Vintage Computing

Historical Past Of The Computerflip-Flops - A Primary Counter

Flip-Flops - A basic counter

We appeared on the Binary system, and fundamental computer logic parts, in previous articles, "It's a binary world - how computer systems count" and "How computer systems add - a logical strategy".

Now we can combine two elements of these articles to have a look at a counter. Another frequent logic element in a pc is a counter or timer. This will b to rely items going previous a sensor on an meeting line, or probably a count-down timer. For instance, if in case you have a late mannequin washer it should have a simple laptop using a depend down timer to give 10 minute wash cycle, etc.

There are a number of types of counter, almost all of which use a primary component of electronics, the Flip-Flop. And you thought they have been rubber sneakers English individuals put on to the shower or the beach. (At this point Australians say "I thought they had been referred to as thongs").

OK back on topic. The flip-flop is as previous as electronics, and is a classic example of the binary system. It has potential steady states, A or B, and could be 'toggled' from one state to the other, similar to a 'push-on, push-off' switch. It was originally made with vacuum tubes (or one, for instance a double triode).

It usually has outputs, one being the complement of the other. That's,if one output(A) is a logic 0, the other(B) is a logic 1, and vice-versa. The enter, or Toggle(T) is at logic zero until a pulse from a sensor, for example, comes along. This pulse takes the logic state to 1, then again to 0. The toggle impact, causing the Flip-Flop to flip, is actually the CHANGE from zero to 1.

In logic terms the flip-flop is made up using AND and OR gates, in logic cicuitry it's just a 'black field' labelled FF. A number of FFs may be grouped into yet one more black field, a counter, timer, or multivibrator.

We are able to make up a Truth Table, which we've got used before. Should you recall, a truth desk tells you what the Output will be for all potential Inputs.

TRUTH TABLE for Flip Flop - Toggle (C)hange,- Outputs A and B.

INITIAL STATE

T B A zero 1 0 'A' output is 0

PULSE 1

T B A C 0 1 'A' output is 1

PULSE 2

T B A C 1 zero 'A' output is 0

Now we string some flip-flops collectively to make a counter. Say we've got a sensor on a beer bottling machine, which has to count 5 bottles before switching the feed, we have to count as much as 5, or one hundred and one in Binary. We will want three flip-flops, for binary bits zero,1 and a pair of, comparable to decimal bit worth of 1,2 and 4.

We are going to take the A output of the 3 flip-flops to a decoder black box, which we will use to detect after we get to 5, then change the feed. The B output of flip-flop zero is handed to the toggle input of flip-flop 1 by way of an AND gate, so the following pulse from the sensor (which works to all 3 flip-flops) at this AND gate will toggle the flip-flop, relying on the value of the B output, 0 or 1. Equally the B output of flip-flop 1 goes to the toggle of flip-flop three through an AND gate.

Our three Flip-Flops now give you a fact desk like this:-

INITIAL STATE

FF2 FF1 FF0

TBA TBA TBA

010 010 010 'A' outputs 000 - 0

PULSE 1

FF2 FF1 FF0

TBA TBA TBA

C10 C10 C01 'A' outputs 001 - 1

[The (C)hange flips FF0 (always). FF1 & FF2 are blocked by the AND gate which wants a zero input from the previous FF 'B' output AND the heartbeat change.]

PULSE 2

FF2 FF1 FF0

TBA TBA TBA

C10 C01 C10 'A' outputs 010 - 2

[The (C)hange flips FF0 (always). FF1 flips beacause the 'B' output from FF0 is a 0 when the Pulse arrives. FF2 is blocked as before.]

PULSE 3

FF2 FF1 FF0

TBA TBA TBA

C10 C01 C01 'A' outputs 011 - three

[FF0 flips, FF1 is blocked once more,as is FF2.]

PULSE four

FF2 FF1 FF0

TBA TBA TBA

C01 C10 C10 'A' outputs 100 - four

(FF0 flips, FF1 flips, FF2 flips.)

PULSE 5

FF2 FF1 FF0

TBA TBA TBA

C01 C10 C01 'A' outputs one zero one - 5

count complete!

[FF0 flips, FF1 and FF2 are blocked.]

This counter can depend up to 111, 7 decimal, it then resets to 0. A few attention-grabbing factors to notice are:-

1. FF0 flips every pulse. FF1 flips every 2 pulses. FF2 flips each four pulses etc. These details can be used to make up a divider, which might be cascaded. For instance the 4 pulse output can go to a second counter which also provides a four pulse output, totalling 16. This can be expanded to make up a decadic counter by decoding a count of 1010 (10 decimal) and utilizing this to toggle the next counter, etc. What about 60 and 12 in your digital watch?

2. Look at the 'B' outputs from the counter. In sequence the

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