MrMarkus
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Xilinx FPGA expansion anyone?

Fri Sep 28, 2012 1:08 pm

Hi all,

Firstly a quick intro… I’m an electronics engineer working in the professional broadcast industry with a fairly significant background in both board-level and FPGA design. I received my Raspberry Pi a little while ago and immediately became interested in developing a simple plug-on board to expand it’s basic I/O capability and add a few extra features. Initially just for my own interest but I started to think maybe there’s a few others out here that might like one…

So I’ve come up with the PiXi board… So named because it uses a Xilinx FPGA to expand the I/O through a connection to the existing I/O port (P1) on the Raspberry Pi.

The PiXi board offers two 24 port GPIO connectors, one of which is basically 24 user-configurable I/O pins. The second GPIO connector offers 8 open-collector (pull to GND) 2amp drivers, 8 open-collector (PULL to V+) 2amp drivers and 8 user-configurable I/O (as in GPIO1). The pins are arranged on the connector so that GPIO2 can also directly connect to upto 8 servos of the kind used in R/C vehicles to provide 8 PWM servo (or speed control) drivers.

The board also has an oscillator (frequency is TBD), an interface designed to drive a vacuum fluorescent display, a small EEPROM memory (size is TBD) a fan connector with PWM control & spin status and a temperature monitor (TBC).

I’m also looking at adding multi-channel analogue to digital and digital to analogue converters if there's enough interest and I can do this without adding to the cost too much.

In fact there’s a whole heap of things I could add to this board at this stage, smart-phone based accelerometers, magnetometers & GPS for a few ideas, but I’m trying to keep the cost down. If it’s of interest to many people here then I’d plan to put this into production. I’ve worked in the electronics industry for many years and I have excellent contacts in the electronics manufacturing industry and believe I can get some prototypes together soon.

One of the main ideas behind this board was that it would offer the user the ability to learn about digital electronics, FPGAs, and develop their own FPGA using VHDL or Verilog based FPGA design tools. The reason I’ve chosen a Xilinx FPGA is that Xilinx offers a ‘WebPack’ tool that is freely available on the Xilinx website which will allow a user to complete design an FPGA at no extra cost. Programming the FPGA would be done through the Raspberry Pi GPIO using a simple programming executable. I should perhaps stress at this point that I'm not employed by Xilinx, I just like using them!

A basic block diagram of the board is available at http://www.cantrills.com/astro-designs/ ... xi-200.pdf

Assuming I can get this production (if there’s enough interest), this board would be offered ready assembled and fully tested for a purchase price of around £25 - £35. That’s the target at least…

So I’ll stop waffling now and open this up for comment… What do you think, would anyone here be interested in one for £25-£35?

Also, is there anything in particular anyone would like to see added to the specification?

Thanks :)

Mark

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johnbeetem
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Re: Xilinx FPGA expansion anyone?

Fri Sep 28, 2012 7:15 pm

If you're interested, there's been previous discussion here and at element14 regarding using RasPi with FPGAs:

http://www.raspberrypi.org/phpBB3/viewt ... =63&t=2316
http://www.element14.com/community/thread/17692

My own input is to make the board as cheap as possible, with lots of I/O pins. It might be a good idea to have the I/Os match an existing standard such as Arduino or BeagleBone. Special Computing had started on a BeagleBone FPGA cape, but I haven't seen any progress lately: https://specialcomp.com/beaglebone/BeagleBone_FPGA.html. The cheaper the board, the more likely schools and teen-agers will be able to afford them.

In your block diagram, is that an XC3S200 or XC3S200A? Might as well go with the newer one. For that matter, it may be that Spartan 6 is now available for as cheap or cheaper than Spartan 3.

MrMarkus
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Re: Xilinx FPGA expansion anyone?

Fri Sep 28, 2012 8:36 pm

Thanks John, i'll take a look at those discussions.

It was going to be a Spartan 6 but at the time I was able to get a much better price on the Spartan 3 - and it was the 3A. The Spartan 3A is going to be in production for long enough to make such a product last a few years so it potentially offers an opportunity to do this and keep the cost at rock-bottom - for exactly the reasons you describe.

Having said that, i've not finished pushing for the best price on the Xilinx yet so the exact choice of FPGA is not set in stone yet and yes it would be nice to get the S6 on the board...

I'll take a look at the I/O standards on the Beaglebone & Arduino too.

Cheers,

Mark

Serac
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Re: Xilinx FPGA expansion anyone?

