Overview
This article reviews the in-progress design and verification of Gigabit data rate shields 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 bi-directional channels of high-speed data (e.g., SGMII Ethernet) between a shield and an FPGA resident on our Darsena development board.
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, which accommodates the RJ-45 jack on the base and provides for air flow & visibility.
Figure 2 shows the resulting waveform from the measurement shown above.
Figure 3 below shows our 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.
We have also created a v0.2 version of the card that has some minor modifications, such as additional plated holes for ground posts. The design files for this prototype SMA shield v0.2 have been released under an open source license: Solder Pad 2.0. You can find the design files here. At this point, we're releasing this concept design as a schedmatic in pdf form and the PCB files in Gerber form. We're actively researching the best way to release hardware designs as open source and would welcome your input via our comment form at the bottom of this page.
Figure 4 shows a close up of our new SMA Shield on Darsena with six of the SMA connectors being utilized.
Figure 5 shows the SMA shield in action. Darsena's Lattice ECP5 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.
The final figure below shows Darsena with our brand new GigE Shield interfacing to a Raspberry Pi.
We continue to test our two existing shields and pursue the development of new ones, such as M.2.
Terms
- LVDS: Low Voltage Differential Signaling
- SGMII: Serial Gigabit Media Independent Interface
- SMA: SubMinature version A connectors, basically a very high quality 50 ohm RF/ high frequency single ended connector to ground
