The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 9
Figure 9-3. The comparator ’s output switches at the light level determined
You can use the same basic circuit with other sensors that vary in resistance. Replace the
photocell with your sensor, and adjust R2 for the switching level you want. Connect VOUT
to any input port pins.
Measuring Analog Signals
Sometimes you need something more sophisticated than a simple level detector. An
analog-to-digital converter (ADC) enables you to measure the precise value of an analog
voltage.
Some versions of the 8052 microcontroller, including Philips’ 80C562, include an on-chip
ADC, but the 8052-BASIC doesn’t have this feature, so you have to add it externally. There
are dozens of converters available, with varying resolution, accuracy, speed, method of
conversion, number of analog inputs, and so on. Another option is to use an integrated sensor
that contains its own ADC and has a digital output.
National Semiconductor’s ADC0848 is an easy-to-use, low-cost, general-purpose, eightchannel
ADC. In many ways, the ADC0848 is similar to National’s long-popular
ADC0808/9 A/D converters, but with some advantages. The ADC0848 does not require an
external clock; its control signals interface directly to many microcontrollers; and it is faster,
with a typical conversion time of 30 microseconds.
National’s data sheet for the ADC0848 has complete specifications, applications information,
and example circuits. You’ll want a copy of the data sheet if you plan to use the chip.
158 The Microcontroller Idea Book