Designing a shield for Alambra

[Jesus Arias]

Hi,

I'm planing to build a board to provide the Alhambra2 with the convenience connectors for video and audio, mainly because my ratnest of wires disintegrates every time I touch it. This board will include:

- An AP-VGA video intergface.

- Stereo audio output with low-pass filters for either PWM or Sigma-Delta waveforms.

- Mono audio input with an analog sigma-delta modulator.

- PS2 keyboard / mouse connector with level shifters.

- An SD-card holder connected as SPI. (It uses the 4 analog pins of the Alhambra)

Let me attach some 3D views of the board:

And also the schematic.

This is still in a design phase, so, comments (and bug reports ;) are welcome. Mi idea is to order some bare PCBs from PCBWay and to build a prototype.

Good night

[charli va]

I want one of these! If I can help in any way, please let me know!

It looks so good, and it's going to save us so many headaches :)

A lot of thanks !!

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[charli va]

Hi Jesús! I really love this project — it has great potential for implementing a bunch of different systems very easy.

I’m throwing out a few ideas that overlap with things I’m working on and that I think could fit in somehow — but as always, just disregard them if they don’t make sense.

On one hand, since you’re already including the PS/2 connector, maybe you could add a USB connector in parallel (so you could use either USB or PS/2), for example to connect a USB joystick. I published the joystick controller some time ago along with the USB mouse one — maybe it’s trying to squeeze too much in and doesn’t really make sense, but I’ll throw it out there in case you see any potential in it.

On the other hand, what I do see as more interesting would be adding a PSRAM. I’m using this model:
https://w.electrodragon.com/w/images/0/04/LY68L6400_0.3.pdf
I’ll share some blocks with you soon. The results seem excellent considering how inexpensive they are (I still need to test with other PSRAM chips that theoretically use the same protocol — I already have them but haven’t soldered them yet).

The idea would be to literally place it in parallel with the microSD, with the CS inverted, so we can select either the microSD or the PSRAM. It’s not a huge memory, but 64 Mbit is already enough for many things, and the read speed comfortably doubles that of the Alhambra’s flash, while the write speed is literally the same as the read speed (which would be a big improvement).

The only pity is not having two extra lines available to use it in QSPI mode, which makes a big difference (we’d get very interesting speeds from the memory there, but oh well — it is what it is). Although the I²C pins from the ADC are still free, with their pull-ups and the risk of the ADC recognizing a command by accident, contention would be guaranteed.

To make everything fit, what comes to mind is using vertical connectors for the microSD and for the USB (or only for the PSRAM, or only for the USB).

I used this one not long ago and the result was excellent — it’s easy to solder by hand and insertion and removal are super convenient:
https://www.digikey.es/es/products/detail/aces-connectors/MSDV-2108-BK33T/22208760?gad_source=1&gad_campaignid=20199916455&gbraid=0AAAAADrbLli-krHhKF9oRUCs5n11i2zR6&gclid=CjwKCAjw6vHHBhBwEiwAq4zvA5SFw1x14raSUUmMCzUaXamZVd8pWAU29zEOVTLHj6kSHs4aANStoxoCi7sQAvD_BwE&gclsrc=aw.ds

By placing it vertically, I think the PSRAM could fit perfectly in parallel within the same footprint where you currently have the microSD lying flat.

These are just a few simple ideas in case you find them interesting.

Good afternoon!

[charli va]

I made a mistake in combining the CS, I would have to use a high SPDT type switch, but I still think it would fit.

[Jesus Arias]

Hola Carlos,

I think I chose the wrong language for this tread ;) But anyway lets keep it... 

The components you mention are very interesting, but the big problem we're facing, apart from the small card size, is the reduced number of pins. So, some signals have to be shared / multiplexed. This has to be carefully studied. A hand-controlled switch for device selection is something I don't like very much...

The USB is another interface I had in mind, but we have to chose between Host or Device, and between fast or slow for the host case. We can share the PS2 pins (connect one or another, but not the two interfaces) And the 200ohm resistors in series with the pins can also degrade the signals (I don't think this is going to be a problem for slow USBs, for for full-speed ones I'm not so sure. Ideally these resistors should be around 22ohm, that added to about the same resistance in the FPGA pin results in 45ohm in series with D+/D-, or 90ohm for differential signals) An USB in Device mode can also require an extra pin for pull-up activation...

