Month: July 2016

Now, we are sending six dots, the red trace showing the signal obtained at the dot contact, and the yellow trace out of a simple debounce circuit in an FPGA.

It is easy to include a counter in the device for measuring the length of marks and spaces.

By manually reading the numbers in the above figure, you can write a short script in R.

% R
> dat_v=c(1034, 883, 1028, 889, 1003, 923, 1053, 871, 1024, 901, 1048)
> barplot(dat_v, col=c("blue", "green"))

The blue bars correspond to the mark (=low) length, and the green ones to the space (=high). You will immediately notice that the mark to space ratio is not 1.0, and some more adjustment is required.

Although the current logic to eliminate the bouncing is far from perfect, let us go on to the next step.

The signals captured using SignalTap II can be exported as, say, a CSV file. Therefore, you can manipulate the data in various ways.

The waveform shows the first eight dots, the last one imcomplete, of consecutive dots.

Each length of marks and spaces is shown as a bar graph. Note that the value of 1,000 corresponds to 50mS.

If your bug key is connected to an input port of an FPGA, you will observe the signals such as in the figure above. (Please click the figure to enlarge.)

You will notice that in some cases (see the green circles) we do not have contact bounce at the rising edges, but you can not always expect it to be true. Also note that (see the yellow circle) sometimes we have relatively long bouncing.

A simple logic is employed to eliminate those bouncing without introducing a delay. See the signal OUT_KEY.