Maxim Integrated MAXQ7667 - Manual (Page 285)

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MAXQ7667 User’s Guide
SECTION 17: ULTRASONIC DISTANCE MEASUREMENT
MODULE—BURST TRANSMISSION AND ECHO RECEPTION
The MAXQ7667 was developed for ultrasonic time-of-flight distance measurement. To accomplish this task several hardware blocks
were added to the basic MAXQ. These blocks can be divided into two functioning units: burst transmission and echo reception.
Typically, the burst transmission stage sends out an ultrasonic sound wave to a surface, from which the distance is to be measured,
and the echo reception stage listens to the reflected burst frequency (echo). The echo reception stage is synchronized to the burst fre-
quency and only processes the echo signals that are of the same frequency as the burst signal.
The burst transmission stage has the following features:
• The burst frequency signal can be generated using the system clock (SYSCLK) or the phase-locked loop (PLL) oscillator.
• The PLL is programmable and allows any frequency between 18.8kHz and 1.67MHz (with a worst-case resolution of 0.13%).
• Selectable polarity of the output pulses.
• Programmable duty cycle.
• Programmable number of pulses (1 to 255) in a single burst.
• The burst output pin can be set to a three-state mode.
The echo reception stage has the following features:
• The circuit is synchronized with the burst transmission unit to receive echoes at the burst frequency for burst frequencies in the
range of 25kHz to 100kHz.
• Detects echoes down to 10µV P-P .
• Programmable amplification with 23.5dB range.
• 16-bit sigma-delta ADC to digitize the echo signal.
• Bandpass filter (BPF) to reduce noise.
• Full-wave detector and lowpass filter (LPF) (with several stages) produce a clean echo envelope.
• FIFO stores up to 8 LPF readings.
• FIFO can be loaded by software, LPF data ready, or a timer.
• Digital comparator with interrupt to indicate when output of the LPF exceeds a programmable threshold.
• Simulated echo signal (derived from burst signal) for testing and debugging.