Revision 1.0
Thank you for your purchase of the SCRWL-COMM secure wireless communications device. Featuring advanced, time-tested processing hardware and an extended range, your purchase assures you the ultimate in affordable secure wireless communications.
Please follow the instructions listed below to operate your new transceiver.
Safety notes:
Do not operate this transceiver while driving
Do not subject the communication device to temperature or humidity extremes.
Using a power supply other than the approved listed power supplies may result in damage not covered by the warranty
This communications device does not comply with FCC regulations. Please seek FCC approval before use
SCWRL-COMM Inc. and Vanderbilt University assume no liabilities for the consequences of situations in which the security codes have been defeated.
Initialization
1.
Power the speaker and microphone controllers with a 12-V/Ground DC line.
2.
Power the DAC with an +/- 18V reference voltage.
3.
Power the microcontroller board with a 5VDC line
4.
The device is now ready.
Sending information:
Press and hold the push-button switch marked “send” to communicate. Use as a “push-totalk” system. Once you have completed your transmission, release the button.
Receiving information:
Your transmitter is automatically configured to receive data. Once acceptable data is received, the transmitter will automatically broadcast the transmission.

The wireless communications device is composed of the following systems:
1.
Serial transceiver
2.
Analog signal input circuitry
3.
analog output circuitry
4.
ADC/DAC and transmission control flow software
5.
RSA encryption software component
6.
Operating system kernel
Operational Concept
The system is designed to allow for wireless communications given input voice data. A standard telephone receiver is powered and the voltage differential from the microphone is voltage-divided and received by an ADC on the 9S12DP256 microcontroller. At this point, the data is encrypted, serialized and then sent via a 5V RS-
232 serial interface to a wireless transceiver device operating at 115200bps. Data received from the transceiver is decrypted and sent to a DAC by means of a parallel port on the microcontroller. The output form the DAC is amplified by means of a simple operational amplifier driver circuit.
Serial Transceiver
The serial transceiver used in each unit is a Parallax Sure-Link 900 wireless transceiver unit. These devices allow for serial-based interfacing at datarates of upto
115200bps. The device was connected to the system according to the diagram below:
The diagram to the left is for a
“point A” transceiver.

Analog input circuitry
The voice input system is based on a powered microphone setup in which a potential difference is applied to a condenser microphone. Changes in voltage are amplified by an LM386 driver IC. The diagram may be seen below.
Output Circuitry
The output from the DAC is fed to a simple driver circuit, as seen below in which an LM380 audio amplifier performs current output/buffering.

DAC
An external DAC (National Instruments 0808 8-bit DAC) is attached to port B of the 9S12DP256 microcontroller. The data direction register on port B is configured to allow for write access to the DAC. The schematic can be seen below.
Transmission Control software
The core of the transmit/receive program is based on serial read/write operations.
A reading task polls the ADC at specified intervals and then passes the read byte into the
RSA encryption function. This function then passes an encrypted value to the serial port, sending the data in RS-232 format to the transceiver.
The send-receive cycle can be seen below
1.
Read byte from ADC
2.
Encrypt
3.
Send to SCI1 serial port
This completes the read routine.
The receive/output task performs the inverse of the above routine, decrypting data from SCI1 serial input, then writing the result to an external DAC.
RSA Encryption algorithms
The RSA encryption algorithm uses a method of encryption relying on the difficulty of factoring two large relatively prime numbers. Due to overhead issues, this algorithm has been shortened to an 8-bit implementation.

Operating system kernel
To synchronize the sending/receiving and encryption tasks, the µCOS-II RTOS kernel was used as the basis for the real time system. Two tasks were created for the purposes of this project to minimize context-switch overhead. The first task was the read/encrypt/transmit, while the second task implemented a receive/decrypt/output task.