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Analog Sensor with digital input?

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http://forum.allaboutcircuits.com – I have a TI cc2540 to which I would like to attach a variety of photoresistors. The cells resistance decreases with light intensity. I would like to trigger a specific action when the resistance decreases past a certain threshold. I am already using the analog inputs on the device for other purposes. Yet it occurs to me that it might be possible to use the pwm digital inputs as all I am trying to accomplish is a simple on/off type activity. (Yes, I know I could use an ADC for this purpose, however I am trying to minimize chip count and cost. ) The range on the photo resistor is 135 Ohm (Embedded)