Clock Multiplication vs Division: Which to Use and Why
In short: Designs need many clock frequencies from one reference. Division is simple but only gives integer ratios below the reference and can pile up jitter across a long chain. Multiplication, usually via a PLL, reaches higher frequencies, supports non-integer ratios, and often has lower jitter, which is why it suits high-speed links, CPU cores, and clock tree synthesis. In a testbench you model the target period directly.
Digital designs need several clock frequencies from one reference. The usual way is to divide the reference clock. A second way is to multiply it, usually with a phase-locked loop. Multiplication is not always better, but for high frequencies and non-integer ratios it often wins. This post compares the two and shows where each one fits.
The division approach and its limits
Dividing the reference clock is simple: toggle an output every few reference edges to get a lower frequency. It works well for clean integer ratios such as divide-by-two or divide-by-four. Two limits show up as designs grow. Division only gives integer ratios, so many target frequencies are simply out of reach. And a long chain of dividers for many clock domains becomes hard to manage and can accumulate timing error.
The multiplication approach
Clock multiplication raises the frequency by multiplying the reference, typically through a PLL or similar clock synthesis. It reaches frequencies a divider cannot, and it supports non-integer ratios, so you get finer control over the exact frequency each block needs. A good PLL also cleans up the output, giving lower jitter than a long divider chain.
Advantages of multiplication
Three benefits stand out. Jitter, the small variation in a clock’s period, tends to be lower with a well-designed PLL than with a deep divider chain, giving a steadier clock. High-performance blocks often need clocks faster than a divider can make from a modest reference, and multiplication provides them. And because multiplication allows non-integer ratios, it gives finer control than the integer-only steps of division.
Modeling both in a testbench
In real silicon a PLL creates the multiplied clock. In a testbench you usually model the target period directly rather than build a PLL. The snippet below shows a reference clock, an integer divide-by-two, and a behavioral 2.5x multiplied clock, which a divider alone could not produce.
// Simple behavioral clock generation in a testbench.
// Reference clock: 100 MHz -> 10 ns period.
timeunit 1ns; timeprecision 1ps;
bit ref_clk;
always #5 ref_clk = ~ref_clk; // 100 MHz reference (10 ns period)
// Divided clock by 2 -> 50 MHz. Only integer ratios are easy this way.
bit div_clk;
always @(posedge ref_clk) div_clk = ~div_clk; // toggles every ref edge
// Behavioral "multiplied" clock by 2.5 -> 250 MHz (2 ns period).
// In real silicon a PLL does this; in a TB we model the target period.
real mult_period_ns = 2.0; // 250 MHz target
bit mult_clk;
initial forever #(mult_period_ns/2) mult_clk = ~mult_clk;Multiplication versus division at a glance
| Aspect | Clock division | Clock multiplication |
|---|---|---|
| Ratio | Integer only | Integer and non-integer |
| Reachable frequency | Only lower than reference | Higher than reference |
| Jitter | Grows with divider depth | Lower with a good PLL |
| Complexity | Simple logic, no PLL | Needs a PLL or synthesis |
| Best for | Simple lower-rate clocks | High-speed and exact rates |
Where multiplication is used
Multiplication shows up wherever high or precise clocks are needed: the fast clocks behind high-speed serial links such as PCIe, SATA, and USB, the core clocks of microprocessors, and clock tree synthesis that spreads several frequencies and phases across a chip. In a multi-clock SoC it lets you generate each domain directly instead of cascading dividers.
A simple way to picture it
Think of gears on a bicycle. Division is like only ever gearing down from one fixed pedal speed: you can go slower in neat steps, but you can never go faster than your legs, and only in whole-tooth jumps. Multiplication is like adding a gear that spins the wheel faster than you pedal, and lets you land on in-between ratios too. When you need real speed or an exact ratio, gearing up beats only ever gearing down.
Common mistakes
- Forcing division for a target that needs a non-integer ratio.
- Cascading many dividers and letting timing error and jitter build up.
- Ignoring PLL lock time when the multiplied clock is first brought up.
- Assuming a testbench clock has real PLL jitter; a behavioral model does not.
- Mixing clock domains without checking the crossings between them.
Key takeaways
- Division is simple but limited to integer ratios below the reference.
- Multiplication reaches higher and non-integer frequencies through a PLL.
- A good PLL usually gives lower jitter than a deep divider chain.
- Multiplication suits high-speed links, CPU cores, and clock tree synthesis.
- In a testbench you model the target period rather than build a PLL.
The concepts here follow standard digital-design practice, and the code models clocks behaviorally for a testbench rather than a real PLL. Confirm timing, jitter, and lock behavior against your actual clocking hardware and simulator.
Keep learning
Once you are generating clocks, you also want to watch them. The companion piece on clock monitors in SoC verification shows how to check a clock stays healthy at runtime. For the checking tools around clocking, see assertions, and for background on the wider effort, the verification plan decides how each clock domain gets covered.
Frequently asked questions
What is clock multiplication?
It is generating a clock faster than the reference by multiplying its frequency, usually with a phase-locked loop. It can reach frequencies and non-integer ratios that a divider cannot.
How is it different from clock division?
Division toggles an output every few reference edges to make a lower, integer-ratio frequency. Multiplication raises the frequency and allows non-integer ratios, at the cost of needing a PLL.
Why does multiplication often have lower jitter?
A well-designed PLL cleans up its output, while a long chain of dividers can accumulate period variation. So for the same target, the multiplied clock is usually steadier.
When should I still use division?
When you need a simple lower-rate clock at a clean integer ratio and want to avoid the cost and lock time of a PLL. Division is perfectly good for those cases.
Where is clock multiplication used?
In high-speed serial interfaces like PCIe, SATA, and USB, in microprocessor core clocks, and in clock tree synthesis that distributes several frequencies and phases across a chip.
How do I model a multiplied clock in a testbench?
You usually model the target period directly with a timed toggle rather than building a PLL, since the goal is to exercise the design at the right frequency, not to replicate PLL internals.
