74LS161 Synchronous 4-bit binary counter, asynchronous clear
The 74LS161 is a synchronous 4-bit binary counter: all four outputs change together on the rising clock edge. It can load a starting value from A–D, count up when both enables are high, and hold when either is low. CLR′ clears it at once, without waiting for the clock.
74LS161 pinout
Top view of the 16-pin DIP package. Pin 1 is at the bottom left, beside the notch, and the numbers run anticlockwise. Power is on pin 16 (VCC, +5 V) and pin 8 (GND).
| Pin | Name | Type | What it is |
|---|---|---|---|
| 1 | CLR′ | Input | Input, active-low |
| 2 | CLK | Input | Input |
| 3 | A | Input | Input |
| 4 | B | Input | Input |
| 5 | C | Input | Input |
| 6 | D | Input | Input |
| 7 | ENP | Input | Input |
| 8 | GND | Ground | 0 V |
| 9 | LOAD′ | Input | Input, active-low |
| 10 | ENT | Input | Input |
| 11 | QD | Output | Totem-pole output |
| 12 | QC | Output | Totem-pole output |
| 13 | QB | Output | Totem-pole output |
| 14 | QA | Output | Totem-pole output |
| 15 | RCO | Output | Totem-pole output |
| 16 | VCC | Power | +5 V supply |
74LS161 function table
| CLR′ | LOAD′ | ENP | ENT | CLK | What happens |
|---|---|---|---|---|---|
| 0 | X | X | X | X | Clear to 0 at once |
| 1 | 0 | X | X | ↑ | Load A–D |
| 1 | 1 | 1 | 1 | ↑ | Count up |
| 1 | 1 | 0 | X | X | Hold |
| 1 | 1 | X | 0 | X | Hold |
RCO, the ripple carry out, goes high while the count is 15 and ENT is high; it feeds ENT of the next counter to make a longer one.
What the 74LS161 is used for
- Counting 0 to 15 with every output changing at the same moment
- Program counters and address generators
- Counters of any length, chained through RCO and ENT
- Counting to any value, by loading a start value when RCO goes high
Electrical summary
| Package | 16-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 74HC161 and 74HCT161 use the same pin numbers, with different electrical values.
The 74LS161 and the Breadboard Simulator
The 74LS161 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 74LS161 can be planned and wired there.