Nuts and Volts - March 2018

To detect the state of a button, the I/O line to that button is brought low, while all remaining I/O lines in the LED matrix are left in the high-impedance input state.

For example, to read S1, CP01 (which is connected to S1 via D1) would be set to low output. If the button is not pressed, 2.5V from the voltage divider would be fed through R8 into CP04. The LED, DM03 which is CP04, will cause the voltage divider output to drop to the operating voltage of that LED — between 2.0V and 2.4V.

This voltage appears at the microcontroller at RA2 via R4. The current going through DM03 is greatly limited by R6 and R8, and since the button is read in a few microseconds, not enough current will flow through the LED to make it appear to illuminate to the human eye.

If S1 is pressed, current through D1 will force the output of the voltage divider to drop to 0.7 volts. That voltage is passed through R8 and R4 and appears on RA2. So, to determine if a button is being pressed, the microcontroller brings that button’s I/O line low and then looks at the resulting voltage on RA2.

If it’s around 0.7 volts, then the button is being pressed; if it’s two or more volts, then it is not. Each button is read one at a time by bringing that button’s line low while leaving the other Charlieplexed lines in their high-impedance state.

The microcontroller has a built-in comparator and a built-in programmable voltage reference that can be used to detect the small voltage change. I programmed the microcontroller so that RA2 is connected to the inverting (-) input of the comparator, and the noninverting (+) input is connected to the programmable voltage reference. I set the voltage reference to 1.042V.

If the button is pressed, the voltage at the I/O pin is 0.7V and the comparator’s output goes high. If the button is not pressed, the input voltage is at least 2.0V and the comparator’s output goes low. The output of the comparator is read as a 1 or 0 by the microcontroller.

Time’s Up

That wraps it up for this month. In Part 2, we’ll finish the circuit, make the printed circuit board, build the clock, and take a look at how the software works. In the meantime, you can find the software source files with the article downloads if you want to take a peek. See you next time! NV

30 March 2018

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