I think the scheme is made unnecessarily complex.

I also dont know why a external latch is required when it is built into the ADC itself. All that is required is this :

Step 1: Assert the 3 bits based on the channel that you want to read.

Step 2: Pulse the ALE to lock in this channel address into the ADC.

Step 3: Pulse the SOC bit to start a conversion. When using ADC 0808 normally it is easier to tie both START and ALE and pulse them with a common signal like the chip -select in a memory mapped I/O or directly from a port pin otherwise.

Step 4: Either use the EOC bit of the ADC or allow a fixed delay and read data.


Thats all there is to it.

Raghu