2  Transmitter
Figure 2 below shows a circuit diagram for the transmitter portion of the communication system.  The MING transmitter board has only three input pins (+V, Ground, and Data Line).   The MING systems comes pre-tuned at 300MHz and ready to operate out of the package.  The MING transmitter/receiver units are designed to accept RS232 serial data, but with a few modifications, they can be easily interfaced to the MCU of your choice as we shall see below for the MIT Handy Board.

In Figure 2, the top transistor circuit is simply an inverting circuit that inverts the transmit data from the Handy Board microcontroller to the MING transmitter module.  Power the MING system with standard +5V and add a 0.1uF bypass capacitor to smooth out any possible voltage spikes coming from the power source.  Notice that the optional LED indicator cirucit is not required for proper operation.  The LED in the circuit flashes when data is being transmitted by the MCU, which is useful for debugging purposes.  To transmit data, have either the Handy Board or other MCU send out data at 1200 baud with 1 stop bit and no parity.   The range of the RF transmitter/receiver system is roughly 35-70 feet depending if you are indoors or outdoors--outdoor range is higher.  Sample code for the Handy Board is included at the bottom as well as assembly code for the Motorola HC11 MCU.

Figure 2.  RF Transmitter Circuit for the Handy Board

2.1  Sample Tx code for the MIT Handy Board
I highly recommend that the following code be downloaded into the Handy Board and used for testing.  The sample code simply configures the serial transmit line for 1200 baud and sends out an 8-bit number from 0 to 255 every second and then repeats.  Use this code with the receiver below unit for testing.

/*    TX.C  Kam Leang  1999  Serial Transmit Program for HB*/
void main(){
        int i;
        printf("Tx: Press Start\n");
        while(!start_button()){}     /*Wait for user to push start button on HB*/
        poke(0x102b,0x33);         /*Set serial baud rate to 1200*/
        i=0;        /*Initialize counter*/
        while(1){    /*Infinite loop for transmitting*/
            if (i>255) /*Reset the counter when i>255*/
                i=0;
            while (!(peek(0x102e) & 0x80)){}  /* Wait for serial transmit register to be empty */
            poke(0x102f, i);  /* send character */
            printf("Tx Char=%d\n",i);    /*Print Tx value to LCD*/
            i=i+1;    /*Increment counter by 1*/
            sleep(1.);  /*Delay for 1 second*/
            }
}

3  Receiver
Figure 3 below shows the circuit diagram for the RF receiver module and the additional external circuitry required for interfacing it to the MIT Handy Board microcontroller.  Similar to the transmitter module from above, the top transistor circuit inverts the received RF serial data from the MING Rx module before going into the Handy Board.   The optional LED indicator circuit is used for bebugging and testing.  The LED flashes when data is being received by the RF module.  The range of the receiver can be increased by incorporating an antenna based on the manufactures specifications. Sample code for both the MIT Handy Board microcontroller and Motorola 68HC11 MCU is included below.

Figure 3.  RF Receiver Circuit Diagram for the Handy Board.

3.1  Sample Rx code for the MIT Handy Board
The sample code below simply waits for a data and then displays it on the LCD.  Download the following code into the Handy Board to test RF receiver module.

/*    RX.C  Kam Leang  1999  Serial Transmit Program for HB*/
void main(){
        int i;
        printf("Rx: Press Start\n");
        while(!start_button()){}     /*Wait for user to push start button on HB*/
        poke(0x3c, 1);    /*Disable pcode character receive mode*/
        poke(0x102b,0x33);         /*Set serial baud rate to 1200*/
        poke(0x102d,0x04); /*Enable receiver*/
        i=0;    /*Initialize counter*/
        while(1){ /*Infinite loop*/
            while (!(peek(0x102e) & 0x20));     /* Loop until character is received*/
            i=peek(0x102f);    /*Transfer data from receive buffer*/
            printf("Char Rx=%d\n",i);    /*Display character on LCD*/
            sleep(.2);    /*Delay 0.2 seconds*/
        }
}

4  References
The author would like to acknowledge the following sources of information related to this article.

1. Sheldon, Ryan Naked Data Nuts & Volts Magazine, May 1998, pp. 85-89
2. Motorla M68HC11 Reference Manual Rev. 3 M68HC11RM/AD, Motorola 1991
3. The MIT Handy Board web site  http://handyboard.com

1. Digikey  MING Microsystems distributor
2. Radio Shack  Basic electronic components such as resistors, capacitors, LEDs, etc.
3. Gleason Research  Preassembled Handy Board distributor.
4. Douglas Electronics  Distributor of blank Handy Board PCBs