The trigger is the simplest device, which is a digital machine. It has two states of stability. One of these states is assigned the value “1”, and the other “0”. The status of the device, as well as the value of the binary information that is stored in it, is determined by the output signals: direct and inverse. In the case when a potential that corresponds to a logical unit is established at the direct output, in this case the trigger state is called unity (in this case, the potential at the inverse output corresponds to a logical zero). If there is no potential at the direct output, then the state of the trigger is called zero.
Triggers are classified according to the following criteria:
1. By the method of recorded information (asynchronous and synchronous).
2. By the method of information management (statistical, dynamic, single-stage, multi-stage).
3. By the method of implementing logical connections (JK-trigger, RS-triggers, T-trigger, D-trigger and other types).
The main parameters of all types of triggers are: the maximum duration of the input signal, the delay time required to switch the trigger, and also allowing the response time.
In this article we will talk about a type of device such as a T-trigger. Such triggers have only one information (T) input, which is called a counting input. It changes its state after entering the counting (T) input of each control signal.
According to the transition table, the law of operation of such triggers is described by the characteristic equation: Q (t + 1) = TtQ't V T'tQt. It follows from the equation that when logic zero arrives at the input (T), the T-trigger will retain its state, and when a logical unit is supplied, it will change to the opposite.
| Q t | T t | Q (t + 1) |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
The table shows that the T-trigger performs the addition operation, this led to the name of such a trigger as a countable one, its informational (T) input as a counting input. The signal level at the input of such a trigger appears twice as often as at its output (Q). Accordingly, the T-trigger is used as a frequency divider.
An asynchronous type T-flip-flop can be constructed on the basis of a two-stage RS flip-flop with additional connections, namely: the trigger output (Q) must be connected to the input (R), and the output (Q ') to the input (S). The information input (T) will be the synchronous input (C).
The photo shows a T-trigger. Functional scheme.
In the initial state, the logic zero level is applied to the information inputs of the trigger (R and S), when the logic zero is applied to the counting (T) input, the state of the first trigger will be constantly copied by the second trigger, because the AND element will not give the level of the logical unit to input of the second trigger. If the T-trigger was in the state of unity, then the inputs (R and S) will be fed levels of zero and unity, respectively. When the first signal arrives at the counting input equal to a logical unit, the logical unit is written to the first trigger. The state of the second trigger does not change, because the zero level from the output of the logical element AND-NOT blocks its state. After removing the counting pulse at the input (T), zero is set, and the second trigger switches to the state of the logical unit.
The photo shows a synchronous T-trigger. Functional scheme.
Synchronous T-flip-flops are used, if necessary, to represent with potential a sequence of logical units at the input of a T-flip-flop.