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From: krw on 18 May 2010 18:52
On Tue, 18 May 2010 14:45:15 +0200, "aleksa" <aleksazr(a)gmail.com> wrote:

>"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:b734v55bqgpfdrp97cb011jauogvcqelj9(a)4ax.com...
>> It's going to be interesting to try to get an ARM or an FPGA to work
>> on a 2-layer board. The Vcc and ground current spikes can be horrific
>> for a fast, many-pin chip, and a wimpy routed (non-plane) ground
>> structure will cause nasty ground bounce noise.
>> Make the ground and Vcc as wide as you can, and stitch them with lots
>> of bypass caps. Lots of luck.
>
>I have a working FPGA board, Spartan II XC2S50 TQFP144.
>If I replace the PC with ARM, I could even use VQFP100.
>The freq is not much, 40 MHz, and I doubt I'll go higher than that.

The clock frequency isn't (usually) the issue; how fast are your edges?

>I've placed vias on every VCC/GND pair and placed caps on the bottom
>side. The bottom side is almost free of signals, so there's no problem
>connecting the power.

The total distance from the pin to the cap and cap to ground is what matters.

>As for the ARM @ 180 MHz... well let's wait and see.
>I hear other people made it.

Again, the core frequency doesn't matter, much. The package should be
designed well enough to handle that part. Your problem is managing the
delta-I on the power, ground, and signals.
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