Good night

[Jesus Arias]

Hi again,

I carried out some simulations about the impedance mismatch in the USB cable. Let me present the results, first with a proper 45ohm series resistance and a 12 Mbit/s datastream through a 4 meter cable:

The green curve is the signal at the receiver side.

But now our resistance is 200ohm plus another 25ohm from the FPGA pin:

Here the RX signal is very degraded. So, this full-speed USB is hardly going to work.

But that was for a 4 meter cable. With shorter cables the degradation is going to be lower. Next simulation is for a 1 meter cable:

So, here the RX signal is still quite easy to decode and the USB is going to work at full-speed. We just have to avoid long cables. Of course, for low-speed USBs the cables can be up to 8 times longer, so they are no problem.

After this test I'm going to modify the board in order to include an USB connector with both the host and device possibilities. I think a vertical female type A conector is a good one because there are lots of adapter cables around.

I'm also going to squeeze the PSRAM into the board, and with a 4-bit interface. This will require more pins, so I think I'm going to remove one of the two output audio channels. And, of course, the SDA and SCL pins are going to be used as chip selects. Because only one of the two pins are going to be active (low) at a given time, there will be no unintentional commands sent to the Alhambra I2C converter. 

I'll post another schematic when more or less complete...

Nice day.

[charli va]

Thank you so much, Jesús, for this detailed analysis! If I can help you with anything, please let me know. I'm giving it some more thought to see if I can contribute anything.

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[Jesus Arias]

Hi,

I'm attaching a revised schematic with the changes.

At the end I'm using two configuration jumpers: One for activating the PS2 level converter and another for USB termination (in device mode)

And some look at the vertical connectors and their possible placement (the 3D models are for different but similar parts, so they don't fit the footprints very well, yet, they can show more or less how the board will look)

Good night

[charli va]

It's going to be a super useful board! One thing I can't estimate with the 3D render: leave a few millimeters of space around the USB connectors so the plastic is thick around the male USB connector. Otherwise, in many cases, it will hit the microSD card (which will also stick out upwards).

Have a good start to the day!

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[Obijuan]

Hola Jesus!!

La placa tiene una pintaza increible! Muchísimas gracias por diseñarla... ¡Necesito una! :-)

Saludos, Obijuan

[charli va]

I'm sorry about the quality of the photo. I'm doing a proof of concept following the HAT schematic, and although I've had to improvise the microSD reader and the contacts fail from time to time, reading and writing are good (currently only at 6MHz).

As soon as I get a decent SD connector or can solder, I'll close the module and send it to you. Right now, it's very inefficient because it writes or reads byte by byte, requiring you to set up the rest of the block in a completely inefficient way. But I've already thought about how to invert a BRAM intermediate cache block. This could be very useful!

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[Jesus Arias]

Hola de nuevo

Ya tengo la PCB completa

Adjunto también los PDFs con el esquemático y las dos caras del layout

Y también he subido los fuentes a Github. (El diseño está hecho con Proteus, de Labcenter Electronics)

https://github.com/jesari-git/Alhambra2media

Si alguien ve algún bug, inconveniente, etc.  please comment...

(por cierto, acabo de ver que tal vez las 6 resistencias de la VGA estarían mejor en la cara de debajo, con el resto de los SMDs)

Saludos

[charli va]

Jesus, that's a silly idea. I was thinking that for assembly, this type of connector might be useful instead of the normal pins. This way, it would be practical for debugging both the board and anything else you're doing, and it would make inserting probes much easier.

https://tienda.bricogeek.com/varios/780-conector-hembra-arduino-10-pines.html?srsltid=AfmBOorMqhdWYJbEYfGdtmiCHYxkY-JtDX8dE3F2rBSLVVB0i7cEHC7B

If the idea "worked," I think you'd have to move the PS2 a few mm to the right because I think it touches the pin footprint, and if you used this type of connector, it wouldn't fit. XD

Personally, I don't like the pinout as such, but at least for some debug boards, I think it could be interesting.

