Signal detection circuit design principle

The signal detection circuit is shown in Figure 3 (a), and the waveform is shown in Figure 3 (b):

In Figure 3 (a), LM393 and other components form two Schmitt voltage comparators, which are used to detect the zero
points of two AC signals respectively. The two voltage signals that are approximately sinusoidal are converted into square wave signals, as shown in waveforms A, B, C, and D in Figure 3 (b).
The circuit composed of D1 and D2 triggers (a 74HC74) completes the detection of periodic signals under the control of microcontrollers P1.0 and P1.1. When P1.0 = 0, Q1 = Q2 = 0; when P1.0 changes from 0 to 1, and the signal at point B changes from 0 to 1, D1 flips, Q1 = 1, and Q2 is still 0 at this time. When the signal at point B changes from 0 to 1 for the second time, D1 flips again, Q1 = 0, and D2 also flips, Q2 = 1. When 80C31 finds P1.1 = 1, it sets P1.0 = 0 and completes a detection. Its waveform is shown in Figure 3 (b) B, P1.0, T, Q2. This circuit detects the rising edge and improves the detection accuracy.

The circuit composed of D3 and D4 triggers (a 74HC74) completes the time difference signal detection under the control of the single-chip microcomputer P1. 0.
When P1. 0 = 0, the circuit does not work, Q3 = Q4 = 0; when the signal at point B changes from 0 to 1, Q3 = 1, Q4 is still 0; when the signal at point D changes from 0 to 1, Q4 = 1, and Q3 is cleared at the same time, thereby detecting the time difference between the two signals.

The waveforms are shown in B, D, P1.0, Δt in Figure 3 (b).

According to the definition of phase difference and the staggered half-tooth installation of the sensor,
the relationship between the phase difference Δ< of the two signals and the period T and time difference Δt is: Δ< = (360°/ T) Δt - 180°. Using the T0 and T1 timers inside the 80C31, T and Δt can be calculated more accurately. Specifically: set T0 and T1 to the internal counter form (C/ T = 0), work in mode 2 state, GA TE = 1, TR = 1, so that when TN T0 and TN T1 = 1, T0 and T1 count, using a 12MHz crystal oscillator, each count time is 1μs. In the interrupt service, use R7 and R5 to record the number of T0 and T1 interrupts to expand the counting range, and finally obtain T and Δt:
Δt = R7 ×28 + (TL 0)
T = R5 ×28 + (TL)