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Document routing a four-pad differential pair through ESD and test points #1020

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Document routing a four-pad differential pair through ESD and test points #1020

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36 changes: 34 additions & 2 deletions docs/elements/differential-pair.mdx
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Original file line number Diff line number Diff line change
Expand Up @@ -48,8 +48,8 @@ signal arrive at different times.
form the pair and how much their routed lengths may differ. It does not create
electrical connections; add those with [`<trace />`](./trace.mdx).

This element currently constrains length matching only. It does not configure
differential impedance, trace width, pair spacing, or parallel routing.
Use `pcbTraceGap` to request the edge-to-edge copper spacing between the two
traces. The pad pitch can require a wider separation at the endpoints.

## Quick start with trace names

Expand Down Expand Up @@ -98,6 +98,37 @@ to avoid an ambiguous match. In
[phased autorouting](./autoroutingphase.mdx), the `routingPhaseIndex` prop
assigns a `<trace />` to a phase. Both traces in a pair must use the same value.

## Route a pair through several pads

Give each two-pad section a positive and negative trace name. The example
routes a connector through an ESD device and test points to an IC. Each
polarity keeps its electrical net across all three sections. Each phase routes
one pair and sees the copper from the preceding phase.

export const usbFourPadExample = `export default () => (
<board width={24} height={12} schematicDisabled routeRemaining={false}>
<pinheader name="J1" pinCount={2} pcbX={-8} pcbRotation={90} />
<chip name="D1" footprint="sot23" pcbX={-4} />
<testpoint name="TP1" footprintVariant="pad" pcbX={1} pcbY={1.5} />
<testpoint name="TP2" footprintVariant="pad" pcbX={1} pcbY={-1.5} />
<chip name="U1" footprint="soic8" pcbX={7} />
<autoroutingphase name="USB0" phaseIndex={0} autorouter="auto_local" />
<trace name="P0" path={["J1.pin1", "D1.pin1", "net.USB_P"]} routingPhaseIndex={0} />
<trace name="N0" path={["J1.pin2", "D1.pin2", "net.USB_N"]} routingPhaseIndex={0} />
<differentialpair name="PAIR0" positiveConnection="P0" negativeConnection="N0" pcbTraceGap={0.2} />
<autoroutingphase name="USB1" phaseIndex={1} autorouter="auto_local" />
<trace name="P1" path={["D1.pin1", "TP1.pin1", "net.USB_P"]} routingPhaseIndex={1} />
<trace name="N1" path={["D1.pin2", "TP2.pin1", "net.USB_N"]} routingPhaseIndex={1} />
<differentialpair name="PAIR1" positiveConnection="P1" negativeConnection="N1" pcbTraceGap={0.2} />
<autoroutingphase name="USB2" phaseIndex={2} autorouter="auto_local" />
<trace name="P2" path={["TP1.pin1", "U1.pin1", "net.USB_P"]} routingPhaseIndex={2} />
<trace name="N2" path={["TP2.pin1", "U1.pin2", "net.USB_N"]} routingPhaseIndex={2} />
<differentialpair name="PAIR2" positiveConnection="P2" negativeConnection="N2" pcbTraceGap={0.2} />
</board>
)`

<CircuitPreview defaultView="pcb" code={usbFourPadExample} />

## Properties

| Property | Type | Description |
Expand All @@ -106,3 +137,4 @@ assigns a `<trace />` to a phase. Both traces in a pair must use the same value.
| `positiveConnection` | `string` | Exact positive trace name or port selector. |
| `negativeConnection` | `string` | Exact negative trace name or port selector. |
| `maxLengthSkew` | `number` | Maximum absolute difference between the routed lengths, in millimeters. Accepts any non-negative distance and defaults to `0.1` mm. |
| `pcbTraceGap` | `number \| string` | Requested copper edge-to-edge gap between the pair. A number is in millimeters. |
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