Hi Paul,
A great article entitled AN449 measuring resistance and capactance without an a/d by Don Sherman can be seen at the below address. This method uses a LM324 opamp set up as a comparator which is monitored by a 8051 port pin to determine when a rising voltage has passed a reference point. This article helped me when I was building a 8 bit 16 channel A/D using a 4067 CMOS IC and 89C2051. I ran into two big problems which I never solved. The offset of input opamp's were not compensated which left every channel with an output off zero reading (typically 0.123 VDC). The other problems was I was trying to get 4 digits of precision out of the measuring method which it could not produce. The value was bouncing around 35 millivolts. I took and nulled out both of these problems using software the customer was happy with the results. Years later I compared the results to a 12 bit A/D IIC IC and found it didn't give any better results.
The web site is http://www.semiconductors.philips.com/acrobat/applicationnotes/AN449.pdf
Regards,
Charles Bannister