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Now that i’ve finished porting icky C++ code I can return back to thoughts of sweet sweet hardware design. Next up is a second MIDIsense board.
The next board is pretty simple: An analog and digital IO board. This is what most MIDI interface boards are used for. The atmega8 chip has 6 10-bit analog inputs so there will definately be 6 analog inputs. Then I decided to put in 5 digital I/O too. Not only will you be able to get digital inputs (switches) but also control the pins as outputs including PWM outputs. OK not very interesting, I admit.
So then I thought about improving that a little bit:
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For the 5 digital I/Os, instead of just having the pin go to the microcontroller and thats it, I put in spot for installing a capacitor to +5V and a series resistor. (To install the resistor a trace must be cut).
Of course, the RC pair can be used as an output filter: so that PWM output can be smoothed out nicely (the output will be PWM’d at around 16Khz)
The resistor can also be used to choke the output, for driving an LED for example.
But of course the real trick is that now the board has 5 extra analog inputs for measuring resistive sensors: by setting the output of the pin to +5V (discharging the capacitor, its tied to 5V) and then measuring how long it takes for the capacitor to discharge through the resistor! Its a poor method of measurement and takes a while to get a stable result (as compared to the onboard A/D). However, it certainly works and for many situations, and its good enough for most MIDI sensor applications where the 10-bit A/D may be overkill.
(You can read more about the RC timing A/D trick from this old-school app note from phillips although its rather outdated, you can get an idea of how to implement it!)
So now the board has: 11 Analog inputs, or 11 digital/PWM outputs (5 with RC filters).