Figure 1 shows the recommended etch lengths from the driver to the Robinson Nugent connector. Note that the difference in trace lengths between the data links and the clock lines must be matched within ±0.02 in. Also, the trace length difference between the differential pairs is limited to ±0.02 in. Overall, trace length from the driver to the connector is defined as under 4 in.
To assist the designer in optimizing the design, the impedance values for the traces are identified as well. The minimum signal levels at the input side of the 226-Ω tip resistors should be 550-mV nominal, as per the HyperTransport specification. The values given are based on a clock frequency of 1 GHz. Longer trace lengths can be accommodated, if the clock frequency is lower than 1 GHz.
Predictions from a simulation show the signal behavior for a data line with no connector or tip resistors in the design (Fig. 2a) and with the connector and 226-Ω tip resistors added to the design (Fig. 2b). The scope-like waveforms reveal that adding the connector, 226-Ω tip resistors, and our analysis probe results in less than 7% signal reduction.
As with everything in engineering, this solution involves tradeoffs. It requires space on the board for the connector, including a keep-out area to allow mating of the FS2240 test-connector shell. One reason to select the 80-pin Robinson Nugent connector was its small size and high-frequency characteristics. By far, the largest tradeoff is the effort required to design in the connector.
Adding a test connector requires planning. Not thinking ahead could mean that you won't be able to test the circuit, or you will need to add a connector later on. The first situation can result in shipping a product that hasn't been fully characterized. In the second scenario, adding a connector can significantly delay product development. Like most things in life, a little planning at the beginning can save a great deal of time and money in the end.
AGP3.0: This generation calls for an 800-mV voltage swing, with data speeds of 266 MT/s (4X) and 533 MT/s (8X). The previous-generation AGP2.0 had a maximum data rate of 266 MT/s, but the voltage swing was 1.5 V. With a speed of 533 MT/s, a 50% reduction in voltage makes using an interposer with AGP 4X difficult, and maybe impossible.
Designing in another connector similar to what was done for HyperTransport would be very difficult given the nature of the AGP signals and the bus width. So we developed a unique probing solution that takes advantage of the existing PCI-style connector, while preventing the signal delays and loading that an interposer would introduce.
The connection solution is a flex/stiffened-flex pc board that's soldered directly to the backside (solder side) of the target's AGP connector. As Figure 3 shows, the probe adapter will contain the isolating tip resistors and connector for the probe's cable adapter, illustrated in Figure 4.
All components of the probe adapter are mounted on the side opposing the target. This enables flush mounting of the adapter to the target. Employing a flex circuit to attach to the connector maximizes the use of board space and maintains signal integrity. It also provides a connection system that can be used on production systems and prototype boards.
This solution necessitates a small component keep-out area near the target's AGP connector. Due to the increased density of new motherboards, this requirement may be difficult to implement for some production products. Additionally, the through-hole leads from the connector must be long enough for the flex circuit to solder to. Once the flex circuit is attached, it may be difficult to remove, without inadvertently damaging the probe adapter. Thus, for all practical purposes, the probe should be considered consumable.