Expansion interface: rules

Before Christmas Kevin and I discussed the expansion header rule-set that I posted last time. I ended up giving him a little clarification. What follows is a summary. Keep in mind that this is a work in progress. Once we finish the new expansion routing for rev C we will post a summary and use case examples.

Figure 1. A View of the MEC8-RA Expansion Header on Titan.

Trace Lengths and Matching All signal trace lengths must be matched to within 20mils and may not exceed 2000mils. Additionally each differential pair in the bank will be routed as a differential pair each pair's N and P net will be length tuned within 5mils. This tuning is intentionally over-kill to reserve as much margin as possible for the routing on modules.

TX and RX pair assignments All LVDS transmit pairs must be connected to the same side of the expansion connector. Figure 2 shows and mapping that meets this rule. This rule is included because the Samtec MEC8-RA connector as longer pins on one side of the connector. Figure 3 was taken from the MEC8-RA's STEP file. My rough measurements show that the top pins are about 100mils longer than the bottom pins. We can avoid accounting for this difference by routing the ECP5's true LVDS transmitter pairs on one side (labelled as pads A and B) and the receiver pairs (labelled as pads C and D) on the other side.

Figure 2. An TX/RX Mapping Example (TX on the Long Side, RX on the Short Side).

Figure 3. A Top and Bottom Pin Pair from the MEC8-RA.

DQ Pin assignments Each DQ group must be matched to the same set of diff pairs; Within the each DQ group the DQS pair must be on a fixed pin. Figure 4 shows an example DQ mapping. This rule means that the individual DQ pair assignments are not fixed and may be routed differently to ease routing.

Figure 4. An Example Mapping of the Two DQ Groups from an ECP5 Bank.

Other Fixed Pin Assignments The bank's VREF net and PCLK on C/D pads must be routed to fixed pins.