Asynchronous Counters


Asynchronous Counters

Video Lectures created by Tim Feiegenbaum at North Seattle Community College.

In the previous section, we looked at asynchronous counters and recall what an asynchronous counter was. It was a counter, and remember that we had a clock pulse coming in and that clock pulse was simultaneously fed to all of the Flip Flops in the given circuit and all of the Flip Flops changed state at the same moment. With an asynchronous counter, they ... and the asynchronous simply mean that they're not in synch with each other. The way that these will work is here we have the clock impulse ... the clock pulse coming in and with this, it's not going anywhere. This particular line over here is just one over to the logic analyzer to display it. The only input is going to this first Flip Flop and this Flip Flop as it is this clock as it changes state, the Q output is fed into the next clock input on the next Flip Flop. It will cause this one to change state, so we'll have a change of state here which is fed to the next one and then this changing state here and it will be fed to the next to the next to the next. It's kind of like the dominoes, one falls and then they just fall in order.

These ... they work well. There is really not a problem with these except that at higher frequencies they can become problematic and it's because of something called Propagation Delay. Propagation delay - your text mentioned it earlier, I didn't mention it then but I will mention it now - propagation delay is the time that it takes a signal to move from the input to the output. Now typically we think of that as being instantaneous because it is so rapid, but actually, it does take an amount of time. Typically it's measured in nanoseconds; you may have a gate that has maybe three nanoseconds from the time it goes from input to output, which isn't very long.

However, if you have in this case three Flip Flops, well then it's going to jump up to nine nanoseconds; the propagation delay, and that's what they call delay from the time of the input till the time of the last output - in this case, it would be nine nanoseconds. If we have a large circuit, say we have 20 or 30 Flip Flops in a row then suddenly the nanoseconds begin to count up. If we have a high-frequency input ... there is the very real possibility that you begin to send pulses in and before you get to the end of the cycle, before it's completely clocked, a new cycle is occurring. In this case, you can lose counts and that can become problematic. For this reason, the asynchronous variety of counters is actually more popular.

Now, we're not going to go into the detail that we did in the previous circuit … addressing counting. What we will do here ... I do want to mention the ... this is actually the spectrum analyzer display and these are the three counter display. We had to see ... this would be the C, B, and the A input and this is the clock down here. Let's do look at how this one actually counts because of this one ... The previous one that we had looked at was an up counter. This is actually what we call a down counter. This is something that counts down from the specific value. In this one if we look at the inputs here, we'll see we have a low, low, low ... that would be zero and then when it resets, it jumps up to a high, high, high that would be seven and then here we have a high, high, low which would be six and then a high, low, high, which would be five and then four, three, two, one and at this point we're at three lows and then here we reset to seven. This is a down counter and if you want to do the simulation on this, look at it a little bit closer, it is a circuit 15 underline 29.


Integrated Counters

Integrated counters are very commonly used and they come in many different configurations and with many different counting sequences. They will include counting in binary as well as B, C, D. With a binary counter - you know the counting sequence - you would have zero, zero, and then you would have one and then you would have two and this is the way it would display in binary. This is all well and good if you understand binary. I had a student one time built a binary clock and it displayed in binary and it was kind of an interesting to watch. If he wanted to figure out what time it was, it was quite challenging because he had an hour, minutes and seconds LED display but it was all in zeros and ones so it was quite an awkward clock, though it was very interesting.

This B, C, D - well this is binary coded decimals - so what this would do it will take that ... you'd have all your binary counting. What it does is it would display it in decimals so it is more understandable. Now, this particular device here, this is an integrated counter. By integrating, what they mean is here you have a single chip and this single chip could contain 40, 50, a 100 Flip Flops possibly and it's all on a single chip. Rather than having this long circuit with 20 or 30 Flip Flops and AND gates and OR gates and whatever, you just simply have an integrated chip that does the same thing. You don't see all of the Flip Flops but you just plug it in. In this case, here we have a signal ... a signal generator input and then our outputs here are simply the ... in this case the four bits.


Frequency Division

I have on this next screen... we see this is the same circuit and this is the display of the outputs that would come out of it. Now recall ... we mentioned earlier about how that, the counters will count but the way that they count is they actually do it by means of frequency division. This is another common use of counters by the way. In this particular case here we have a 32 megahertz input and the outputs would be 16, 8, 4, and 2. Systems like this are commonly used in computer timing systems. It's just one example where you have a given frequency that the computer operates at and then you need lower frequencies for various busses. Oftentimes you want the buses to be synchronized together but they operate at lower speed. This would be one way to do that to have different frequencies but have the frequencies actually sync together. Anyway, that is another use for frequency division in computers.

This is a student project. A guy named Bryan Norton was in my class a while back and he built a student project. This is it right here. We jokingly referred to it as Bryan's bomb because it looked like some explosive device. It had the readouts here and this thing would count down or count up. It was rather ... you could do lots of different counting sequences. Then he had some connections here where you could connect something. At the end of a counting sequence, you would have to do something. Most commonly to activate an alarm or set off some type of, not a bomb, okay. Anyway, we're kidding him about it because it did work like a bomb. Anyway, this is a device -  a sophisticated device that utilized counters. Then you had several integrated counters built within this device.


State Machines

Then this final circuit is a state machine. This state machine utilizes Flip Flops. It is a three input entry system. If the correct sequence is entered the solenoid will activate granting access. Here we have this circuit. You'll notice over here on the far left side, we have switches S1, S2 and S3 and at the crack sequence it is entered here, then this solenoid will activate it in. In this case, it could be the grant access.

Brinks Home Security Systems are activated and deactivated with a system similar to this. If you've seen Brinks Systems, they had a keypad. In order to turn them on, you input three values and if you want to deactivate them you enter three values. Also in this particular circuit, whenever you press the key the little LED down here will activate. We're not going to go into the theory of this one. This particular circuit is in your circuit package but an I would invite you to play with it and it does utilize a couple Flip Flops.

Okay. In this section, we're wrapping up our discussion on counters. We ended with this state machine. We looked at the student project. We looked at the concept of frequency division and we talked about integrated counters which are probably the most common building block that you would use just today to build counters. Then we looked at asynchronous counters and that concludes our discussion on counters.

Video Lectures created by Tim Fiegenbaum at North Seattle Community College.