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　　详细图片(PDF, 325kB)

　　图5. 输出曲线图示例

　　附录A. 软件例程

　　MAXQ7667传感器校准文件

　　// This routine measures the transducer damping at the specified frequency (PLLfreq).

　　// The system timer is used to measure when the LPF output (LPFD) has dropped to 1/2 the peak value.

　　// This is the settling time that will be used later when doing the frequency sweep.

　　unsigned damping_half_time (unsigned PLLfreq, unsigned pulse_width)

　　{

　　unsigned short i;

　　unsigned short peak = 0;

　　unsigned short half_peak = 0;

　　unsigned short temp = 0;

　　SCNT_bit.STIME = 0; // Make sure system timer is off.

　　STIM = 0; // Clear the system timer.

　　SCNT_bit.STDIV = 4; // Set system timer prescale divider to 16 (1µs per cycle).

　　PLLF_bit.PLLF = PLLfreq; // Set the PLL frequency.

　　BPH = pulse_width; // Pulse width = BPH/(receive frequency * 400) when BDIV = 0xC.

　　usWaitTimer2(10000); // Let the PLL settle for 10ms.

　　SCNT_bit.STIME = 1; // Start the system timer.

　　BPH_bit .BSTT = 1; // Send a burst.

　　usWaitTimer2(50); // Wait for LPF to partially settle.

　　RunTimer0_us(20); // Start timer # 1 with a reload time of every 20µs.

　　for (i = 0; i 200; i++)

　　{

　　temp = LPFD; // Read the output of the lowpass filter.

　　if (temp > 0x2000 temp > peak) {peak = temp;} // Save the peak value if it is greater than 2000.

　　if (temp peak/2 half_peak == 0) // If LPFD is less than half the peak value and the half peak time

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