Technical Article

T Is for Toggle: Understanding the T Flip-Flop

April 17, 2018 by Robert Keim

This tech brief provides an overview of a somewhat uncommon member of the flip-flop family.

This tech brief provides an overview of a somewhat uncommon member of the flip-flop family.

Related Information

Flip-flops are fundamental components in the world of digital electronics. These devices are used as clock dividers and one-bit storage elements, and by connecting multiple flip-flops in the right way you can make shift registers, storage registers, and counters. Unlike mere logic gates, flip-flops utilize feedback to create circuits (called sequential logic, as opposed to combinational logic) in which the future state is influenced by the previous state.

If you want to make a flip-flop, you start with a gated latch, such as the gated SR latch:



A gated latch is a useful component, but the output can change whenever the enable signal is high. This introduces a lack of precision and reliability into whatever digital interface is built around the latch. It would be better if the latch would respond to input changes only at a specific moment in time. The trouble is, once we have implemented this functionality, the latch is no longer a latch. It’s a flip-flop.


An SR flip-flop.


The pulse transition detector (PTD) converts a rising or falling edge into a short pulse. This pulse becomes the enable signal, such that the latch is enabled only for a short period of time following the transition. The latch-plus-PTD arrangement is what we call a flip-flop, and since we’re usually working with logic circuits that are governed by clock signals, the flip-flop’s enable signal is often referred to as simply the clock.


The T Flip-Flop

The essential characteristic of a flip-flop is that it changes its output state in response to a positive or negative transition on the control signal. But there is more to a flip-flop than this: we also have to define the input-to-output relationship. This is why there are different types of flip-flops; they are all sensitive to clock edges, but they perform different actions in response to the input states.

The “T” in “T flip-flop” stands for “toggle.” When you toggle a light switch, you are changing from one state (on or off) to the other state (off or on). This is equivalent to what happens when you provide a logic-high input to a T flip-flop: if the output is currently logic high, it changes to logic low; if it’s currently logic low, it changes to logic high. A logic-low input causes the T flip-flop to maintain its current output state.

Here is the same information in truth-table form:



From SR or JK to T

You can modify the input-to-output relationship of an existing flip-flop by adding logic gates and appropriate interconnections. AAC already has an abundance of information on this topic; if you want to explore the details, our article on the conversion of flip-flops is a good place to start. In this short article, I’ll simply present two ways to create a T flip-flop from an existing flip-flop.

If you have an SR flip-flop, all you need is two AND gates to turn it into a T flip-flop:



The process is even easier if you’re starting with a JK flip-flop. No additional gates are required; all you need to do is connect the same input signal to both input pins:




T flip-flops are handy when you need to reduce the frequency of a clock signal: If you keep the T input at logic high and use the original clock signal as the flip-flop clock, the output will change state once per clock period (assuming that the flip-flop is not sensitive to both clock edges). Thus, the output clock will be half the frequency of the input clock. If you know of a clever use for a T flip-flop, let us know in the comments.

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    Bhavani Chekuri June 01, 2019

    can u explain about asynchronous active low T flip-flop

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    • M
      Mandeep321 July 02, 2019
      Consider 'S' as an active-low, asynchronous input. Depending upon the output they produce, T flip-flops can be of Two types in behaviour (usually manufacturer dependent) Type 1. when S = 0, output = 1 -- Here we call them that 'S' has 'set' the output when asserted. Type 2. when S = 0, output = 0 -- Here we call them that 'S' has 'reset' the output when asserted. Usually, it is mentioned that what an asynchronous input would do upon assertion. If nothing is mentioned, you can choose either type and mention it at the beginning of your solution.
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    free-bee June 29, 2020

    A clever usage for a T-type flip flop? I have a circuit design that uses a DPST switch to control system power. Issue is, the circuit itself resides on a PCB. Include the part where it has battery backup, I have six individual connections that are external to the PCB. I recently found an IC (FPF1320) that I might be able to use to replace the switch (it’s actually to replace another part). It was designed to control system power using an enable pin. If I can utilize a T flip flop And cleverly manipulate my power rails, I can add a button (with debouncing) and get rid of the switch. Then, my PCB will only have two external connections (battery + and -). My only problem—how I ended up on this page—is that I am in need of a T flip flop.

    But anyway, there’s one creative use for a flip flop: controlling system power.

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