3.2 Synchronized Transmit and Receive Masking Signals with Timer/Counter Type D
In order to effectively mask the PWM transmission from the receive line, correct synchronization of the transmit and receive masking signals is necessary. This is possible by using the "One Ramp" Waveform Generation mode of Timer/Counter Type D (for more information, refer to the device data sheet). As can be seen in the figure below, the compare values can be configured to generate two synchronized outputs able to be used as masks to control transmission and reception utilizing the transducer.
The different compare values have the following roles and should be
customized according to the hardware being used:
- The CMPASET value is set to
'
0', indicating that the receive line should be disabled from the beginning of a measurement cycle (when an ultrasonic burst is being transmitted) - The CMPBSET value indicates the length of a transmission. Decreasing this value will decrease both the minimum and maximum ranges. Increasing this value will increase the range to a certain point, however, once there is a part of the reflected burst with no attenuation, no advantage is gained from increasing transmission time.
- The CMPACLR value indicates when the receive line is enabled. This value should be customized according to the hardware being used, and the situational requirements. It should be long enough that resonance due to nonoptimal hardware is considered, and short enough that the reflected burst is not masked along with the transmission signal.
- The CMPBCLR value defines the length of a measurement cycle. This should be long enough that all reflected signals have been sufficiently attenuated (some signals can bounce back and forth between the sensor and the barrier several times). Decreasing this value will also decrease the resolution of measurement calculations (for example, CMPBCLR = 0xFFF gives a distance measurement resolution of one cm).