I would like to confirm if the implementation of my 74LS191 counter is correct. Here is a diagram of my circuit:

enter image description here

The purpose of the circuit is to count from binary 0011 to binary 1111. However, the circuit above only counts from 0011 to 1110. After 1110 it recycles back to 0011. My expectation is that it would reach 1111 and then recycle back to 0011. Is this the normal operation of the 74LS191, or am I missing something?

  • \$\begingroup\$ It does reach 1111 for a few nano-seconds (or however long it takes to reset the counter). \$\endgroup\$ – brhans Aug 20 '18 at 20:25
  • \$\begingroup\$ @brhans Yes, I did actually notice this \$\endgroup\$ – user10251719 Aug 20 '18 at 20:26
  • \$\begingroup\$ So it seems that you should either reset on 0000 instead or use more logic and/or a flip-flop/latch to gate the reset signal using the clock line. I'm not intimately familiar with the '191 so I'm just throwing out suggestions here ... \$\endgroup\$ – brhans Aug 20 '18 at 20:31
  • \$\begingroup\$ You can probably get a 74LS02 so you don't have to mix LS and F logic. (not that it won't work 95% of the time, but F series has a bad reputation for causing trouble due to being too fast) \$\endgroup\$ – The Photon Aug 20 '18 at 20:35
  • \$\begingroup\$ Thanks for the recommendations. But I need to stick with what's given, as this is a practical assignment. \$\endgroup\$ – user10251719 Aug 21 '18 at 14:25

Using external 4 input NAND for LOAD! is a bad design for race conditions, as it should also include CLK! to occur mid-cycle when a race condition is avoided.

For this reason, RCO! should be used instead,to drive LOAD!.


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