On the other hand, your design has been useful to me to test the VGA palette calculator that I prepared this summer for future things :) and I was delighted to see that everything matched up with your calculations. I have uploaded it so that whoever wants to play can do so. It helped me understand how the handful of resistors were translated into colors (in addition to being useful for being able to adjust custom color palettes at certain times).

And it will also generate a very rich and vivid color spectrum :)

If anyone wants to play or is designing a DAC for VGA, you can try it if it helps, I have in mind to include some other typical ladder-type scheme or similar and increase the number of bits, for now as it is a proof of concept it is with 2 which is what we are using in general:

https://icestudio.io/play/vga-dac-calc

Good night!

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[Jesus Arias]

Hi Carlos

I moved a little the connectors in order to make room for the female pins. (notice the audio jacks are really close to each other, the 3D model is narrower than the real part)

And about your DAC calculator. Maybe you can also include this sort of R-2R DAC:

I'm using this DAC in my SIMRETRO board, and with 4-bit per color component we can achieve 4096 colors. The DAC has a maximum output amplitude of 1.4V and an output resistance of 75ohm, so, when connected to a monitor the final amplitude is 0.7V and the cable impedance is matched on both ends (I was a bit too paranoid about impedance matching at that time).

Good night

[charli va]

Hi Jesús! I'm doing some testing to get a head start on debugging the cards. I think if we set up a test bench, it could serve as usage examples and also as a test of the assembly.

With the microSD card, I'm testing with several cards and I've found that with most of them (I have 4 and it's happened with 3), the reading doesn't work unless I use an external pull-up resistor. If I use the FPGA's internal pull-up resistor, the MISO reading is always 0.

As I mentioned, one card works fine with the FPGA's internal pull-up resistor, and then two of them work fine with and without an external pull-up resistor.

Although the standard theoretically recommends a pull-up resistor, many cards supposedly work without one, but...

I mention this because if that's the case, the pull-up resistor would need to be connected to MISO. I'm not sure if it's specific to the card models I have, but I'm mentioning it just in case. I'll run more thorough tests this weekend and confirm it 100%.

This started happening when ramping up to high speeds, possibly because the internal pull-up resistor is degrading the signal too much or isn't able to sustain or raise the voltage quickly enough. Externally, with a 10k resistor, everything is working perfectly.

If this is confirmed, it might make sense to also add pull-up resistors to CS and MOSI to stabilize them at startup. I had to add a large counter at the start of the circuit because the fluctuating startup peak of the Alhambra caused the card to malfunction in many cases. Once I added a buffer time for the Alhambra to stabilize and the SD card to start up smoothly, everything worked perfectly. Perhaps adding a "crutch" to CS and MOSI would make it more robust against this initial noise.

Although it doesn't add much, I'll leave you with a screenshot of the analyzer showing the traffic between the FPGA and the SD card already flowing smoothly.

Good night!

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[Jesus Arias]

Hi Carlos,

That's weird since all SPI signals are of the push-pull type, MISO included. There are no open-drains here. But MISO is left in high impedance until the card has some data to send back to the FPGA, and a floating MISO pin can pick up noise and get randomly some valid tokem (the start of a data block is a single byte: 0xFE, an easy value to find among noise). Maybe this is the source of the problem. Anyway, a pullup won't hurt either, so I'll add it to the board. 

Good point! 

& Good night

[Jesus Arias]

Hi again,

The pullup resistor is included in the last revision of the board:

https://github.com/jesari-git/Alhambra2media

Also, I found another possible issue when using the USB in device mode. I tough the 5V switch in the Alhambra was a simple switch, but in fact it is a MOSFET, and it can keep conducting when it is supposed to be open if 5V are applied from the USB connector, so, I included a jumper in the board to disconnect the 5V entirely if the board is connected to a host with a double-male USB-A cable.

Nice day

[charli va]

Hi Jesús! I'm improving the DAC calculator with your model (as always, very interesting). I'll update you tomorrow; I still have a few details to add.

