How do you start up an untested circuit board?

Hi Frank,

Interesting topic : ) A proper test plan for bring-up is an excellent idea, and the table looks very good. It is sometimes very easy to design oneself into a hole, with a board that is not easily testable : ) so thinking it through like this makes a lot of sense.

Prior to PCB testing, I try (although admittedly not always) to test all the bits I'm unsure of (either real or use a dev-board, or simulate), before creating the PCB, i.e. I end up with a lot of scraps of stripboard/perfboard etc. But I also scavenge the parts off them once the PCBs arrive.
Sometimes I can't do this, especially recently, where I'm waiting a month or longer for parts to arrive from China, and I sonetimes have to just take the leap, sometimes even just going by measurements of pixels from photos of the parts, to get the dimensioning approximately right. This is only for hobby stuff of course.
For the PCB, in terms of test points, zero-ohm resistors can also be handy for isolating circuits on a board (in third place to 100k and 10k resistors, they are the next most-used resistor for me. I try not to connect inputs/outputs from subsystems on a circuit board together without a resistor in-between, for allowing the isolation, unless I'm certain the circuit portion will work. For digital circuits, I might use a resistor instead of tying pins high or low, in case something else needs to be patched. I don't always do that, but sometimes it's handy. Also I'll sometimes add a resistor in-line, it is usually benign (might help with matching impedances a bit occasionally) but saves too much damage if things get shorted, or if two outputs are accidentally connected together, e.g. RX/TX lines, since sometimes it's easy to get that wrong in a design. Ferrite bead chips are also handy for isolating power to portions of circuitry on a board too, i.e. there's usually no harm to add that in on supply lines to chips. That way, even if a lot of the board is assembled, it is still possible to power off portions for testing (provided inputs/outputs do not cause an issue), or to current-limit using a resistor.
It is also good to stick a polyfuse on each board, especially if it's expensive, I don't do that much on cheap hobby projects, but for the small cost it's worth it for a bit more peace of mind on more expensive boards. With test wires or metal chassis everywhere, it's too easy to accidentally short things, so that bit of extra protection from the polyfuse could help.

EDIT: Also test code helps : ) It is handy to have debug code, extra print statements with a debug flag, to troubleshoot the code and hardware.

Hi Frank,

Interesting topic : ) A proper test plan for bring-up is an excellent idea, and the table looks very good. It is sometimes very easy to design oneself into a hole, with a board that is not easily testable : ) so thinking it through like this makes a lot of sense.

Prior to PCB testing, I try (although admittedly not always) to test all the bits I'm unsure of (either real or use a dev-board, or simulate), before creating the PCB, i.e. I end up with a lot of scraps of stripboard/perfboard etc. But I also scavenge the parts off them once the PCBs arrive.
Sometimes I can't do this, especially recently, where I'm waiting a month or longer for parts to arrive from China, and I sonetimes have to just take the leap, sometimes even just going by measurements of pixels from photos of the parts, to get the dimensioning approximately right. This is only for hobby stuff of course.
For the PCB, in terms of test points, zero-ohm resistors can also be handy for isolating circuits on a board (in third place to 100k and 10k resistors, they are the next most-used resistor for me. I try not to connect inputs/outputs from subsystems on a circuit board together without a resistor in-between, for allowing the isolation, unless I'm certain the circuit portion will work. For digital circuits, I might use a resistor instead of tying pins high or low, in case something else needs to be patched. I don't always do that, but sometimes it's handy. Also I'll sometimes add a resistor in-line, it is usually benign (might help with matching impedances a bit occasionally) but saves too much damage if things get shorted, or if two outputs are accidentally connected together, e.g. RX/TX lines, since sometimes it's easy to get that wrong in a design. Ferrite bead chips are also handy for isolating power to portions of circuitry on a board too, i.e. there's usually no harm to add that in on supply lines to chips. That way, even if a lot of the board is assembled, it is still possible to power off portions for testing (provided inputs/outputs do not cause an issue), or to current-limit using a resistor.
It is also good to stick a polyfuse on each board, especially if it's expensive, I don't do that much on cheap hobby projects, but for the small cost it's worth it for a bit more peace of mind on more expensive boards. With test wires or metal chassis everywhere, it's too easy to accidentally short things, so that bit of extra protection from the polyfuse could help.

EDIT: Also test code helps : ) It is handy to have debug code, extra print statements with a debug flag, to troubleshoot the code and hardware.

Hi Shabaz,

Those are all great suggestions.  I especially like the concept of modeling/testing things beforehand and isolating circuits with zero-ohm resistors.  Can you give an example of a case where you would consider adding a resistor in-line and how you would size it?  I've thought about adding a polyfuse but haven't done that to date.  Ferrite beads are another thing I haven't tried.

Hi Frank, 

For digital logic 33 ohm is often typical (at the output end of the wire), apparently that's good to prevent reflections bouncing.. often I won't over-think it and will just put in a 100 ohm resistor, and see what happens. It will limit current to 33 mA max with 3.3V logic, so it's unlikely to damage chips if an output is accidentally shorted or two outputs are mistakenly connected together, but will hardly slow things down either.

For surface mount, 1206 size is pretty good to select for polyfuses, there's a nice selection then at current ratings which are typical for semiconductor circuits. For ferrite beads, there are curves to indicate the expected impedance at different frequencies, but some datasheets only specify one frequency, in either case I just go for one with high impedance at that frequency... 0805 size is handy, and 600 ohm @ 100 MHz is a typical value feasible for them.. they may have 10 ohm or so at 10 MHz from memory. I used this one: HZ0805E601R-10 but I was running low on them and they are out of stock, so this time ordered PE-0805FB601ST which is similar. They can be treated like a zero-ohm resistor on power lines, but act as a higher value resistor for all the noise on the power line, so they are nicer than a real zero-ohm resistor for power lines, and the same/similar price.

Thanks Shabaz,

Got it and thanks for the clear explanations. I suppose I have been getting away without some of these things as my circuits are not critical and are low speed. But then again I have a box of things I never made work as well.