Emulation

Emulation

Even using the described techniques, it cannot be stated that there will never be a need for additional help. There will be times when instrumentation, such as emulation and logic analysis, are necessary to resolve problems within a design quickly. Timing and intermittent problems cannot be easily solved without access to further information about the proces-sor and other system signals. Even so, the recognition of a potential problem source, such as a specific software module or hardware, allows more productive use and a speedier resolu-tion. The adoption of a methodical design approach and the use of ready built boards as the final system, at best remove the need for emulation and, at worst, reduce the amount of time required to debug the system.

There are some problems with using emulation within a board-based system or any rack mounted system. The first is how to get the emulation or logic analysis probe onto the board in the first place. Often the gap between the processor and adjacent boards is too small to cope with the height of the probe. It may be possible to move adjacent boards to other slots, but this can be very difficult or impossible in densely populated racks. The answer is to use an extender board to move the target board out of the rack for easier access. Another problem is the lack of a socketed processor chip which effec-tively prevents the CPU from being removed and the emulator probe from being plugged in. With the move towards surface mount and high pin count packages, this problem is likely to increase. If you are designing your own board, I would recom-mend that sockets are used for the processor to allow an emulator to be used. If possible, and the board space allows it, use a zero insertion force socket. Even with low insertion force sockets, the high pin count can make the insertion force quite large. One option that can be used, but only if the hardware has been designed to do so, is to leave the existing processor in situ and tri-state all its external signals. The emulator is then connected to the processor bus via another connector or socket and takes over the processor board.

The second problem is the effect that large probes can have on the design especially where high speed buses are used. Large probes and the associated cabling create a lot of addi-tional capacitance loading which can prevent an otherwise sound electronic design from working. As a result, the system speed very often must be downgraded to compensate. This means that the emulator can only work with a slower than originally specified design. If there is a timing problem that only appears while the system is running at high speed, then the emulator is next to useless in providing any help. We will come back to emulation techniques at the end of this chapter.