

bit#15

bit#14

bit#13

bit#12

bit#11

bit#10

bit#9

bit#8

Examples:

command line example	explanation
controlio setdir 65535	set all I/O pins as input
controlio getport	Read the digital I/O port states. The buffer is 16 bit long, and is shown in hexadecimal. Output ports usually read back the state they are set to. Input ports read their externally set state. Command output is: OK: buffer length = 2 Buffer: ff f0
controlio setdir 0	set all I/O pins as output
controlio setport 0	set all output I/O pins LOW
controlio setport 12	set the RC2 and RC3 I/O pins HIGH, other output pins LOW (works only on output pins)
controlio setbit 32768	set the RB7 output pin HIGH, don't change other output pins (works only if RB7 is an output pin)
controlio clearbit 4097	set the RB4 and the RC0 output pins LOW, don't change other output pins (works only if RB4 and RC0 are output pins)

USB requests

The circuit is a full speed (12Mbit) vendor-specific USB device. It uses only the control endpoint 0, and is controlled by vendor-specific control requests. If you want to make your own host interface, the implemented control requests are:

name	setup bmRequestType	setup bRequest	setup wValue	setup wLength	response
GETPORT	USB_TYPE_VENDOR, USB_ENDPOINT_IN, USB_RECIP_DEVICE	0x53	unused	8	2 bytes, I/O port states
SETPORT	USB_TYPE_VENDOR, USB_ENDPOINT_OUT, USB_RECIP_DEVICE	0x54	I/O port states <n>	0	0 bytes ACK
SETDIR	USB_TYPE_VENDOR, USB_ENDPOINT_OUT, USB_RECIP_DEVICE	0x55	I/O port directions <n>	0	0 bytes ACK
SETBIT	USB_TYPE_VENDOR, USB_ENDPOINT_OUT, USB_RECIP_DEVICE	0x56	I/O port pin selection <n>	0	0 bytes ACK
CLEARBIT	USB_TYPE_VENDOR, USB_ENDPOINT_OUT, USB_RECIP_DEVICE	0x57	I/O port pint selection <n>	0	0 bytes ACK

You can find the explanation of these requests under "controlling the I/O extender"

Compiling the host side code under Linux

- install a C compiler and the libusb development headers

yum install gcc libusb-devel # for redhat/fedora
apt-get install gcc libusb-dev # for debian/ubuntu

- compile the code

cd c; cc -o controlio controlio.c -lusb

- check operation with the command

./controlio getport

Compiling the host side code for openwrt

- install the Openwrt SDK on a computer
- a package/usbio/Makefile is being made (not yet ready, please ask for it if you need). Copy files from usbio.zip c/ directory to the SDK package/usbio/src/
- download the libusb library

svn co svn://svn.openwrt.org/openwrt/packages/libs/libusb

- select the libusb library and the usbio package to be built

make menuconfig

- build the libusb and usbio packages

make package/libusb/compile V=99
make package/usbio/compile V=99

Compiling the host side code under windows

- download and install mingw and msys from mingw.org
- extract files from usbio.zip c/ directory to c:\mingw\msys\1.0\home\<user> where user is your user name
- if your OS is Vista, Win 7 or later, modify the controlio.c source code, and delete these 5 lines from it:

    if (usb_claim_interface(udev, 0) < 0)
    {
        printf("Unable to claim interface\n");
        exit(-1);
    }

- download and install libusb-win32 from sourceforge
- copy include/lusb0_usb.h from the libusb package to c:\mingw\msys\1.0\include\usb.h
(if your libusb-win32 version is 1.2.4 or older, then copy include/usb.h to c:\mingw\msys\1.0\include\usb.h)
- copy lib/gcc/libusb.a from the libusb package to c:\mingw\msys\1.0\lib
- start the mingw shell from programs->mingw->mingw shell
- compile the code with

gcc -o controlio controlio.c /lib/libusb.a -I/include

- connect the USB device to the computer
- start bin/inf-wizard from the libusb package
- select the connected device from the list, then create and install a driver for it
- find the executable controlio.exe under c:\mingw\msys\1.0\home\<user> where user is your user name
- check operation with the command

 controlio getport

Download

Download usbio.zip, contents:

directory	description
c/	source code for the USB host, please check section "controlling the I/O extender"
pic18f14k50/	source code and compiled HEX file (firmware) for the Microchip pic18f14k50 microcontroller

Planned improvements

make a package for openwrt
publish PCB for the SMD version
add options to the controlio command to select from multiple devices by serial/VID/PID
add analog A/D input capability to the firmware
add SPI capability to the firmware
add PWM output capability to the firmware
add a command to the firmware to store default port states and directions to use on power-up

Please contact me if you need a feature, or have an idea.

related projects

Multi-drop RS232/RS485 digital I/O extender at http://jap.hu/electronic/pbus.html
The PIC source code is based on the USB framework written by Bradley A. Minch.
You can find the original code at http://pe.ece.olin.edu/ece/projects.html
The USB framework was ported by me to the PIC18f14k50. The ported code is at http://jap.hu/electronic/pic18-usb.html
Teensy, an Atmel AVR based board for similar purposes

References

Microchip PIC18F14k50, PIC18f13k50 microcontroller datasheet
OpenWrt, Linux for embedded devices
USB connector pinouts
USB made simple, tutorial
USB in a nutshell, tutorial
USB Complete - The Developers Guide, Jan Axelson, book
USB Design by Example, John Hyde, book
wireshark, monitor USB packets

feedback

Please contact the author if you found an error, incomplete or missing documentation, or you have trouble making this circuit work.