74LS74 Dual D-type flip-flop with preset and clear
The 74LS74 holds two independent D flip-flops. On each rising edge of its clock, a flip-flop copies D to Q, and holds it there until the next rising edge. Each one also has active-low preset and clear inputs that set or reset Q at once, whatever the clock is doing.
74LS74 pinout
Top view of the 14-pin DIP package. Pin 1 is at the bottom left, beside the notch, and the numbers run anticlockwise. Power is on pin 14 (VCC, +5 V) and pin 7 (GND).
| Pin | Name | Type | What it is |
|---|---|---|---|
| 1 | 1CLR′ | Input | Input, active-low |
| 2 | 1D | Input | Input |
| 3 | 1CLK | Input | Input |
| 4 | 1PRE′ | Input | Input, active-low |
| 5 | 1Q | Output | Totem-pole output |
| 6 | 1Q′ | Output | Totem-pole output, active-low |
| 7 | GND | Ground | 0 V |
| 8 | 2Q′ | Output | Totem-pole output, active-low |
| 9 | 2Q | Output | Totem-pole output |
| 10 | 2PRE′ | Input | Input, active-low |
| 11 | 2CLK | Input | Input |
| 12 | 2D | Input | Input |
| 13 | 2CLR′ | Input | Input, active-low |
| 14 | VCC | Power | +5 V supply |
74LS74 function table
| PRE′ | CLR′ | CLK | D | Q | Q′ |
|---|---|---|---|---|---|
| 0 | 1 | X | X | 1 | 0 |
| 1 | 0 | X | X | 0 | 1 |
| 0 | 0 | X | X | 1* | 1* |
| 1 | 1 | ↑ | 1 | 1 | 0 |
| 1 | 1 | ↑ | 0 | 0 | 1 |
| 1 | 1 | 0 | X | Q0 | Q0′ |
↑ is the clock's rising edge. 1* — both outputs high while PRE′ and CLR′ are both low, which does not last once they are released.
What the 74LS74 is used for
- Storing one bit, such as the state of a finite state machine
- Dividing a clock by two, by wiring Q′ back to D
- Synchronising an outside signal to the system clock
- Building shift registers and ripple counters
Electrical summary
| Package | 14-pin DIP |
| Supply voltage | 5 V (4.75 V to 5.25 V) |
| Output type | Totem-pole — drives both high and low |
| Output current, low | sinks up to 8 mA |
| Output current, high | sources up to 0.4 mA |
| Input current, low | 0.4 mA out of each input held low |
| Fan-out | one output can drive about 20 LS inputs |
Typical figures for quick reference when planning a board. Check the manufacturer's datasheet for limits and timing before committing a design. The 74HC74 and 74HCT74 use the same pin numbers, with different electrical values.
The 74LS74 and the Breadboard Simulator
The 74LS74 is not in the Breadboard Simulator's parts library yet — this page is a reference. The simulator seats the gates, decoders, multiplexers, adders and buffers marked on the chips page, so the logic around a 74LS74 can be planned and wired there.