IR Remote Control to PS/2 Keyboard Adapter

PS/2-IR is a receiver for infrared remote controls masquerading as a standard PS/2 computer keyboard. With this small and cheap device, one can use a common TV/DVD/SAT or universal remote control to control a desktop computer or HTPC.

Although any wireless input device may be used to control a media center PC, few if any can be used to actually power on the computer. This is where PS/2-IR comes in. Although the PS/2 port is a legacy interface, with many mainboards it is the only option that allows the computer to be powered on via the keyboard. Another advantage is that it can reliably be used to navigate the BIOS setup or a bootloader menu.

The PS/2-IR adapter supports any infrared remote control using the common NEC or Extended NEC protocol with a carrier frequency of 38 kHz. Any IR function code sent by the remote can be mapped to an arbitrary PC keyboard scancode. Mainboards that support power-on by PS/2 keyboard will boot up the system in response to the ACPI Power On scancode.

The PS/2-IR adapter is based on the tiny Microchip PIC12F1571 microcontroller. Although relatively recent, it is one of the cheapest 8-bit microcontrollers on the market and very easily available.

The infrared sensor is a standard TSOP4838 with integrated pre-amplifier and demodulator. Its power input is connected through a simple RC filter.

The complete circuit is powered directly from the 5V line the PS/2 interface, with just two bypass capacitors added. Although nominally 5V, it should also work just fine with non-standard 3.3V PS/2 interfaces.

The layout fits comfortably on a tiny and cheap 32x30mm 2-layer PCB. Mounted in a translucent case, the device does not even need an opening for the IR sensor.

The firmware is written exclusively in assembler. Components are implemented in separate source files, and assembled into relocatable objects combined by the linker. The GNU PIC assembler was used for development, but MPASM should work as well.

The IR decoder is completely interrupt driven, making use of a hardware timer to measure pulse duration. The internal low-power RC oscillator is used to clock the timer, which allows the device to go to sleep during measurements. In order to cope with the low accuracy of the uncalibrated RC oscillator, the bit detection allows for fairly large tolerances in timing. This also helps to ensure compatibility with various models of remote controls.

The PS/2 synchronous serial I/O is implemented in software, by bit-banging tri-state GPIO pins. Transmission is done at a clock rate of 25 kHz, using busy delay loops to control the signal timing, with compensation for cycles spent by sourrounding instructions. The CPU clock rate of 4 MHz has been selected to minimize the number of delay cycles needed. During bus idle time, a pin change interrupt is set up to detect host requests, allowing the CPU to be powered off.

The PC keyboard control state, command protocol and scancode generation are implemented in a separate component, layered on top of the serial communication interface. Although this firmware implements only a subset of the PC keyboard functionality, all standard control commands are recognized and will elicit a response in the expected format. Most commands which control unavailable functionality are silently ignored. One exception is the command to select the scancode set, which will be responded to with an error code if an attempt is made to set a scancode set other than set 2.

A look-up table with 256 positions is used to map IR function codes to PC keyboard scancodes. This table is defined in its own separate source file so it can easily be exchanged for another, enabling different types of remote controls to be supported with minimal changes. In addition to the function code mapping, this remote control definition file also sets the device address used by the remote.

In addition to the base scancode, each entry in the look-up table also allows for various bits to configure the scancode type, whether to send break codes on key release, and which modifier key scancodes to send along with the base scancode.