Micah Dowty wrote:
> I am interested in openhardware.net, and I'm sure the schematics would be
> released if I make them open hardware. The hardware isn't very
> complicated, but there are so many small problems that have to be sorted
> out. For example, I wasted about 2 weekends trying to debug a problem in
> my Flash memory that was caused by insufficient decoupling capacitors.
> And because of a missing resistor, I fried the PLL on my first CPU.
> Luckily the CPUs are free samples :)
I have a rule when I design a PCB, one 0.1 ceramic cap per device. I can always take
them off later, the board at openhardware.net exhibits 20mv of noise on the 3.3v
supply. After being bitten by the noise bug a number of times I find it hard NOT to
get excessive with ceramic & tantalum caps (tants should be 1..5uf is fine).
Also, I agree about the resistor, I didn't understand its purpose in the Motorola
schematic where they had the wrong value (24 ohm), I had seen a number of analog
designs where a low ohm resistor was used to decouple a device from a potentially
noisy power buss and this is what I thought it was. By pure chance, I had asked an
engineer at motorola tech support to look at my clock circuit (driven from external
TTL source) and he remarked about the low resistance and sent me a PDF outlining the
criteria for the choice of values... (I am going to check to see if I posted that PDF
on the site, this is a must have).
-- Tom Walsh - WN3L - Embedded Systems Consultant - 'www.openhardware.net', 'www.cyberiansoftware.com' "Windows? No thanks, I have work to do..."
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