SIDshield prototype: test results

For those interested in this project (both of you!) :-) I finished the testing of the SIDshield prototype. Summary: It works! (Yay!) but with caveats and a few fixes are needed (booo!)

Arduino (under Protoshield) with SIDshield prototype fully populated

After cleaning, visual inspection and continuity check (see yesterday's post) I found I forgot 3 wires - one to connect the grounds together, and 2 for getting the 12V to the SID chip. Thus fixed, I populated the board with all chips except for the SID and applied power. Seeing no magic blue smoke escape, I measured the 12V supply, which looked very good (without load of course) giving me ~12.4V. All other signals on the SID socket looked good so I put it in, applied power again and verified the voltages again. Same as without load.

Power Considerations
The SID needs up to 100mA on the 12V line (according to the datasheet), so about 1.2W of power. The TPS6734 has to produce that power from 5V, and thus needs to draw more than twice the current off that supply. Clearly, this is too much for a USB port, thus the Arduino must be connected to a power supply. Also take note that the barrel connector is going through a regulator with associated voltage drop (at least on my Arduino Diecimila) so your supply needs to give more than 5V.

A further complication is that the TPS6734 regulates really well for a range of input voltages; this means it has an effective negative resistance: if the input voltage drops (say, to 4V) it will draw more current; this could lower the voltage even further. We have a positive feedback loop, ending when the supply voltage drops below what the boost converter can use and we loose the 12V. Milliseconds later, the converter starts up again (since it did not draw any current and the supply recovered) and the process begins anew. Needless to say, this does not sound good on the output. So, give the poor SID a good supply!

The multimeter shows the draw on the 5V line, including the Arduino.  The scope shows the noise on the 12V line, which is 1Vp-p and about 100mV RMS.  Noise control needs to be improved.

One could argue of course that the better solution is to provide a good stable 12V supply and drop in a good old 7805 to supply the Arduino - this is why we build prototypes, right?

BTW, with all this power being sucked up, the SID gets pretty warm, I tell you...

Corrections and Changes to the Schematic
The circuit even when supplied with good power on the Arduino external power barrel connector gives a bit of noise when playing large amplitude sounds. This probably needs some resizing of capacitors when compared to the circuit given in the TPS6734 datasheet.

I also moved the reset line of the SID to a programmable (Digital Out) pin, and toggle that line in the SID library constructor.

The biggest error in the schematic posted in a previous post is that the capacitors on the input and output lines are far too small.  22pF?  I have no idea how that got in there - they need to be about .1uF!  I need to fix that on the board.

The hardware is thus effectively finished, needing just a few corrections. I will update the schematic eventually, including the boost converter. The main to-do is now some more code, especially to get steady timing, which is vital for good sounding vibrato, tremolo,  and arpeggio effects - not to mention basic song timing.

However, there are a pile of other projects I've been itching to do...

SIDshield prototype almost done

I've finally gotten around to doing some electronics again, and finished wiring up the full prototype of the SIDshield for Arduino.  Soldering and wiring got done yesterday, later today I will go over the whole thing very carefully to check for shorts and broken connections/wires, make sure GND and power are everywhere they're supposed to be.  Then, I will plug in everything except for the SID and apply power, to make sure the magic smoke stays in; only then will I plug in the SID since it's a not-easily-replaced chip.

Component side

It's not a "real" schield in this format, but I decided it was better to separate it from the Arduino using a ribbon cable.  Still, the whole thing is in a 8cm x 5cm rectangle.

Solder side

I'll post the results of checking and testing either tonight or tomorrow.
Update: results here.

SIDshield on hiatus for a short time

For the few folks who have been looking at the SIDshield project, just a quick note that I will not be updating anything about it for the next few weeks.  At the end of this month (March 2011) I will be defending my Ph.D. thesis, so I need to prepare for that.  Afterwards, I'll be taking a couple of weeks of vacation, away from my tools (but closer to mountains and forests and stuff).

Once I'm back, I'll be ordering the parts to build a properly soldered prototype.