Fri Sep 28, 2012 8:42 pm

Been looking at a Cyclone III device from Altera myself, so you are not alone in your thinking... However, is an RS232 port really necessary as one already exists on the Pi ?

What would be useful to me would be high resolution D/A converters and at least eight I/O pins with differential TTL inputs - All other I/O pins would need to be 5V tolerant which is the downside to the Altera Cyclone devices.

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johnbeetem
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Re: Xilinx FPGA expansion anyone?

Fri Sep 28, 2012 9:13 pm

Serac wrote:All other I/O pins would need to be 5V tolerant which is the downside to the Altera Cyclone devices.
Why do you need 5V tolerance in the 21st Century? If I'm not mistaken, the last Xilinx Spartan family to have truly 5V tolerant I/Os was the Spartan-II. I think it had Zener diodes on the I/Os, which slowed them down. Spartan-IIE and Spartan-3/3E have clamping diodes, so you can attach 5V devices using current-limiting resistors. However, Spartan-3A does not have clamping diodes, so it requires other tricks like bus switches and resistive voltage dividers.

MrMarkus
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Re: Xilinx FPGA expansion anyone?

Fri Sep 28, 2012 9:38 pm

Hi Serac,

As far as I know the serial port on the Pi only becomes 'RS232 serial port' with the addition of a external level converter.

The serial RS232 port i'm describing on the PiXi board is a dual standard RS232 / RS422 port that includes the voltage level conversion on-board - much like a board I've seen that's referred to as the 'Pi232' except the interface on the PiXi board may be configured as either a RS232 (+/- 9V interface) or RS422 (differential +/-5v) and only requires a wired connection to (say) a 9-pin D connector to allow it to connect directly to other serial RS232 or RS422 peripherals.

It can offer a number of 5V capable output through the open-collector outputs on the 'GPIO2' connector. It also offers eleven 5V TTL/CMOS outputs on the 'VFD' connector since vacuum florescent displays typically require a 5v interface. But the 'VFD' connector may be also considered a general-purpose IO. I'd not planned to make the VFD connector bi-directional but this is possible to a certain extent with no effect on component cost so i'll look into that which will then give it eleven 5V tollerant inputs.

Regards,

Mark

Serac
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Re: Xilinx FPGA expansion anyone?

Sat Sep 29, 2012 9:24 am

johnbeetem wrote:Why do you need 5V tolerance in the 21st Century?
Not everything in the real world runs at 3.3V and lower - Much of what I interface to uses 5V TTL or 24V DC. Raw I/O speed is not a priority in this situation, but tolerance to high levels of noise over long distance (5-10m) is, hence the use of higher voltage signaling.

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Arjan
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Re: Xilinx FPGA expansion anyone?

Sat Sep 29, 2012 1:28 pm

With respect to D/A, the MCP4922 (http://ww1.microchip.com/downloads/en/D ... 22250A.pdf) might be of interest.
The MCP4922 device is a dual channel 12-bit DAC which uses an external voltage reference. This device offers high accuracy and low power consumption, and is available in various packages. Communication with the device is accomplished via a simple serial interface using SPI protocols. The MCP4922 device is a part of the MCP4902/MCP4912/MCP4922 product family, which are dualchannel 8-bit/10-bit/12-bit DACs which use external voltage reference (VREF).
I am working on a D/A prototype board with 1* 4922 and 2* 4902. I've added a third /CS by means of a 74HC139
http://www.raspberrypi-dmx.org/
Open Source DMX/RDM/MIDI/OSC/Art-Net/sACN solutions

cutterjohn
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Re: Xilinx FPGA expansion anyone?

Sat Sep 29, 2012 2:32 pm

Would be interesting if it had enough gates to say emulate(in hw) some older 8bit and 16bit comps(support chips, cpu, etc.), e.g. Atari 800, C64, Atari ST, etc. wire up some real peripherals to GPIO(e.g. disk drive), possibly displays to(or send the gfx back to the pi to disp)...

[EDIT]
e.g. http://c64upgra.de/c-one/
[/EDIT]

Although I suspect at that cheap of a price that this is a very basic xilinx FPGA, so it would be mainly for experimenting with some hw emulation in hw.

milhouse
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Re: Xilinx FPGA expansion anyone?

Sun Sep 30, 2012 3:59 am

With the recent sad demise of the Squeezebox range, killed off by Logitech, would a shield with optical (spdif)/coax digital output be possible using this FPGA?

I'm wondering if the R-Pi plus a digital output shield would make a passable replacement for a headless audio playback device akin to the Squeezebox Receiver (control with either the Squeezebox Controller or a smartphone app).

