Crosspoint switch (signal crosspoint switch IC)

01

Fix the matrix size, signal standard and data rate

TI's DS10CP154A is a 4×4 LVDS crosspoint switch for DC to 1.5 Gbps signal routing over FR-4 backplanes and balanced cables. Its fully differential, non-blocking architecture allows any input to connect to any output or outputs; a wide input common-mode range accepts LVDS, CML and LVPECL levels while outputs are LVDS; every input and output has an on-chip 100 Ω termination; loss-of-signal monitors can flag an open input such as a disconnected cable; and the LVDS pins are rated 8 kV HBM ESD. It runs from 3.0–3.6 V (3.3 V nominal) at −40 to 85 °C, in a 6 × 6 mm WQFN-40, and TI lists channel select, clock and data buffering and muxing, and SD/HD-SDI routers as uses. [1]

The SN65LVDS250 is a 4×4 non-blocking crosspoint in which each output driver has a 4:1 multiplexer so that any input can be routed to any output; TI rates it above 2.0 Gbps, with peak-to-peak jitter of 60 ps (typ.) at 2.0 Gbps and 110 ps (typ.) at 2.5 Gbps, 800 ps (typ.) propagation delay, inputs compatible with LVPECL, CML and LVDS, a single 3.3 V supply drawing 110 mA (typ.), a 38-pin TSSOP, −40 to 85 °C operation and ±3 kV HBM on all pins; the SN65LVDT250 version adds 110 Ω line termination. [3]

For lossy channels, TI's DS25CP104A extends the 4×4 LVDS crosspoint to 3.125 Gbps and adds four levels (off, low, medium, high) of transmit pre-emphasis and receive equalisation, set over SMBus (two levels by pins), to remove inter-symbol-interference jitter; TI lists SD/HD/3G-SDI routers, OC-48/STM-16, InfiniBand and FireWire as uses. [2]

Give the supplier the number of inputs and outputs, signal standards in and out (LVDS, CML, LVPECL), data rate or clock frequency, whether broadcast (one input to all outputs) is needed, channel length and loss, termination preference, control method, supply rail, temperature range, package and quantity.

02

Set the jitter, skew, switching time and control

For the DS10CP154A, TI gives maximum total jitter (PRBS-23) of 0.036 UI at 270 Mbps rising to 0.072 UI at 1.5 Gbps, deterministic jitter of 24–30 ps peak to peak maximum across 270 Mbps to 1.5 Gbps, and random jitter of 1.0–2.0 ps RMS maximum on clocks from 135 to 750 MHz. Differential propagation delay is 460–675 ps, channel-to-channel skew 40 ps (typ.) and 125 ps maximum, part-to-part skew 225 ps maximum, and rise and fall times 145–350 ps. [1] A select-pin change reaches the output in 8 ns (typ.) and 12 ns maximum; power-up takes 7–20 µs. [1]

Its supply current is 40 mA (typ.) with outputs powered down, 103 mA (typ.) and 125 mA maximum in 1:4 broadcast mode, and 115 mA (typ.) and 140 mA maximum as a 4:4 quad buffer; the SMBus interface runs at 10–100 kHz. [1] The SN65LVDS250 specifies select-to-output switch time of 1.2 ns nominal and 1.6 ns maximum. [1] [3]

Editorial calculation: at 1.5 Gbps one unit interval is 1/1.5 GHz ≈ 667 ps, so the DS10CP154A's 0.072 UI maximum total jitter is about 0.072 × 667 ≈ 48 ps; and at 3.3 V its 115 mA (typ.) quad-buffer current is about 3.3 × 0.115 ≈ 0.38 W (0.46 W at the 140 mA maximum). [1] Add the switch's jitter and skew to the transmitter, channel and receiver budget before choosing it. [1]

03

Scale up: what changes in a large crosspoint matrix

MACOM lists the M21605, M21601 and M21080 as 160x160, 120x120 and 80x80 asynchronous, fully nonblocking crosspoint switches, with each path running independently "at any data rate of up to 12.5 Gbps". Each input has a programmable equalizer compensating up to 27 dB of loss at 12.5 Gbps (6.25 GHz), and outputs offer up to 12 dB of post-tap and 5 dB of pre-tap de-emphasis. The M21605 is listed at 32 W power consumption in a 45x45mm package, with a single 1.8 V core and I/O supply, latency of less than 2 ns, four temperature sensors with programmable alarm, per-lane loss-of-signal detection and squelch, and 2-wire, 4-wire serial or 8-bit parallel configuration. [4]

The large matrix adds functions that the 4×4 data sheets on this page do not need: lane-by-lane power-down, loss-of-signal squelch, temperature alarms and JTAG boundary scan [4]. MACOM names serial digital video (SDI) and UHDTV/4K routers, DWDM switches and storage fabrics as applications. For a router design, ask for the equalisation and de-emphasis settings used to reach the stated data rate over your channel, and for the power at the number of lanes you will enable.

Power is the largest difference: the M21605 is listed at 32 W [4], against tens to about 140 mA at 3.3 V for TI's 4×4 LVDS parts [1]. TI's DS25CP104A shows pre-emphasis and equalisation at 3.125 Gbps [2]; at 12.5 Gbps MACOM specifies 27 dB of input equalisation, so ask how the switch is cooled and what loss your backplane traces present at 6.25 GHz.

Editorial calculation: 160 paths at 12.5 Gbps is 2,000 Gbps (2 Tbps) of switching capacity; 32 W over 160 channels is 0.2 W per channel; a TI 4×4 at 140 mA and 3.3 V draws about 0.46 W, or about 0.12 W per channel. [4] [1]

Values are from MACOM's M21605 product page; the data sheet was not reviewed.

04

TI and MACOM parts cited, no carded supplier yet

No company carded on CNFX has been checked for crosspoint switch ICs yet; the Texas Instruments and MACOM parts on this page are cited as examples of what to specify, not as a supplier list.

05

Receivers, matrix, drivers, equaliser and control

The equaliser row applies to the DS25CP104A, which sets four levels of receive equalisation over SMBus; ask whether the part offered for your channel has one. [2]

06

Crosspoint figures to quote back to a supplier

Insertion loss and isolation are the one row the cited data sheets leave blank for these digital switches; every other row has a TI or MACOM example.

07

Send the path, timing and control data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Paths: [inputs; outputs; broadcast]
Signals: [standard in and out; data rate or clock]
Channel: [length; loss; EQ or pre-emphasis]
Timing: [jitter budget; skew; switching time]
Control and power: [pins or SMBus; supply; power budget]
Package: [type; temperature; quantity]

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