Mind Chasers Inc.
Mind Chasers Inc.

High Speed Shield for Darsena

An overview of LVDS routing across standard 100-mil connectors for high-speed Darsena shields

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Overview

This article discusses the initial stages of design and verification of a high-speed shield concept for Arduino form-factor boards. Testing with our FPGA-based Darsena card shows that 625 MHz LVDS signals can be reliably routed across high-quality 100-mil connectors. On Darsena, we re-define four pairs of adjacent pins to route two bi-directional channels of high-speed data (e.g., SGMII Ethernet) between shield and an FPGA resident on Darsena.

Figure 1 below shows a differential probe placed across a pair of pins on a Samtec TSW/SSW connector pair. Note the long posts of the TSW connector to accommodate the RJ-45 jack on the base and to provide for air flow & visibility.

probing Arduino connector
Figure 1. Probing 100-mil SSQ/TSW combination

Figure 2 shows the resulting waveform from the measurement shown above. We were encouraged enough at this point to build our first shield for further testing.

eye diagram
Figure 2. Eye Diagram

Figure 3 below shows our first-article high-speed SMA Shield for Darsena. The TSW connector shown in Figure 1 has been soldered to the card, and the SMA connectors are Samtec SMA-J-P-H-ST-TH1. Each connector routes either the positive or negative signal of an LVDS pair. As shown, we have TX2 / RX2 and TX3 / RX3. The 0 and 1 pairs are utilized on the Darsena base.

first article shield
Figure 3. First Article SMA Shield

Figure 4 shows a close up of our new SMA Shield on Darsena with six of the SMA connectors being utilized.

close up
Figure 4. SMA Shield on Darsena

Figure 5 below shows the SMA shield in action. Darsena's FPGA is configured to switch one of the Ethernet port's traffic onto the shield. A pair of SMA cables connect into our lab bench scope, and the decoded Ethernet stream is shown being successfully decoded.

shield on lab bench
Figure 5. Decoding an Ethernet Stream from the High-speed Shield

The final figure below shows our SMA shield now being used to switch two different Ethernet streams simultaneously. One full-duplex channel (2 pairs / 4 cables) is routed to a prototype Ethernet card we developed, which in turn connects to a Raspberry Pi. Another pair of cables on the SMA shield continues to stream Etherent into our scope, so we can observe the traffic in real-time in a fully deterministic manner.

shield being used as part of a system
Figure 6. Two Active Ethernet Streams onto our Shield

We have several other shields in development, and we'll be posting about these soon.

Terms

  • LVDS: Low Voltage Differential Signaling
  • SGMII: Serial Gigabit Media Independent Interface

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