This Design Idea describes a way to increase the number of analog inputs to your microcontroller for cases in which adding an analog-multiplexer chip or upgrading to a microcontroller with more inputs might be impractical. If the microcontroller you're using has some spare I/O pins and at least one of them is bidirectional or is amenable to tristating, you can configure a simple analog multiplexer using switched capacitors. Figure 1 shows a two-input multiplexer. A typical switched-capacitor multiplexer completely disconnects the capacitor from the sensed voltage before sampling the voltage across the capacitor.

To use a typical microcontroller's I/O ports, one terminal of the capacitor remains connected to the input source through a resistor. During most of the operating time, pins 12, 14, and 15 are configured as output pins and are held at logic 0. Diodes D₁ and D₂ do not conduct, so capacitors C₁ and C₂ charge to the values of the input voltages V₁ and V₂, respectively. To sample the voltage stored in the capacitors, pin 12 becomes an input, and the pin associated with each channel switches high while the microcontroller's comparator compares the voltage on Pin 12 with the reference voltage. Listing 1 gives the code fragment that samples the inputs.

The voltage on Pin 12 is V_PIN12=V_DD–V_DIODE–V_IN, where V_PIN12 is the voltage applied to the analog input of the comparator; V_DD is the power-supply voltage (5V in this example); V_DIODE is the voltage across the diode, and V_IN is the voltage applied to input of the RC filter. During the sampling of one input, the voltage on the positive terminals of the capacitors exceeds V_DD; thus, D₁ and D₂ are in series with microcontroller pins 14 and 15 to block voltages above V_DD and prevent C₁ and C₂ from discharging into the power supply. Also during sampling, C₁ and C₂ are in series with the filter resistor of the input undergoing sampling, causing the capacitors to discharge through the resistor. For this reason, it is important to keep the RC time constant with respect to the sampling period. The worst-case voltage error occurs in the second channel to be sampled, when both V₁ and V₂ are at 0V:

where T_SAMPLE is the time one of the diodes' anodes switches to V_DD (3 µsec in this example). This sampling time uses the assumptions R₁=R₂, C₁=C₂, and the fact that the sampling periods for each channel are the same.

With the 1-MHz controller in Figure 1, sampling time is a total of 6 µsec for the two channels; using a 16-MHz controller, the total sampling time would be only 375 nsec. When you expand the circuit for more inputs (for example, using the four-input multiplexer in Figure 2a), you must take the extra sampling time into account. To maintain a low duty cycle and thus allow the RC filters to charge to the full input voltage, the software should infrequently call the sampling routine. An interrupt every 2048 clock cycles calls the sampling routine in this example. The voltage at Pin 12 in Figure 2a is inverted, and, because of the isolation diodes, the maximum input voltage is a diode drop below V_DD (approximately 4.4V). If you multiplex both inputs, the circuit compensates for both the polarity and the diode drop (Figure 2b). Listing 2 gives the microcontroller assembly code for the multiplexer scheme.

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