The first model is very simple to calculate, but after receiving your model, I thought it would be interesting to expand it to easily incorporate more possible models in the future, extend the tool, or make it usable for other things (it's clear that any simulation program like SPICE or KiCad itself can be used, but the idea is to have some visualization beyond just voltage calculations, for example, in this case, to see the color palette. For me, it's an interesting exercise that I think could be useful for other things in the future).

To get to the point, since I'm getting lost, as your model is more complex, I used a computational model (Kirchhoff's current law + solving systems of equations by Gaussian elimination) and I got 0.622V as the final output voltage after accounting for impedances, load resistance, etc. I don't know if I've overlooked some pre-existing resistance that you've included, or if that's the value we should use (or maybe I made a mistake in the calculation).

The palette looks good, but I wanted to know if you've calculated or measured the actual final value of the channel to see if I'm doing something wrong and to fine-tune the model.

Thanks!!

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[Jesus Arias]

Hi,

The R-2R DAC has an output resistance of value R, that is 180ohm, and its maximum voltage is Vdd * 15/16 = 3.09V.

Now, connecting the 130ohm resistor in parallel with the output we get an output resistance of Req=180*130/(180+130) = 75,48ohm, and the max. voltage is Vmax= 3.09 * 130/(130+180) = 1.3V.

So, when matched to the monitor the voltage is going to be half that value, or 0.647V.

Of course, the 2R value isn't the exact 360ohm, but but about 330+25 = 355ohm, and therefore, the DAC isn't very linear (but for 4 bits it is enough). All resistors are rounded to the nearest available value.

I'm attaching a C program for the exact calculation.

Good night

[Jesus Arias]

Sorry, I did a mistake in the previous code (in the function  "genDAC()") I'm attaching it again

[charli va]

Thank you very much! Reviewing your simulator, it was very similar to what I had done, but I had the elephant in the room and couldn't see it: I had slipped in and entered R31 in series instead of parallel, which was the source of the biggest discrepancy.

I've simplified it to use only one value for the pin's resistance, as can be configured to approximate the resulting palette. I think that's enougth.

Now the calculations are closer to the actual theoretical value:

| Parameter | Obtained Value | Expected Value | Error |
|----------------|----------------------|----------------------|---------|
| V_max       |   0.656V            |.             0.647V |  1.4% |
| R_eq          | 176.5Ω             | ~180Ω               |     0    |

I'll post it in a new thread later so as not to clutter this one and to avoid going off on a tangent about our rhizomatic trends.

Have a good day!

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[Jesus Arias]

Hola a todos

Gracias a la entusiasta colaboración de Charliva ya tenemos los primeros prototipos montados y las pruebas que hemos hecho han salido todas bien:

A la izquierda sobre fondo rojo las placas montadas por Carlos, con pines macho-hembra para facilitar la depuración. A la derecha la mía, con pines sólo macho, y enchufada a la Alhambra-II

Saludos

[Jo mo]

Congratulation Jesus  e Charli,

Beautiful and compact design !

You now have plenty of interfaces to play with! ;-)

i am impatient to see the future designs that you will load on your "expanded Alhambra boards"

Have a great week guys !

[Obijuan]

Enhorabuena! La placa tiene una pinta tremenda! Muchísimas gracias!

¿Cómo podría conseguir una? 😀

La placa es genial para temas educativos, sin duda

Saludos, Obijuan

[charli va]

Todo un placer Jesús!, que me hayas dejado participar en "la criatura"  ha sido toda una pasada y en lo que he podido aportar he disfrutado del camino y eso que acaba de empezar!

Va ser una tarjeta que nos va a dar muchísimas posibilidades, se vienen demo en las próximas semanas!!!

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[Democrito]

Quiero una!

[charli va]

In English below:

hola a todos!

Comunicaros que entre Jesús y yo, vamos a donar a algunas personas de la lista un prototipo de la shield de Jesús, las hemos producido a mano con mucho cariño con la idea de que esta experiencia sea super enriquecedora para todos y vivirla en comunidad. Espero que la experiencia sea muy buena y sea la primera de muchas más de este tipo :) 

No vamos a poder enviar placas a todas las personas que nos gustaría, cada placa lleva un importante esfuerzo personal y económico y algunas personas se nos quedan fuera de nuestras posibilidades ahora mismo.

