Custom Spartan-7 FPGA board for beginners

[asmi]

@asmi - just wondering, did you place the DDR2 further away from the FPGA in the PCB for a reason?  Looks like it could have been placed a lot closer?

There are few reasons:
1. This board was designed for hand-assembly, I wanted to leave more space between BGA devices for easier hot air gun soldering.
2. I only had 2 signal layers to work with (and even those are not fully accessible due to part placement), length matching requires quite a bit of space, so I wanted to have lots of it.
3. Making compact board was not a design goal (as it in most cases goes against point 1), anything at 10x10 cm or below was good enough. I could've made it more compact, but in that case it would be next to impossible to assemble it without a reflow oven.

Also, they are not that far away from each other - only about 12.5 mm apart. The scale on photos and in eCAD can be very deceptive if you don't have some reference to see the actual scale. All DDR2 traces are only about 36 mm long, which is far enough from critical length (interface is running at 333 MHz due to package limitations) to be reasonably sure it will work with no termination for address/control lines without doing actual board-level simulations (which I can can't do in KiCAD, I would have to import the PCB into Orcad Pro somehow).

[moreasm]

Hi asmi,

I did a DDR2 simulation (400 MT/s) with HyperLynx and I'd say the result is excellent.

Attached you will find the report.

Enjoy the reading.

moreasm

[asmi]

Hi asmi,

I did a DDR2 simulation (400 MT/s) with HyperLynx and I'd say the result is excellent.

Attached you will find the report.

Enjoy the reading.

moreasm

Thanks! I wish I would have access to tools like this... But they are waaaay outside of my budget.
It looks like under/overshoot peaks are a bit out of hand, which is kind of expected given the lack of termination, but the over/undershoot areas still within the limits.

[moreasm]

No problem.

I enjoyed taking the test.

The hardest part was successfully exporting the "HyperLynx" format from KiCad. HyperLyinx generated an import error due to the absence of the stackup in the exported file, once added manually (with a little effort) the import was successful.

Then I assigned the IBIS models and the DDRx batch tool did the rest (once configured).

Tomorrow I will make another report with all the images of the signals not just the ones that don't go through.

I will also see to do the "Power Integrity" test.

Congratulations on your work ... 

moreasm

[moreasm]

Hi,

this is a full DDR2 simulation (400 MT/s) with HyperLynx.

The PDF is large, use this link for download: HyperLynx DDR2 Spartan-7 Custom Board Full Report

moreasm

[koba30]

On this board I use TPS65581 triple DC-DC buck converter IC, which is TSSOP-20EP with exposed thermal pad, which sits on a solid chunk of ground plane with a bunch of thermal vias to ensure good thermal conductivity, so soldering it using lead-free solder would be somewhat tricky with hot air gun, but since these are prototypes, you can use leaded solder and that will make things much easier. These ground planes are obviously not an issue for an oven since entire board is heated up to soldering temperature. This chip is great for many reasons - very easy to use due to no need for external compensation, reasonably good efficiency (it's mostly 70+% for main 1.0 V rail and 80+% for other rails), but my experience with it shows it can heat up pretty severely under full load on all rails - hence the need for robust thermal path.

Hi, is this TPS65581 recommended to power the board with a 5v power supply? Or should it be replaced with another converter with a lower input voltage range?

[asmi]

Hi, is this TPS65581 recommended to power the board with a 5v power supply? Or should it be replaced with another converter with a lower input voltage range?

The datasheet says it should be OK, however there is an LDO which generates 5 V from 12 V for HDMI out and a voltage level converter. If you plan to power the board from 5 V, you can remove that LDO as you can provide 5 V to HDMI out and a voltage level converter directly from your input.

[koba30]

The datasheet says it should be OK, however there is an LDO which generates 5 V from 12 V for HDMI out and a voltage level converter. If you plan to power the board from 5 V, you can remove that LDO as you can provide 5 V to HDMI out and a voltage level converter directly from your input.

That’s great, thanks for your advice, I do not plan to use the HDMI so there is not required the 5v power supply.