A trigger is an elementary device, which is a digital machine with two stability states, one of which is assigned the value “1”, and the other - “0”.
By the method of implementing logical connections, the following types of devices are distinguished: T-trigger, D-trigger, JK-trigger, RS-triggers. Naturally, the most common options are listed here, but besides them, there are other types of automatic devices.
In this article, we will take a closer look at the D-trigger. The mentioned automaton has a single information (D) input, so it is designed to implement the time delay function.
Principle of operation
The characteristic equation Q ( t + 1 ) = Dt describes the operation of such a device type as a D-trigger. The truth table ( conversion table ) for this digital machine is shown below.
| Q t | D t | Q (t + 1) |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
As you can see, in the first and fourth lines, the values of the Q signals at time t and t + 1 coincide. That is, the D-trigger is an element of signal delay. As a result, the considered devices of the asynchronous type did not find their application, since the input signal with a small time delay will be repeated at the output.
A synchronous type D-flip-flop is built from single-level (single-stage) and two-level (two-stage) RS devices of the same type. The mentioned machines operate according to the transition table.
A single-stage D-flip-flop can be made of a single-level synchronous RS-device and one I-NOT1 element, which connects both inverse inputs of the D-flip-flop into a single information (D) input.
When logical zero arrives at the synchronizing input, the RS type machine is blocked by the level of the logical unit from the outputs of the AND-NOT 2 and AND-NOT 3 elements. When the synchronization signal is changed, the level applied to the information input will create a logical zero either at the input S (at D = 1) or at the input R (at D = 0) of the asynchronous trigger T. It will switch to the state corresponding to the logical level D. One-stage A D-type trigger delays the propagation of the input during a pause between the clock signals.
D-trigger with dynamic control. Description of work, functional diagram
An automatic device of this kind is constructed of three RS-triggers of the asynchronous type. They are built on AND-NOT elements, while two of them perform a switching function, and the third is the output. The output signals of the switching triggers are designed to control the output trigger.
When signal level C is equal to logical zero, a neutral combination of signals arrives at the inputs of the output trigger, and it switches to storage mode. When the information signal changes, the switching triggers go into standby mode, and as soon as the signal of the logical unit arrives at the enable input of trigger C, the output automaton is set to a new state, which corresponds to the information signal at the D-input in the previous clock cycle.
If the change in the level of the information signal takes place during the installation of the output trigger, then the switching devices will not miss the signal. It turns out that the purpose of the switching triggers is to receive information signals, transfer them to the input of the output device at the moment the signal at the control input C changes from a logical zero to a logical unit and self-blocking from the effect of the signal at the information input.