EDN — 01.18.96 Probing system simplifies acquiring signals from PQFPs

Out in Front: January 18, 1996

Probing system simplifies acquiring signals from PQFPs

For many designers of high-performance digital pc boards, a probing system from Hewlett-Packard ends the nightmare of trying to acquire true pictures of waveshapes on the pins of high-pin-count ICs. Not only do these devices themselves have hundreds of pins, but also pc boards usually contain several such devices. All devices can have pin spacing that is almost invisible—as small as 0.5 mm. Getting an undistorted picture also requires making a reliable connection to a ground pin located as close as possible to the signal pin. In addition, when you start debugging, you can’t predict which signals might hold the key to an elusive problem.

HP’s MultiProbe system eases these problems by simultaneously contacting as many as 960 pins and connecting any pair of signals to your scope’s inputs without your having to move a probe. The probing system also eliminates the requirement that you note signal names on printouts of waveforms you’ve captured. In addition, the system extends the benefits of active probing to points only a few millimeters from the signal pins. Input impedance is 20 k(ohm) in parallel with less than 4 pF. Rise time is less than 0.5 nsec, and skew between any pair of inputs is less than 250 psec.

The system uses one of three types of pods. One mounts on 240-pin PQFPs. A second mounts on 160-pin PQFPs with 0.65-mm lead spacing or 208-pin PQFPs with leads on 0.5-mm centers. The third type has nine flying leads. This pod has 0.35-nsec rise time and input impedance of 10 k(ohm) in parallel with 1 pF.

Use of the system requires an HP 16500B logic-analyzer mainframe, which houses the plug-in control module. You connect one to four pods to this control module. The mainframe can also house either of two plug-in, real-time-sampling, digital-scope modules. One of these modules takes 1G samples/sec/channel and has 250-MHz bandwidth; the other samples twice as fast and offers twice the bandwidth. The control module provides outputs that you can connect to an external scope, however.

To make a reliable connection to a PQFP device, you must attach a retainer to the top of the IC. HP’s retainer kits include an alignment tool and glue. Retainer attachment takes about 15 minutes. A threaded post projecting from the retainer passes through a hole in the pod. You secure the pod to the IC by placing a thumbscrew over the post. The pods make reliable contact to all IC leads. Because the thumbscrew holds the pod in place, the pod can’t fall off the IC.

To configure a pod for an IC type and a pod orientation, you use a calibration fixture, which performs several functions. (Because the ICs are square, four pod orientations are possible, but some may not work, because tall devices on a pc board can interfere with the rectangular pods.) The fixture blows fusible links in the personality adapter, which contacts the device leads. Three screws attach the adapter to the underside of the pod. Before you blow the fusible links, all of the adapter’s leads are grounded. Blowing the links disconnects the signal leads from ground. Calibration also trims small channel-to-channel gain, offset, and timing differences. A nonvolatile RAM chip in the pod retains the calibration values.

The final step is identifying the name of the signal on each pin. You can do this by creating an ASCII file with a word processor on a PC and loading the file into the control module via the 16500B mainframe’s floppy-disk drive or network connection. You can also enter the names on a keyboard plugged into the mainframe. If you use one of the logic analyzer’s plug-in scope modules, you can select signals for viewing by scrolling through the signal list and using the analyzer’s touch-sensitive color display.

The control module for one to four pods costs $5000. A pod with an adapter kit for 240-pin PQFPs costs $5500. A pod with an adapter kit for either 160- or 208-pin PQFPs costs $4700. A nine-channel flying-lead pod costs $3700. The 16500B mainframe costs $8980. Two-channel, 1G-sample/sec/channel scope plug-ins cost $8950; the 2G-sample/sec/channel version costs $11,800. — by Dan Strassberg

Hewlett-Packard Co,
Santa Clara, CA. (800) 452-4844.