FPGAARCADE minimig compatible board, comments?

[mikej]

Thanks,
There are a few switching inverters which will do the job, but most are switched cap. Even if the switching frequency is quite high the supply will need some extensive filtering.

The DAC has quite a good build in filter, so is there any benefit in adding a line driver - especially if the op-amps power supply is not the best?
/Mike

[mikej]

At the moment the DAC output is capacitive coupled directly to the stereo jack - which is the cheapest and smallest solution for the main card. The DAC has an output filter so it will work fine.

I have more space on the breakout board, and Wolfson talk about an optional low pass filter. It would also have low output impedance good for driving long cable runs.

Downsides are the cost (small) and the need to produce a -ve supply. It can be done with an inverting reg but they are a bit noisy, so without going overboard with the filtering the buffer may not improve the quality of the output, which is the aim for this optional card.

Thoughts?

/Mike

[mikej]

yes, the "big" expansion board will probably have USB, Ethernet and SP-dif. I just did the board symbol for the optical transmitter.

I can't use HDMI because of licensing issues. You can however connect my DVI out with a cable to a HDMI input on a TV, but no audio.
/Mike

[mikej]

jkonstan wrote:
Mikej,

DACs tend to have poor output drive capability; thus, adding buffers (after the series cap) to the DAC outputs is a good idea. The simpliest approach is using a single ended MC33204 (VCC and GND)with a "virtual ground (VCC/2)" as a buffer or as a non-inverting amplifier so that you will not need a -VEE rail.

Thanks, this was my thinking.
If I use a single ended amp then I will require an additional output capacitor? I was trying to avoid this as it degrades the low freq phase response further?

Any tips on how to modify the example WM8729 filter to run single ended? I've mailed Wolfson but no response as yet. I haven't done any analog design for a while, I avoid any components with less than 200 pins
/Mike

[mikej]

The problem is the DVI driver chips I can use will not embed the audio, you can't buy them without a HDMI license. Nothing to stop you wiring the DVI up to a HDMI and using either a separate analog or the digital connection for the sound.

In the future I will look at direct driving the DVI from a Spartan6 with SerDes.

The standard stereo jack works fine, I have it wired up to my amplified PC speakers. Sounds rather nice

As soon as I have finished testing the ARM, memory controller and basic video I'll get some more boards made.

/Mike

[mikej]

Display port would have been nice, although it didn't exist when I started this
/Mike

[mikej]

jkonstan, great thanks for the advice.

One small problem I have is the 47KR resistor is on the bottom board and the feed to the expansion board is taken from there. Perhaps if I do a board respin I can take the audio connection directly from the DAC before the cap, but then it is biased around the DAC vmid which I don't have on the top board. mmm

Having said that, I could add another 1K to VCC and 1K to GND at the left hand side of the 1.8KR, and ignore the 47K to GND. I would also need an output capacitor as well after the 51R, or would you not bother?

The DAC maximum output is 0.726V RMS so 1.1V peak. With a gain of 2 we are getting close the the op amp supplies.

I think I need to simulate this.

/Mike

[mikej]

Not really in this case but same sort of idea. The main reason here is to better drive the cable and whatever is connected to the board, as well as further attenuate any nasty aliasing from the DAC.

The filters on the A500 had a much lower cut-off freq and this could be approximated with a digital filter in the FPGA.

/Mike

[mikej]

Possible, we have gone as far as a 68030.
To be honest, the soft cores are likely to be able to significantly out perform even the 68060 given some time.
Best,
Mike

[mikej]

Exactly. The aim is to get cycle accurate 68000 & 68020 cores. Not sure what the limits are yet but 30-40MHz should be possible.

I think it is unlikely we can clone the 68060 exactly, and I also don't think it is the right way forward. It is much easier to design a software compatible processor with the implementation targeted to the FPGAs architecture.

I am all for adding new video modes etc which can be used easily. Adding new processor instructions is less useful, who is going to code for it now? Anybody fancy doing the compiler backend work?

/Mike

[mikej]

Hi,

The board can be fitted with 128MB of ram. I really thought that would be enough for anybody!

256MByte is possible with a flip mounted chip underneath the top DRAM. This would not normally be mounted so it would not increase cost. I have some spare wires for chip selects. I'll consider this if I have to do a full board respin.

The other possibility as we discussed is memory on the daughter card. The problem here is the IO is 3.3V. You would need a slave memory controller FPGA on the daughter board. I could imagine a 68060, small FPGA and DIMM slot.

Then the main board would be used as the "chipset" and the lower memory as video display buffer.

It's going to be an interesting ride

Best,
Mike.

[mikej]

xyzzy,

USB and Ethernet are planned for the "real" daughter board. At the moment I'm busy making debug/test cards for production testing the main card and for the 68030.

I would love to borrow a 060 if you have a known good working one, that would really help.

Best,
Mike.

[mikej]

I've sent you a pm.
Cheers,
Mike

[mikej]

I've sent you a pm.
Cheers,
Mike

[mikej]

The video out is sort of complex.

There is a link which selects 15K/30K scan like the Minimig board - basically "TV" or "VGA" mode. This lets you at least see the boot menu on whatever you are connected to.

If the composite/SVHS out module is plugged in, then it only works in 15K (TV) mode.

Whatever mode it's in, the analogue output come out on the DVI analogue pins, so you can use a DVI to VGA / SCART adapter for TV or VGA monitors - same as the Minimig.

The digital signals also always come out the DVI pins. If the refresh rate is 50Hz then some computer LCDs won't display it - but if you use a DVI to HDMI cable then LCD TV's will work nicely.

Most LCD TV's will only accept 60Hz on the DVI/VGA input, but are happy with 50Hz on the HDMI / composite / SVHS inputs.

Best,
Mike.

[mikej]

Hi,
yes - I've been on holiday for the last two weeks. I'm now in China and will be back to work on the Replay bring up as soon as possible. Several boards are with other developers who are also making good progress
/Mike