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Connecting Processors

What is the best way to connect an emulator to your target hardware? These days when you are starting a new design, not only do you have to consider which processor to use, but also what processor package: DIP, PLCC, PGA, PQFP, TQFP. The list is incredible.

It used to be that the most popular way to connect an emulator to a target was to have a socket on the target hardware and some kind of male adaptor on the emulator. Today that method is still used extensively, but what about when the processor you are using is only available in a surface mount package? Sure, you might, and I stress MIGHT, be able to get some kind of socket for the processor, such as from AMP or Textool, but not always. In addition, shallow sockets do not always allow for a good connection. Not to mention the fact that, in many cases, you want the target hardware to match the final production boards; therefore, a socket on the board may not be what you are looking for.

On a recent visit to a client's office, I found another reason not to put a socket on a non-socketed production board. The board had not been built to work with an emulator, but the client wanted to do some work with one anyway. A production board was pulled from stock and a socket was used to replace the previously surface mounted processor. When the emulator was connected to the target, nothing worked. Through troubleshooting we discovered that the target had some serious problems. We needed another target board, but none was available because a socket needed to be installed in place of the processor before we could continue. When one of the engineers tried to install the socket, some of the pads on the board were pulled up. It was three days before there was another board available.

So what do you do if you have a surface mount processor and there is no socket available for that part? You could use a solder-on adaptor. In this case, the processor is removed from the target hardware and an adaptor is soldered to the board in the processor's place. In my view, this is probably the worst way to go. First off, if you want to test the target hardware using just the processor and not the emulator, often you have to remove the adaptor and then reattach the processor. Repeated soldering and desoldering will surely weaken the pads on the board. It is true, though, that some solder-on adaptors have a method by which you can install the processor in the adaptor instead of having to remove the adaptor. Second, all emulator pods have some kind of weight associated with them. (That pesky gravity and all) With a solder-on adaptor attached to a pod, if the pod's weight is not centered over the adaptor, the pod will want to tip in one direction or another. The result is a force that is pulling up on the surface mount pads. This could result in just an unconnected pin, or even worse, you could have pulled up pads. My suggestion is to avoid using this method, if at all possible.

So what's left? ONCE mode and a clip-on adaptor, that's what. Some chip manufacturers, like Intel and AMD, have developed a special processor mode called ONCE, or ON Circuit Emulation. In this mode, the output and input/output pins are driven to a high-impedance state. This allows for bed-of-nails testing but also allows for the use of an emulator without having to remove the target processor. During reset, the processor checks the state of certain pins, usually at least one of the address lines. If the tested lines match what the processor is looking for, generally a low on the line, then the processor goes into ONCE mode. An adaptor that clips onto the target processor is used to connect the ICE to the target. The emulator, during reset, is responsible for sending down to the target the signals that are needed to put the target processor into ONCE mode. There are several types of clip-on adaptors, but the best ones have some sort of locking mechanism that helps to hold the adaptor in place, ensuring a solid, reliable connection to the target processor.

The benefits of ONCE mode are numerous for those wishing to use an in-circuit emulator. First, there is no need to remove the processor from the target hardware ever (as long as you don't blow it up). Second, it makes switching between running the target using the emulator and using the on-board processor a breeze; just remove the adaptor. The clip-on adaptor is non-intrusive to the target system in the sense that no modifications need to be made to the target in order to work with the emulator. The adaptor requires little clearance around the processor. In fact, I have yet to hear of a problem with using a clip-on adaptor that was caused by the board lay-out. Actually, I have yet to hear of any problems caused by using a clip-on adaptor.

Among other factors like size and cost of the processor, the tools you will use on a new design can help you to determine which processor package is right for you. Every target connection scheme has its merits, but if the processor you will be using has ONCE mode, or something similar, consider making using of this special mode and a clip-on adaptor. It is the easiest connection scheme to implement. You don't even have to worry about removing the processor from a socket. I should point out that clip-on adaptors also make it easy to connect a logic analyzer and you don't need ONCE mode to do that.