Digital Inputs

A Four button Digital Input Board

Quite often in Electronics we need input from the user.

Despite touch screens being the norm, I believe tactile push buttons still have a place in the world.
They’re much cheaper. Easy to understand and implement.

But what’s the best way to get them into the uP?

Please, take a vote!

Lets take a look at some pros and cons for each:

Dedicated Digital Inputs

Pros

  • Simple and cheap
  • Interruptible
  • Multiple Touch

Cons

  • Only one input per Input on uP

Multiplexed Digital Inputs

Uses the Rows and Columns approach to have row*column number of inputs.

Pros

  • Simple & Cheap
  • Allows higher input density and better utilisation of uP I/O
  • Interruptible

Cons

  • No Multitouch. (This is due to the matrix setup, you can get false signals)
  • Inefficient for low numbers of inputs. eg. 2 rows * 2 columns requires 4 inputs, why not just use dedicated digital inputs?

Analogue Multiplexed Digital Inputs

Uses an Analogue to Digital Converter (ADC) to determine what input was pushed.

Pros

  • Requires only one input (ADC)
  • Number of digital inputs depends on the ADC resolution

Cons

  • Requires ADC on the uP
  • Not easily interruptable
  • Difficult to multi-touch
  • Slower
  • Need to take into account resistor tolerances.

Personally, I believe if you have less than ~10 inputs, try to use an Analogue Multiplexed input method. This requires only one input pin. Downside is of course, is that you require a uP which has an ADC.

Less than 4 inputs, you can get away with Dedicated Digital Inputs, but only if you have ‘spare’ I/O. But lets face it! The more available I/O the better!

If you need more than 10 keys, say for a 12 or 16 keypad, then I suggest a Row/Col approach. You get better input density than the dedicated DI and you won’t get issues around the ADC not having a high enough resolution.

That’s my thought! What are yours?? Next I’ll write about whether we should Poll or use Interrupts for input detection! Stay Tuned

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