Are you sure you're generating the proper timing? What is the duration of nWR and nRD from the 18 MHz 805x? Does that generate a long enough START strobe? What about the relationship between OE and the nRD strobe? Does that allow data to propagate before nRD goes away?

Have you measured any of these timings?

My recollection of the standard 12 MHz 805x is that it barely produces 150 ns nRD and nWR strobes (about two 12 MHz clocks) and from that I'd deduce that AT89C51 clocked at 18 MHz might come up just a bit short.

Please describe these timing measurements. Before you do that, however, please examine the AT89C51 datasheet and compute the CS, START, and OE strobe duration and timing with respect to the 18.432 MHz AT89C51 controls. Are the numbers satisfactory with respect to the timing restrictions described in the ADC0808 datasheet?

RE