Test Data for Your Decimal Adder

The best way to test your decimal adder is to feed it all possible (valid) inputs, and the easiest way to do that is with a counter for as many bits as you need independent inputs. This paper shows how to cascade two 4-bit counters to get eight independent inputs. It also shows how to turn the (binary) Cnt4 built-in macro into a decimal counter, which does not generate (except possibly in transit) invalid decimal values.

An 8-Bit Counter

The Cnt4 built-in GateSim macro only provides four independent outputs when you connect it up to the system clock:
A  Cnt4 0 0 0 0 RESET CLK
These four outputs are named A.0 through A.3, and you can trace them in the simulator by the line:
A  HEX A.3 A.2 A.1 A.0
which gives a single hexadecimal digit in the trace. If you want to see the bits separately, change the HEX line to:
A3210 HEX A.3 0 0 0 A.2 0 0 0 A.1 0 0 0 A.0
Adding a second counter based on CLK would only replicate the same outputs. Instead we need to drive the clock input to the second counter with a signal that changes only 1/16th as often as CLK. The most significant bit out of the first counter, A.3 serves this purpose:
B  Cnt4 0 0 0 0 RESET A.3
The equivalent circuit diagram for this pair of counters is:

Note that with this circuit, the outputs from counter B will change somewhat later than the outputs from counter A, typically the propagation delay through the counter (two or three gate delays), as shown in this trace fragment:

CR BA   #
11 0F   164
01 0F   165
01 0F   166
01 0F   167
01 00   168
01 00   169
11 00   170
11 10   171
11 10   172
11 10   173
11 10   174
01 10   175
01 10   176
01 11   177
01 11   178
The above trace was obtained with the clock info and trace lines set as follows:
20,5,2,1 ; run,ClkRate,PonDly,Trate
BA HEX B.3 B.2 B.1 B.0  A.3 A.2 A.1 A.0
Ordinarily you would set the trace rate (the 4th number on this line) to twice the clock rate (that is, 10 for a clock rate of 5 as shown), in order to see only one trace line for each output value. However it is a good idea to run the trace on every line like this at least once or twice to verify that your circuit is not too slow for your specified clock.

Counting in Decimal

Your decimal adder is not required to give correct results for invalid inputs, and more than half of the binary counts in the 8-bit counter above are not valid decimal inputs. You can connect the outputs of a binary counter back to the Load control through a gate in such a way as to make it count in decimal, that is, wrap around to 0000 after it reaches 9 (1001). Here is a single decimal digit counter:

In this counter, when A3 and A0 are both 1 (that is the counter is 1001 or any higher odd number), the gate fires, sending a low to the (low-active) Load control, so the next clock resets the counter to its (all zero) input instead of continuing to count. The GateSim specification for this circuit is the following two lines (each part is one line):

Cnt9 Cnt4 0 0 0 0 is9 CLK
is9  gate Cnt9.0 Cnt9.3

Rev. 2002 October 22