This design idea creates a wireless universal serial bus (USB) presentation helper called the “Presentation Pal” that remotely “flips” the slides on your computer. The utility of this idea can be extended to other wireless devices that need to communicate with a PC through a USB serial port. The best part about this design idea is that no complicated device drivers need to be written for the PC.

When a short-range wireless design needs to communicate with a PC, often the serial method of choice has been the ubiquitous RS-232 serial port. An RS-232 serial port is relatively easy to implement from a hardware and software point of view. However, RS-232 serial ports are slowly disappearing from newer PCs and are being replaced by USB ports.

The USB serial port was designed for ease of use. The circuitry is simple. However, more programming effort is required by the designer for the embedded microcontroller and the PC. Perhaps the most complicated aspect in designing a USB device is writing the software driver for the PC.

To ease the burden of writing USB drivers, similar devices are grouped together into device classes. One particular device class called the Human Interface Device (HID) class supports keyboards, mice, joysticks or any device that transfers blocks of information at a low speed. Low speed in the USB specification is defined as a maximum speed of 1.5 Mbps. This is fast enough for a short-range wireless application.

The benefit of using the HID-device class for a USB device is that the software driver is already included in the operating system. This makes the task of writing an application program on the PC easier. Thus, we have two choices. We can write a custom application program that communicates with the HID-device class driver already installed on the PC. Or, we can emulate a keyboard and communicate with any application that takes its input from the keyboard. The latter choice is how the Presentation Pal works.

When plugged into a PC, the Presentation Pal enumerates as a keyboard. When a command to flip the slide comes from the wireless remote, a keyboard “page-up” to advance the slide or “page-down” to go to the previous slide command is sent to the PC.

To demonstrate the design idea, a PICkit 1 Flash Starter Kit was used as a low-cost USB development platform. The figure shows a block diagram of the entire system and a detailed schematic diagram of the USB circuitry. A PIC16C745 microcontroller unit with a USB peripheral is the central microcontroller for the Presentation Pal.

All the necessary circuitry required to implement a USB device is shown in the figure. As can be seen in the schematic, little hardware is required to implement a USB serial port. The PIC16C745 micro-controller unit was programmed to recognize push-button commands received by the remote transmitter and convert them into keyboard commands. The remainder of the Presentation Pal is an ASK receiver module based on the rfRXD0420 receiver IC that is plugged into the expansion header J3 of the PICkit 1 Flash Starter Kit. The transmitter module is constructed using the rfPIC12F675.

The goal of this design idea is to demonstrate that designing a USB device does not have to be complicated. The Presentation Pal avoided complicated software device drivers by using HID-class drivers already installed by the operating system. By enumerating as a keyboard, the Presentation Pal looks like a keyboard to the PC. Thus, any application that responds to keyboard commands can be used. Or, an application program can be written to communicate with the HID-class drivers. With either choice, the task of writing a custom USB driver is avoided. All that remains is design and development of the project that communicates with a USB serial port.

1. AN258 “Low-cost USB Microcontroller Programmer, The Building of the PICkit^™ 1 Flash Starter Kit,” Microchip Technology Inc. (http://www.microchip.com).

2. Jan Axelson, USB Complete, Lakeview Research, Madison, WI, Second Edition, 2001.

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