Take a standard 51 and build an RC based ADC yourself. (I've made it using atmel 89C2051 using internal voltage comparator).

You need additional voltage comparator ic, one resistor and capacitor. Measure the time needed to charge capacitor C to the same lavel as analog input voltage. Then comparator switches and you stop the timer.

It is very accurate and you can even increase the accuracy by increasing the measuring interval-charging time of C by increasing R.

For discharging C, you just connect it to one P0 pin (open drain, higher current capacity).

Take a look at www.atmel.com, I think they have an application note.
You also need to convert exponential C charging curve to linear - best do it with a table (for 8 bits 256 byte table is sufficient)

Franc