Making embedded system debug easier: useful hardware & software tips
Connecting Tools
Orient the CPU chip so that it’s possible to connect an emulator, if you’re using one. Sometimes the target board is so buried inside of a cabinet that access is limited at best. Most emulator pods have form factors that favor a particular direction of insertion.
Watch out for vertical clearance, too! A pod stacked atop a large SMT adaptor might need 4 to 6 inches of space above the board. Be sure there’s nothing over the top of the board that will interfere with the pod.
Don’t use a “clip-on” adaptor on a SMT package. They are simply not reliable (the one exception is PLCC packages, which have a large lead pitch). A butterfly waving its wings in Brazil creates enough air movement to topple the thing over. Better, remove the CPU and install a soldered-down adaptor. The PCB will be a prototype forever, but at least it will be a reliable prototype.
Leave margin in the system’s timing. If every nanosecond is accounted for, no emulator will work reliably. An extra 5 nsec or so in the read and write cycle—and especially in wait state circuits—does not impact most designs.
If your processor has a BDM or JTAG debug port, be sure to add the appropriate connector on the PCB. Even if you’re planning to use a full-blown emulator or some other development tool, at least add PCB pads and wiring for the BDM connector. The connector’s cost approaches zero and may save a project suffering from tool woes.
A logic analyzer is a fantastic debugging tool yet is always a source of tremendous frustration. By the time you’ve finished connecting 100 clip leads, the first 50 have popped off.
There’s a better solution: Surround your CPU with AMP’s Mictor connectors. These are high-den-sity, controlled impedance parts that can propagate the system’s address, data, and control buses off-board. Both Tektronix and Agilent support the Mictor. Both companies sell cables that lead directly from the logic analyzer to a Mictor. No clip leads, no need to make custom cables, and a guaranteed reliable connection in just seconds. Remove the connectors from production versions of the board or just leave the PCB pads without loading the parts.
Some signals are especially prone to distortion when we connect tools. Address latch enable (ALE), also known as address strobe (AS) on Motorola parts, distinguishes address from data on multiplexed buses. The tiniest bit of noise induced from an emulator or even a probe on this signal will cause the system to crash.
Ditto for any edge-triggered interrupt input (like NMI on many CPUs). Terminate these signals with a twin-resistor network. Though your design may be perfect without the terminations, connecting tools and probing signals may corrupt the signals.
Add test points! Unless its ground connection is very short, a scope cannot accurately display the high-speed signals endemic to our modern designs. In the good old days it was easy to solder a bit of wire to a logic device’s pins to create an instant ground connection. With SMT this is either difficult or impossible, so distribute plenty of accessible ground points around the board.
Other signals we’ll probe a lot and that must be accessible include clock, read, write, and all interrupt inputs. Make sure these each have either test points or a via of sufficient size that a developer can solder a wire (usually a resistor lead) to the signal.
Do add a Vcc test point. Logic probes are old but still very useful tools. Most need a power connection.
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