Kyle L. Carithers
REU 2006
Sonar
The Devantech SRF05 is a sonar module that features two modes of operation and a four meter maximum range. One mode uses two digital pins on a microcontroller. One pin is a trigger pin and is held high for 10µs to trigger sonar. The second pin is the output of the sonar and is held high for a length of time proportional to the distance the sonar is away from an object. In this mode, a SRF05 is a drop in replacement for the previous SRF04 sonar module. Aside from a led that blinks every time the sonar is triggered, a feature very useful when debugging, the SRF05 offers an additional feature not found on the SRF04, a second mode of operation. In this mode, the trigger line and output of the sonar are sent on a single line to and from the microcontroller. This mode requires the microcontroller to switch the pin from an output for the trigger to an input for the response. This mode is beneficial because it allows each sonar module to be placed on a single pin. Additionally, each sonar module requires only three wires, VCC, GND, and signal.
Although the SRF05’s second mode of operation is convenient, the first mode was used in conjunction with a digital eight to one multiplexer from Texas Instruments. Port A on the MAVRIC was used to trigger the sonar. The output lines of all five sonar modules were connected to the multiplexer, or mux. The mux used three input pins on the MAVRIC to determine which sonar signal to send to the MAVRIC. The output of the mux was connected to an external interrupt pin on the MAVRIC. The mux could have been bypassed and all five of the sonar could have been connected to separate external interrupt pins. There are eight external interrupts though and many share the same Atmega128 pins as critical timer related functions such as serial transmission. The limited number of available external interrupts necessitated the use of the multiplexer. Using the sonar modules in the second mode along with a multiplexer/de-multiplexer may be possible. This would decrease the required number of pins to as few as four for up to eight modules.