MrMarkus
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Re: Xilinx FPGA expansion anyone?

Mon Oct 01, 2012 9:42 am

cutterjohn wrote:Would be interesting if it had enough gates to say emulate(in hw) some older 8bit and 16bit comps(support chips, cpu, etc.), e.g. Atari 800...
A few years ago i'd probably be up for doing that... I wouldn't be surprised at all if someone had already ported something like the 6502 based Atari to VHDL. It's already been ported to an emulator that runs under Android so the information must be out there somewhere!

Didn't I hear that there's a version of Android that runs on the Pi? So there may already be a solution for achieving this but getting something like the Atari ported to an FPGA through VHDL (or verilog) is still an interesting concept... Even if it could be done (and it probably needs a bigger FPGA) I think the bandwidth available over the Pi's GPIO would limit the quality of the graphics & sound.

I think the Android App was called "Colleen" or somethign like that - if you could get Android on your Pi then it may be possible to get it to emulate the Atari. Then you could use the PiXi (or the Pi's GPIO directly) to add a couple of joystics etc.

Like the idea though!

Mark

MrMarkus
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Re: Xilinx FPGA expansion anyone?

Thu Oct 25, 2012 4:40 pm

I have an updated specification for the board i'm working on together with a 3D CAD drawing of the board taken from the board development tools - so it's definitely moving on... I need to finalise the bill of materials and get this off to the guys who are going to handle it's manufacture so ai can get a final quotation, then finish the board design and get some prototypes made...

I have a very basic page to offer some description of the board (it's really quite laughable but it'll do for now - i'm not a web designer as you can see...) http://www.cantrills.com/astro-designs/pixi

I'm still open to any thoughts anyone might have so if you have any please let me know?

Ta!

Mark

tufty
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Re: Xilinx FPGA expansion anyone?

Thu Oct 25, 2012 7:28 pm

Rather than an fpga, I'd be enormously interested in seeing something with one of chuck moore's greenarrays forth multicore chips.

Yeah I know, not your remit, but if it floats your boat, please contact me...

Simon.

mung
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Re: Xilinx FPGA expansion anyone?

Fri Oct 26, 2012 10:19 am

I would love to play with fpga expansion, not something I have ever worked with before, so I don't really have much to say.

Only thing I can think of is that I hope the board would minimise impact on other I/O (i.e. still allow i2c and spi to function as normal).

I would definitly buy one at $35 (unless something better or cheaper were available) but more than that and I would need to actually have a good reason to invest in it.

summers
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Re: Xilinx FPGA expansion anyone?

Fri Oct 26, 2012 10:44 am

Sounds like the concept is well aligned to the RPi aims, viz a low cost platform, that encorages users to get their hands dirty with hardware. In this case whats bing offered is experience in playing with FPGAs, so yes having it so it can be programmed from the Pi is a great idea. Would one be able to develop VHDL on the Pi? Know here at work any VHDL simulation needs powerful workstations - so that would be out on the pi; so guess the question is can the Pi compile VHDL into the code needed for the Xilinx?

As to how much interst there would be. I would hope lots, but lets face it, developing FPGAs isn't as mainstream as playing with computers and IOs - so level of demand isn't clear to me.

Heater
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Re: Xilinx FPGA expansion anyone?

Fri Oct 26, 2012 10:48 am

MrMarkus,

Interesting project.

You have to do something with that web page though. That small yellow text on a black background is almost unreadable here. Google Chrome, no zoom, LCD monitor.
Memory in C++ is a leaky abstraction .

cutterjohn
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Re: Xilinx FPGA expansion anyone?

Fri Oct 26, 2012 8:01 pm

MrMarkus wrote:
cutterjohn wrote:Would be interesting if it had enough gates to say emulate(in hw) some older 8bit and 16bit comps(support chips, cpu, etc.), e.g. Atari 800...
A few years ago i'd probably be up for doing that... I wouldn't be surprised at all if someone had already ported something like the 6502 based Atari to VHDL. It's already been ported to an emulator that runs under Android so the information must be out there somewhere!

Didn't I hear that there's a version of Android that runs on the Pi? So there may already be a solution for achieving this but getting something like the Atari ported to an FPGA through VHDL (or verilog) is still an interesting concept... Even if it could be done (and it probably needs a bigger FPGA) I think the bandwidth available over the Pi's GPIO would limit the quality of the graphics & sound.

I think the Android App was called "Colleen" or somethign like that - if you could get Android on your Pi then it may be possible to get it to emulate the Atari. Then you could use the PiXi (or the Pi's GPIO directly) to add a couple of joystics etc.