I might still make the occasional post about MATLAB, LaTeX, Python, Mercurial or whatever else takes my fancy but not too much time.

...oh, and anyone knowing of open post-doc positions with focus on audio signal processing (preferably in Europe, Canada or Australia/NZ) feel free to shoot me a line... :-)

SIDshield code updated

I have updated the code for the SIDshield, now it's actually useable to play some music in a relatively easy way.  Here is the new library, and here is a simple player with a little melody.  The player code is basically ported from the SIDcog, including the stored melody - so that remains copyright "Ahle2"! (Who deserves lots of respect for writing SIDcog)

The biggest problem with my code is that I didn't spend much effort in getting the timing correct so it's a bit wonky.

NOTE the Arduino environment puts static tables in RAM, which is very limited especially on the Atmega168.  I would constantly get mysterious malfunctions, including the inability to upload new sketches - effecitvely "bricking" the Arduino.  This can be fixed by carefully removing and resetting the power using the jumper and timing the upload of a new sketch.  It took me quite a while to figure out the solution.

The solution is to ensure the music data is in program memory: using the PROGMEM keyword.  Its use is a bit involved, since one can't simply access it as arrays but has to explicitly copy it using special functions.  See the player app.

UPDATE: The files got moved to my homepage, I adjusted the links accordingly.

Arduino SIDshield initial schematic and code

This is the schematic for the Arduino SIDshield I've been working on, drawn using Eagle. This is NOT the circuit on the breadboard, since I don't have a couple of 74LS164's handy - instead, I used a pair of 74LS323's that are slightly more flexible.  They operate in a simple parallel out mode though.  Also, as mentioned in the previous post, the 1MHz clock is provided by a function generator buffered by a 74LS04.  Hence, regard this schematic as untested!

I still haven't gotten around adding the 5V-to-12V boost circuit.  I was planning on using the LT1109CZ-12 - a nice, small TO-92 packaged chip.  Unfortunately it's not easy to get, it seems.  My 6581 draws 27mA from the 12V supply, well within the limits promised bu Commodore (datasheet says 25mA typ, 40mA max).

Software

The library to drive the chip is a modification of the Spi library, reduced to the constructor and a single function.  I will add more functions later, for now all it does is take an address and data value and set the appropriate SID register.  This first version is on my website.  A simple sketch to test it:

#include <SID.h>

void setup()
{
  sid.setReg(24,15); // full volume
  sid.setReg(6,0xf0); // ADSR=0,0,15,0
  sid.setReg(1,0x1c); // set frequency to 440Hz
  sid.setReg(0,0xd6);
}

void loop()
{
  // .8s beep at 1s interval
  sid.setReg(4,17); // gate on
  delay(800);
  sid.setReg(4,16); // gate off
  delay(200);
}

Performance

The speed with which registers are written is pretty good.  I don't want to make it any shorter actually, since the chip select is not synchronized to the clock signal - so I have to make sure a few cycles happen during CS low.  The shift registers get loaded within 5us, and the CS signal is asserted for slightly less than that.  Overall the entire operation of setReg is about 15us.

More to come.
UPDATE: see here.

Arduino SIDshield: it's ALIVE!!!!

My most recent little project is to make a good ol' Commodore SID chip (6581) be controlled by an Arduino.  The prototype is now functioning:

The breadboard with the Arduino, the SID chip, a couple of shift registers, and a clock buffer.

The complete setup requires a bit more hardware:

Ok, quite a bit of that stuff is redundant...

On the scope you can see the triangular wave I coaxed out of the chip, which was pulled from my first C64, bought back in 1984 or 1985 in Germany.

This weekend I will finalize the schematics, do a board layout and finish the library for the Arduino.  Note that the design is conceptually very similar to the MIDIbox SID module, but then there are only so many ways to attach a SID to a couple of shift registers.

The final version will use a crystal oscillator (replacing the function generator in the picture) for the 1MHz clock, and a DC-DC boost converter to generate the 12V for the SID (currently provided by the adjustable supply).

I will post the schematic, layout and code as soon as I'm done.