Continued from my last post, the serial interface for the antenna rotator is now up on github.

The RX line on the serial header passes through a simple inverting level-shifter using an NPN transistor and a handful of resistors. This line also has a TVS diode tied to ground to protect it from electrostatic discharge or whatever might befall it during use.

The heart of the interface is an Attiny13 microcontroller which handles receiving and transmitting commands. The microcontroller's tiny and cheap and doesn't actually have a hardware UART interface, but its possible to use the USI system (Atmel's name for SPI) for half-duplex UART communication. According to Atmel's appnote on the subject the USI system should first be inactive and a pin interrupt should be used to detect the start bit condition. The interrupt service routine should then enable the USI system and switch its clocksource from the normal slave-clk pin to an internal timer calibrated to run at the same speed as the as the serial data being sent from the computer. Once all 8 bits and the stop condition have been received the USI system switches back off.

From there its just a simple process of matching up the received ASCII character to a table of commands. One pin on the microcontroller is connected to the control box's IR RX line (the actual IR receiver must be removed to prevent shorting) and data is sent across to the microcontroller inside the control box.

It should be noted that IR transmissions are usually sent on a modulated carrier (frequently 28kHz) and the overall waveform looks like a square wave made up of smaller square waves. The IR receiver is tuned to this specific carrier frequency, which prevents natural infrared radiation from interfering with the control signals. The signal that emerges from the IR receiver is stripped of this carrier frequency and looks like a normal square wave containing the binary data being sent to the receiver (usually a few address bits and a few data bits). By bypassing the IR receiver, my microcontroller will be able to send data without having to generate and modulate a carrier frequency.

The actual code that I'll be flashing to the Attiny hasn't been fully written yet. Until I know exactly what data is being transmitted by the remote control it won't make sense to try designing the program. In my next instalment I'll be probing the IR transmissions coming from the remote and coding the firmware to send commands to the control box.