Like the idea though!

Mark
I'd forgotten about this thread until you updated it.

re Atari in an FPGA: it's been done already, but IIRC on a different setup than the c-one. If you google it'll eventually turn up, i.e. I don't recall the site nor do I have the URL handy o.w. I'd've posted more in my first resp.

MrMarkus
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Re: Xilinx FPGA expansion anyone?

Mon Oct 29, 2012 5:32 pm

Heater wrote:MrMarkus,

You have to do something with that web page though. That small yellow text on a black background is almost unreadable here. Google Chrome, no zoom, LCD monitor.
I absolutely agree - try here: (it's better but I still need to improve it...)
http://astro-designs.com/page14.php

MrMarkus
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Re: Xilinx FPGA expansion anyone?

Mon Oct 29, 2012 5:35 pm

summers wrote:Sounds like the concept is well aligned to the RPi aims, viz a low cost platform, that encorages users to get their hands dirty with hardware. In this case whats bing offered is experience in playing with FPGAs, so yes having it so it can be programmed from the Pi is a great idea. Would one be able to develop VHDL on the Pi? Know here at work any VHDL simulation needs powerful workstations - so that would be out on the pi; so guess the question is can the Pi compile VHDL into the code needed for the Xilinx?

As to how much interst there would be. I would hope lots, but lets face it, developing FPGAs isn't as mainstream as playing with computers and IOs - so level of demand isn't clear to me.
Thanks for the thumbs up! With regard to compiling the VHDL on the Pi, I suspect that would be out of the question. The tools do run under Linux as well as Windows but I think it would really struggle on the Pi.

Regards,

Mark

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johnbeetem
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Re: Xilinx FPGA expansion anyone?

Mon Oct 29, 2012 7:07 pm

summers wrote:Would one be able to develop VHDL on the Pi? Know here at work any VHDL simulation needs powerful workstations - so that would be out on the pi; so guess the question is can the Pi compile VHDL into the code needed for the Xilinx?
You could probably run a VHDL simulator on RasPi for a small design. There are open-source VHDL simulators around, but I've never used them so I don't know about their limitations and/or performance.

OTOH, I don't think there are any open-source tools for FPGA logic synthesis, mapping, placement, and routing so you have to use the free-as-in-beer Xilinx tools on an x86 PC, either under Windows or GNU/Linux. You can run Xilinx tools from the command line, so it would be possible to use RasPi as a front end and run the Xilinx tools on a networked PC. Or you can probably run the GNU/Linux version using X Windows with the X server running on RasPi to get a GUI.

MrMarkus
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Re: Xilinx FPGA expansion anyone?

Mon Nov 05, 2012 9:50 pm

mung wrote:I would love to play with fpga expansion, not something I have ever worked with before, so I don't really have much to say.

Only thing I can think of is that I hope the board would minimise impact on other I/O (i.e. still allow i2c and spi to function as normal).

I would definitly buy one at $35 (unless something better or cheaper were available) but more than that and I would need to actually have a good reason to invest in it.
Sorry for the delay in responding to this question, I thought I'd replied to this one previously. The SPI and I2C and GPIO could in theory all be passed straight through to the 24 port 3.3V GPIO on the PiXi board, in fact this was always an intention. At the moment the example FPGA design hangs off the SPI but it could be equally implemented over I2C if the SPI bandwidth was needed elsewhere. It's not a trivial task to pass SPI or I2C through the FPGA but it is doable and is something I plan to implement by example once I have the prototypes to work with.

One of the things i've done with the GPIO is to place the 3.3V GPIO connector in the opposite corner of the board to the Pi's P1 connector so that (excluding power connections) the connector could provide extensions of the Pi's GPIO, I2C & SPI etc. on identical pin numbers of an identical connector and in a similar location relative to the opposite corner of the board. Not an exact representation of the Pi's GPIO connector but hopefully close enough to make it relatively easy to allow the user to plug existing 'Pi' compatible hardware on to the PiXi board that both can be used simultaneosly.

Regards,

Mark

matib12
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Re: Xilinx FPGA expansion anyone?

Sat Sep 14, 2013 12:44 pm

Hi Everyone,

I see that the topic stopped some time ago. Anyway I would like to present you my design. I didn't have time to choose and build any among all possible sub-circuits like gps, gsm or sensors so I decided to build a simple board with an FPGA and GPIO connectors.

http://mbecho.blogspot.it/2013/09/raspf ... berry.html

I would be happy if it were useful to anyone.

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