As far as I know, thus far we could only multiply the BD counts (See Stormweaver's design in this thread). Having an adder module allows the counting of any number of pulses, even odd or prime numbers.
In my (proof of concept) version I used Harcion's 3-switcher to route the the same pulse to two BD counters. I already have ideas how to make the footprint of the adder much smaller.Design Ideas
To add the number of counted pulses it to necessary to first let one BD count n pulses, then let the second one count m pulses. This is achieved by wiring the pulse input to both counters in parallel but have T-FlipFlops toggle in such a fashion that only one BD at a time receives the input pulses. Once a BD outputs a pulse (counted to it's designated number), the FlipFlops are toggled and the other BD begins counting. When the second BD outputs a pulse, we have counted n+m pulses, the FlipFlops are toggled again, and the process repeats itself.
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For example pictures of how to do it, please refer to the posts made by Harcion and me later in the topic.