Yes. The way this is done is to make the oscillator's frequency dependent on the parameter you want to measure (normally a voltage) and the A/D conversion is done by measuring the osc. frequency. There are temperature sensors and accelerometers that provide output as a varying frequency so they can be fed straight into a digital input.I've been playing with a Maxim temperature sensor that works this way and it works really well.
Alternately, there is an old technique that generates a PWM-like output that is added with the signal-under-test (the thing you want to measure :-) and the output of that addition is monitored by an input pin. Measure the on vs. off time (PWM) at the input pin and you have a fairly accurate ADC. Low resolution, but it works.
But honestly, with so many precise, accurate and cheap ADCs available these days, doing conversions like this in software is relegated to designs where pennies are significant and the manufacturing costs dwarf the development time.
Alternately, there is an old technique that generates a PWM-like output that is added with the signal-under-test (the thing you want to measure :-) and the output of that addition is monitored by an input pin. Measure the on vs. off time (PWM) at the input pin and you have a fairly accurate ADC. Low resolution, but it works.
But honestly, with so many precise, accurate and cheap ADCs available these days, doing conversions like this in software is relegated to designs where pennies are significant and the manufacturing costs dwarf the development time.