PIC Tutorial Twelve - RF Remote Control

For these tutorials you require two Main Boards (16F628), one main board (16F876), LED Board, Switch Board, Joystick Board, RS232 Board, Analogue Board and LCD Board. Download zipped tutorial files.

This tutorial is about using licence free radio modules, most countries now have such modules available at low cost, although the exact frequencies may vary between different countries. If you've tried to send plain RS232 via these modules, you've probably found that it doesn't work?, I found this a number of years ago, and a quick check with a scope soon showed why - the modules are AC coupled, so you can't send a long continuous HIGH level, after a fairly short period it breaks up and falls LOW. The standard RS232 sends a continuous HIGH level when there's no data being sent, with a drop to LOW being the START bit, so this method soon falls foul of the AC coupling - a crude solution, (which I used with my original modules years ago) is to invert the RS232 - hence the normal condition is LOW, and a START bit goes HIGH. To do this using the hardware USART requires an external hardware inverter, but for a software UART you can simply do the inversion in the software (which is what I did).

As I suggested, this is rather crude, and while it worked well for the very short range I was using it for, it's not really recommended, but there are a number of ways to deal with the situation - the most common of which is 'Manchester coding', and it's this that we'll be dealing with in this tutorial. While RS232 works on the width of individual pulses (9600 baud being 105uS wide for each pulse) Manchester coding works on transitions from HIGH to LOW (bit 0), or LOW to HIGH (bit 1), the actual width of the pulses doesn't matter particularly (within reason) - this is good, because neither IR or wireless preserve the exact width of the transmitted pulses. The popular Philips RC5 IR remote control systems actually use this same coding scheme.

This is the way the bits are actually sent, a bit 1 is a transition from 0 to 1, and a bit 0 is a transition from 1 to 0.

I'm basing these tutorials on the routines by Peter JAKAB from http://jap.hu/electronic/codec.html, incorporating his basic encoding/decoding code within these tutorials, please visit his site and check his remote controls based around them.

The first thing we need to discuss is the actual transmission format, we can't just send a single byte like we do for RS232, radio transmission isn't reliable enough, and if it misses the start bit then the complete byte will be useless. So it's normal to use a PACKET system, where a number of different pieces of information are transmitted after each other in the form of a 'packet', this consists of a number of different sections:

Header - This consists of a row of 20 bit one's, followed by a single bit zero. The long sequence of 1's gives the receiver time to settle, and the decoding software time to synchronise.

This is what the header looks like, with the signal level originally being sat at zero, and the first transition HIGH being the first bit ONE. It's the actual transitions between levels that signify the data bit, NOT the width of the pulses. Notice that the first 20 ONE's are a really nice single frequency, this property of Manchester coding allows you to recover the original clock speed from an asynchronous data stream.

Address - This consists of the 8 bits of data in the byte, followed by a single bit one, and a single bit zero.

This is what the Address byte (and all the following bytes) looks like, this particular byte has the binary value '11100110', the two extra bits at the end are always ONE and ZERO.

Data - This is the same as Address, 8 bits followed by a 1 and a 0.
Checksum - This is also the same as Address, 8 bits followed by a 1 and a 0.

This gives a total of 52 bits to send just a single 8 bit data byte, but by transmitting more than one byte in a packet this can be improved, two bytes will take only 62 bits, or four bytes only 82 bits.

This tutorial comprises a transmitter unit (requiring one 16F628 main board and the wireless transmitter board - Tut12_1T), and two receiver units, the first outputs the received bytes via RS232 to your PC (requires one 16F628 main board, one RS232 board and the wireless receiver board - Tut12_1R), the second is essentially the same, but rather than using the PC it uses an LCD display (requires one 16F628 main board, one LCD board and the wireless receiver board - Tut12_1RL). This tutorial uses a single byte packet, so only one byte is sent at a time - the transmitter simply sends a short text string one byte at a time, separated by a short delay.

The Wireless Boards connect to PortA, and the LCD or RS232 Boards connect to PortB.

This tutorial comprises a transmitter unit (requiring one 16F628 main board and the wireless transmitter board - Tut12_2T), and two receiver units, the first outputs the received bytes via RS232 to your PC (requires one 16F628 main board, one RS232 board and the wireless receiver board - Tut12_2R), the second is essentially the same, but rather than using the PC it uses an LCD display (requires one 16F628 main board, one LCD board and the wireless receiver board - Tut12_2RL). This tutorial uses a four byte packet, so four bytes are sent at the same time - the transmitter simply sends a four byte string, which repeats after a short delay.

The Wireless Boards connect to PortA, and the LCD or RS232 Boards connect to PortB.

Notice that the first two tutorials are pretty well identical, with only the packet size being changed, this is done by simply altering the buffer size, and changing the packet size defined near the beginning of each program (packet_len EQU 2) - please note, the packet_len constant includes the address byte, so needs to be one greater than the data packet size.

This tutorial comprises a transmitter unit (requiring one 16F628 main board, a switch board and the wireless transmitter board - TUT12_3T), and one receiver unit (requires one main board, one LED board and the wireless receiver board - TUT12_3R). This tutorial uses a single byte packet, so only one byte is sent at a time - the transmitter simply sends the packet while a transmit button is pressed, different buttons send different data bytes, and you can turn the receiver LED's ON and OFF, two buttons turn them ON, and the other two turn them OFF - a simple radio remote control.

The Wireless Boards connect to PortA, the Switch and LED Boards connect to PortB.

This tutorial comprises a transmitter unit (requiring one 16F628 main board, a switch board and the wireless transmitter board - TUT12_4T), and one receiver unit (requires one main board, one LED board and the wireless receiver board - TUT12_4R). This tutorial uses a single byte packet, so only one byte is sent at a time - the transmitter simply sends the packet while a transmit button is pressed, different buttons send different data bytes, but this time you can toggle the receiver LED's ON and OFF - a simple toggling radio remote control. This gave me a few problems though, initially I tired to use exactly the same transmitter code as the previous example, but I was struggling to identify a new key press of the same button (particularly as they repeat whilst held down). Eventually I hit on the idea of modifying the transmitter code as well - so while you hold the button down the same code repeats, but when you let it go, a different code (actually 0x00) is sent to inform the receiver the button has been released. Incidentally, I mentioned the Philips RC5 IR remote system above, this actually uses an extra bit to signify a new button press or just a repeat signal while the button is held down - rather than implement that type of system I decided on a 'release' transmission instead.

As in the previous tutorial, the Wireless Boards connect to PortA, the Switch and LED Boards connect to PortB.

This tutorial comprises a transmitter unit (requiring one 16F876 main board, the analogue board and the wireless transmitter board - TUT12_5T), and one receiver unit, that outputs the received bytes via RS232 to your PC (requires one 16F628 main board, one RS232 board and the wireless receiver board - TUT12_5R). The transmitter board reads the two analogue inputs, then transmits the two 10 bit readings as four hexadecimal bytes (just as the A2D provides them), it does this repeatedly, with a short delay between each transmission. The receiver converts the two 10 bit values into decimal, and sends them to the PC over RS232 as ASCII characters, separated by a comma, and ended with a CR/LF - this will display nicely in HyperTerminal, and is easily imported into many PC programs. This tutorial uses a four byte packet, so the four bytes are sent in a single packet.

On the transmitter the Wireless Board connects to PortB, and the Analogue Board to PortA (because the Analogue Board MUST use PortA), on the receiver the Wireless Board connects to PortA, and the RS232 Board to PortB.

PIC Tutorial Twelve - RF remote control.