74LS90 Decade counter
The 74LS90 is an asynchronous decade counter: a divide-by-2 stage (CKA to QA) and a divide-by-5 stage (CKB to QB, QC, QD) that count on the falling edge. Connect QA to CKB and clock CKA, and QD QC QB QA counts 0 to 9 in BCD. Two reset-to-0 and two set-to-9 inputs work at once. Note the supply pins: VCC is pin 5 and GND is pin 10.
74LS90 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 5 (VCC, +5 V) and pin 10 (GND).
| Pin | Name | Type | What it is |
|---|---|---|---|
| 1 | CKB | Input | Input |
| 2 | R0(1) | Input | Input |
| 3 | R0(2) | Input | Input |
| 4 | NC | — | Not connected |
| 5 | VCC | Power | +5 V supply |
| 6 | R9(1) | Input | Input |
| 7 | R9(2) | Input | Input |
| 8 | QC | Output | Totem-pole output |
| 9 | QB | Output | Totem-pole output |
| 10 | GND | Ground | 0 V |
| 11 | QD | Output | Totem-pole output |
| 12 | QA | Output | Totem-pole output |
| 13 | NC | — | Not connected |
| 14 | CKA | Input | Input |
74LS90 function table
| R0(1) | R0(2) | R9(1) | R9(2) | QD QC QB QA |
|---|---|---|---|---|
| 1 | 1 | 0 | X | 0 0 0 0 |
| 1 | 1 | X | 0 | 0 0 0 0 |
| X | X | 1 | 1 | 1 0 0 1 |
| X | 0 | X | 0 | counts |
| 0 | X | 0 | X | counts |
| 0 | X | X | 0 | counts |
| X | 0 | 0 | X | counts |
For a BCD count, connect QA to CKB and clock CKA; the outputs step on each falling edge of CKA.
What the 74LS90 is used for
- Counting events in decimal for a 7-segment display, through a 74LS47
- Dividing a frequency by 10, 5 or 2
- Building multi-digit counters by chaining QD into the next CKA
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 74LS90 and the Breadboard Simulator
The 74LS90 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 74LS90 can be planned and wired there.