Serial adder: build and simulate it online
One full adder and a D flip-flop that remembers the carry: add numbers one bit per clock, LSB first.
Build the serial adder in the free online simulator — it checks your circuit automatically.
What you need
- 3 logic switches labelled A, B, CLK
- 1 probes labelled S
- a full adder
- 1 D flip-flop
Step by step
- Inputs and outputs. From the Logic section of the library, drop digital inputs (0/1 boxes) or logic switches for the inputs, and digital outputs or probes for the outputs; give each its label in the inspector (the checker finds them by label). CLK is a switch too — you (or the checker) flip it to step. Switches A, B, CLK; Probe S
- One full adder. Its carry-in comes from the flip-flop, not from a switch. 2 XOR, 2 AND, 1 OR — or an Adder block
- Carry memory. A D flip-flop stores Cout on each clock and feeds it back as Cin for the next bit. D flip-flop: D ← Cout, Q → Cin, CLK ← CLK
- Prove it. The checker adds 3 + 3 = 6 one bit per clock: sums 0, 1, 1 (LSB first). S shows each sum bit before its clock
Expected sequence
| # | inputs | clock | S |
|---|---|---|---|
| 1 | A=1 B=1 | ↑ | 0 |
| 2 | A=1 B=1 | ↑ | 1 |
| 3 | A=0 B=0 | ↑ | 1 |
Frequently asked questions
How do you build a serial adder?
One full adder and a D flip-flop that remembers the carry: add numbers one bit per clock, LSB first. In short: inputs and outputs, then one full adder, then carry memory, then prove it.
How does a serial adder behave clock by clock?
It is checked over 3 steps: bit 0: 1 + 1 = 0, carry 1, bit 1: 1 + 1 + 1 = 1, carry 1, bit 2: 0 + 0 + 1 = 1. The table on this page shows the expected S after each clock.
What parts do you need for a serial adder?
a full adder; 1 D flip-flop — plus logic switches for the inputs and probes for the outputs.
Can I simulate a serial adder online for free?
Yes. Open it in the BitForBytes Embedded Lab, a free browser-based circuit and logic simulator: drag in the gates, wire them, and press "Check my circuit" — every clock cycle is simulated and marked for you. No download or sign-up needed.
Part of DSD 13 · Serial adder in the free BitForBytes digital system design course.