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Part 2 · Medium: the trace is real

11 · Bits and eyes

Status: ✅ Runs today (about 2 minutes per run)

Goal: go from single edges to real traffic, and learn to read an eye.

The ideas​

  • UI (unit interval): the time for one bit. 2 ns per bit is 500 Mb/s.
  • Eye diagram: cut the received waveform into windows and overlay them.
    • Eye height is the voltage margin at the centre of the bit.
    • Eye width is the timing margin at the threshold (VREF = 1.65 V here).
  • ISI (inter-symbol interference): if one bit's reflections are still ringing when the next bit arrives, they close the eye.

Build​

Use a longer line, 72 mm as a serpentine. Drive it with the strong 24 mA buffer and leave the far end unterminated. The shared LineBoard in teaching-board.tsx is lesson 10's board as a component, with props for the bends, the drive and the bit rate.

11-long-line.circuit.tsx
import { LineBoard } from "./teaching-board"

// About 72 mm of serpentine: eleven straight runs, none longer than 13 mm. Strong 24 mA driver, nothing at the far end.
export const serpentine = [
  { x: -6, y: 1 }, { x: -6, y: 7 }, { x: -3, y: 7 }, { x: -3, y: -6 }, { x: 0, y: -6 },
  { x: 0, y: 7 }, { x: 3, y: 7 }, { x: 3, y: -6 }, { x: 6, y: -6 }, { x: 6, y: 1 },
]

export default () => <LineBoard name="LONG_LINE" drive="LVCMOS33_24MA" bends={serpentine} />
11-long-line-fast.circuit.tsx
import { LineBoard } from "./teaching-board"
import { serpentine } from "./11-long-line.circuit"
// Same line and driver, 2.5x the bit rate: 0.8 ns per bit.
export default () => <LineBoard name="LONG_LINE_FAST" drive="LVCMOS33_24MA" bends={serpentine} bitDuration="0.8ns" />

The pipeline straightens the serpentine into one 72 mm stage. Coupling between neighbouring runs of the serpentine is not modelled.

What you see​

Extraction: L = 24.1 nH and C = 11.6 pF, so Z0 = 45.6 Ω and the delay is 529 ps. Per mm that matches lesson 10.

2 ns per bit0.8 ns per bit
RX swing−0.88 V to 4.20 V−0.90 V to 4.20 V
Eye height3.29 V0.04 V (closed)
Eye width2.0 ns of 2 ns0.73 ns of 0.8 ns

It's the same line and the same driver. Only the bit rate changed.

Check by hand​

  • Overshoot: the 15 Ω driver sees a 44.5 Ω line, so the step it launches is 3.3 × 45.6 / (15 + 45.6) ≈ 2.48 V. At the open end it doubles to about 4.9 V. The clamp diodes catch part of that, which gives the 4.20 V you see.
  • Settling: the reflection takes 2 × 529 ps = 1.06 ns to come back and bounce again.
    • At 2 ns per bit it settles before the centre of the bit, so the eye is open.
    • At 0.8 ns per bit, one round trip is already longer than a bit, so every bit inherits the previous bit's ringing, and the eye closes.

Try this​

Switch to drive="LVCMOS33_12MA". Its 30 Ω output is much closer to Z0, so it behaves like the series termination from lesson 9. Predict the eye first; lesson 12 shows the result.