Help me take a look at this circuit about ADC0809! ! ! ! ! !

Two 74LS74 are used to divide the ALE signal of the microcontroller by 4 and serve as the working clock of ADC0809. Because the maximum operating frequency of ADC0809 is 1.28MHz, the frequency of ALE is too high for it, so it needs to be used after frequency division.

74LS02 is a NOR gate, and the logical relationship is "only when both inputs are 0, the output is 1." In this circuit, one NOR gate controls the start of AD conversion (START and ALE), controlled by P3.6 of the microcontroller, and the other NOR gate causes the conversion result to appear on the data bus (OE), controlled by P3.7. These two NOR gates are controlled by the P2.7 pin of the microcontroller at the same time. The specific reason for doing this depends on the software.

I can’t help but say that ADC0809 was made 30 years ago, and I’m afraid you can’t buy it in the electronics market now. I didn’t expect it is still taught in universities. No wonder it is out of touch with work needs. If you want to make something, it's best to take a look at the current chips. In fact, even the AD converter built into the microcontroller is much better than this.