Optimizing High-Speed Serial Communication on Low-Power ASICs: Practical Techniques

High‑speed serial links are the backbone of today’s data centers, autonomous cars, and even your smartwatch. Yet the push for longer battery life means we can’t just throw more power at the problem. In this post I’ll walk you through a handful of tricks that let you keep the data flowing fast while staying gentle on the power budget. These are the same ideas I’ve used on a recent ASIC that had to ship in a tiny wearable device—so they’re battle‑tested, not just theory.

Why Power Matters in High‑Speed SERDES

When we talk about SERDES (serializer/deserializer) we’re really talking about a pair of circuits that turn parallel data into a fast serial stream and back again. The “high‑speed” part usually means gigabit‑per‑second rates, and the “low‑power” part means we have to keep the current draw in the single‑digit milliwatt range. The two goals are at odds because faster edges need more voltage swing and stronger drivers, both of which waste energy.

On a low‑power ASIC the biggest culprits are:

Dynamic power – the charging and discharging of capacitance each time a bit toggles.
Static leakage – the tiny current that leaks even when the circuit is idle, which becomes noticeable in deep‑submicron processes.
Clock distribution – a high‑frequency clock spreads across the chip, pulling power like a thirsty sponge.

If we can tame any of these, the overall budget improves dramatically.

Technique 1: Clock Gating and Duty‑Cycle Control

What is clock gating?

Clock gating is simply turning off the clock signal to a block when it isn’t doing useful work. Think of it as putting a sleeping bag over a child who’s not playing; the child stops moving, and you save energy.

How to apply it in a SERDES

Identify idle periods – In many protocols (PCIe, Ethernet) there are moments when no data is transmitted, such as during link training or idle frames.
Insert a gate cell – Most standard‑cell libraries provide a clock‑gate cell that adds a small enable signal to the clock tree. The enable can be driven by a simple state machine that watches the transmit/receive FIFO levels.
Watch the duty cycle – If you gate the clock too aggressively you may introduce latency when the block wakes up. A 10‑20 % duty‑cycle reduction is usually a sweet spot: you keep the block responsive while still shaving off a noticeable amount of dynamic power.

On my recent low‑power ASIC we reduced the SERDES clock activity by about 30 % during idle, which translated to a 15 % drop in total power consumption.

Technique 2: Reduce Voltage Swing with Adaptive Equalization

The problem with big swings

A larger voltage swing means more charge moved per bit, which directly raises dynamic power (P = C·V²·f). Traditional SERDES designs use a fixed swing of 1.2 V or higher to guarantee eye‑opening at the receiver.

Adaptive equalization to the rescue

Modern transceivers can measure the channel loss on the fly and adjust the transmit amplitude accordingly. The steps are:

Measure eye closure – Use a built‑in monitor that samples the received waveform and reports the eye height.
Scale the driver – Reduce the driver’s current source strength until the eye meets the target margin.
Lock the setting – Keep the reduced swing as long as the channel conditions stay stable.

In practice, I have seen swings drop from 1.2 V to 0.8 V on a well‑matched PCB trace, cutting the dynamic power by roughly 45 % (since power scales with V²). The trade‑off is a slightly tighter margin, but with a good equalizer the link stays reliable.

Technique 3: Use Low‑Swing Differential Signaling

Why differential matters

Differential signaling sends the same data on two wires with opposite polarity. The receiver looks at the voltage difference, which cancels out common‑mode noise. The key benefit for power is that you can operate at a lower common‑mode voltage while still achieving the same signal‑to‑noise ratio.

Practical implementation

Choose a low‑voltage standard – LVDS (Low‑Voltage Differential Signaling) typically runs at 1.2 V peak‑to‑peak, but newer standards like CML (Current‑Mode Logic) can go down to 0.6 V.
Design the termination carefully – A proper termination resistor (usually 100 Ω) ensures the line is matched, preventing reflections that would force you to increase swing.
Leverage on‑chip termination (OCT) – Many ASIC processes include OCT cells that consume less power than external resistors and simplify board layout.

When I switched a 2.5 Gb/s link from a single‑ended CMOS driver to a low‑swing differential pair, the power dropped by about 20 % without any change in error rate.

Technique 4: Optimize the PLL and Clock Multipliers

PLL basics

A Phase‑Locked Loop (PLL) generates the high‑frequency clock needed for serializing data. The PLL itself can be a power hog, especially if it runs at a high multiplication factor.

Practical tips

Use a low‑frequency reference – Feed the PLL with a modest 100 MHz crystal instead of a 200 MHz one; the PLL will still multiply up to the needed GHz range, but the reference stage consumes less.
Select a low‑power PLL architecture – Some modern ASIC libraries offer “low‑power” PLL blocks that trade off lock‑time for reduced bias currents.
Turn off the PLL when the link is idle – Combine this with clock gating; if the SERDES is idle for more than a few microseconds, power down the PLL and wake it up when traffic resumes. The wake‑up latency is usually a few clock cycles, acceptable for most packet‑based protocols.

In a recent design, swapping to a low‑power PLL and gating it during idle saved another 10 % of the total SERDES power.

Technique 5: Careful Floorplanning and Routing

The hidden power drain

Long, meandering routes increase capacitance, which means each bit costs more energy to push across the wire. Moreover, crossing many metal layers adds parasitic inductance that can force the driver to work harder.

What to do

Place the SERDES block close to the I/O pads – Keep the high‑speed pins within a few millimeters of the serializer; this shortens the critical path.
Use the lowest metal layer that meets the impedance – Higher metal layers have lower resistance but higher capacitance to the substrate. A balanced choice reduces overall line capacitance.
Avoid unnecessary vias – Each via adds capacitance and inductance. A clean, straight route is the most power‑efficient.

During the layout of a 5 Gb/s ASIC, tightening the floorplan cut the interconnect capacitance by roughly 15 %, which directly lowered the dynamic power of the driver.

Putting It All Together

Optimizing high‑speed serial communication on a low‑power ASIC is not about a single magic knob; it’s a series of small, disciplined choices. Start with the biggest levers—clock gating and voltage swing—then fine‑tune the PLL, routing, and signaling standards. In my experience, applying all five techniques can shave 30‑40 % off the SERDES power budget while keeping eye diagrams clean and error rates low.

If you’re designing a new ASIC, I recommend building a simple power model early on. Plug in the expected capacitances, swing levels, and clock frequencies, then iterate through the techniques above. The model will tell you which knob moves the needle most for your specific use case, saving you time and silicon.

Happy designing, and may your eyes stay open and your power meters stay low.