Si hubiera interés general en la shield podéis contar conmigo para que organice una remesa a un precio justo para cubrir gastos mínimos, esto lo dejamos en el aire para los próximos meses.

Esta plaquita le va a dar superpoderes a la Alhambra , la va a convertir en una auténtica plataforma de aprendizaje para poder aprender a hacer cosas al más alto nivel, pudiendo trabajar con ella cómodamente desde verilog o Icestudio.

Os dejo un acceso para no sobrecargar el mensaje con fotos del montaje y algún vídeo de inspección de las soldaduras, en el primer prototipo he aprovechado a experimentar con algunos materiales nuevos y la placa es mejorable, pero lo dejo por si a alguno le es de interés:

https://drive.google.com/drive/folders/1axAZUAIw__CJzvaYTYQvh0KHISqWaLon?usp=sharing

English.               :

Hello everyone!

I wanted to let you know that Jesús and I are going to donate a prototype of Jesús’s shield to some people on the list. We’ve made them by hand with great care, hoping this will be a truly enriching experience for everyone and something we can enjoy together as a community. I hope it turns out to be a great experience and the first of many more like this :)

We won’t be able to send boards to everyone we would like to. Each board requires a significant personal and financial effort, and some people unfortunately fall outside of what we can manage right now.

If there is general interest in the shield, you can count on me to organize a production batch at a fair price to cover minimal costs. We’ll leave that possibility open for the coming months.

This little board is going to give the Alhambra superpowers—it will turn it into a true learning platform where you can work at the highest level, comfortably using Verilog or Icestudio.

I’m sharing a link so as not to overload this message with photos of the assembly and a video inspecting the soldering. In the first prototype I took the opportunity to experiment with some new materials, so the board can still be improved, but I’m leaving it here in case it’s of interest to anyone:

If there is general interest in the shield, you can count on me to organize a production batch at a fair price to cover minimal costs. We’ll leave that possibility open for the coming months.

This little board is going to give the Alhambra superpowers—it will turn it into a true learning platform where you can work at the highest level, comfortably using Verilog or Icestudio.

I’m sharing a link so as not to overload this message with photos of the assembly and a video inspecting the soldering. In the first prototype I took the opportunity to experiment with some new materials, so the board can still be improved, but I’m leaving it here in case it’s of interest to anyone:

https://drive.google.com/drive/folders/1axAZUAIw__CJzvaYTYQvh0KHISqWaLon?usp=sharing

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[Alexander Lang]

Hi, 

I would be happy to buy one of the shields for a fair price 😀.  I would cover the shipping too.

Kind regards

Alex

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[charli va]

🙌 We'll see how we can organize ourselves in the coming days, depending on who's interested. Thanks for your interest!

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[Jesus Arias]

Hi,

I uploaded  a design example for the board to Github:

https://github.com/jesari-git/Alhambra2media/blob/main/demo/larvaMM.tgz

It includes:

- VGA Video

- Sound output: sigma-delta DAC, and input: sigma-delta ADC

- SD card: FAT32 format, read-only

- PSRAM: 4-bit mode with mapping controller (a bit buggy, don't try to execute code from the PSRAM yet)

Not yet included:

- PS2 

- USB

Nice day!

[charli va]

Thanks Jesus!! I’m pulishing my psram modules , i’ll send you at the end of the day . This will be direct from the oven but i think is aparently stable .

Tell you later ,have a nice day!

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[Jo mo]

Ola Jesus and Carlos,

Congratulations for this amazing work ( firmware and hardware code )!
That features.txt file is a pleasure to read.
I also had quick lock a the verilog code files and it looks compact and well documented/easy to read !

For sure i will copy-paste parts of that code for my own designs ;-)

Have a great weekend guys

[Democrito]

I just received my super Shield!

Thank you so much to Carlos and Jesús!

[charli va]

I'm sure you'll get a lot out of it ;)

Go for it!

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