Category: electronics

We have this very cheap optical S/PDIF to analog converter which hangs off the back of the TV and feeds the amplifier. We use the TV for audio source switching, since all the other widgets plug into the TV (mostly via HDMI, one with component video and analog audio). This has 2 issues:

* When the converter isn’t getting any input (eg the TV is off), it generates white noise.

* Without the TV on, you can’t listen to music.

I’m contemplating building a better multi-input auto-switching S/PDIF interface, but there seem to be a few hurdles.

Firstly, none of the required chips seem to be available in PDIP packages. More disturbingly, a significant number of useful ones are no longer available in SOIC, but only in even smaller packages like TSSOP.

However, there are a couple of likely SOIC chips out there which seem to be worth investigating. There’s the CS8416, which can select between up to 8 different S/PDIF interfaces (or 4 if you don’t want to interface it with a micro), and spit out an I²S stream (and generate a recovered master clock from the input stream to drive the DAC). There seem to be a few people out there with devices built around this, so it appears to be feasible to make it work as a hobbyist.

DAC selection is a little more complicated. At first sight it would make sense to go with a matching Crystal Semi DAC, but I don’t think any of the newer designs are currently manufactured in SOIC. Eg the CS4392 – the datasheet says you can order it in SOIC, but it seems to only actually be available in TSSOP. So I’m looking at something like the Wolfson WM8521, which has the advantage of producing line-level output straight from the chip (but the disadvantage of requiring either 9V or 12V analog supply). Another advantage of the WM8521 is that it seems to be able to handle a master clock ratio of 128xFs all the way from 8kHz to 192kHz sample rates; this is important because in “hardware mode” the CS8416 can only switch master clock ratios on a hardware reset. An alternative would be the newer WM8726 which can run from 3.3V, but would need an external analog stage to get the output to line level. Or the WM8501 which has a 1.7Vrms line level output, and can run from 5V rather than 9/12V – assuming I can source one.

For the optical inputs, I’d ideally like one which runs off 3.3V and has an output pin to indicate valid data (or at least light) on the input. I’m sure I found one of these once but now I can’t seem to track it down again!

For prototyping this I think I’ll get a couple of the sparkfun SOIC-to-PDIP adapter boards and then I should be able to breadboard it – although I’m a little worried about the CS8416 PLL circuit as it seems to be fussy about capacitance. Hopefully that will just cause the PLL to take a bit longer to lock onto the signal rather than making it fail completely.

For the final object I’ll have to get a board made – I don’t think I can produce a board suitable for SOIC chips by hand.

Of course there are a million other things I should do before this, so I don’t expect it to make progress at all soon. I have however scavenged a board with SOIC chips on it from a broken printer, so I can practice soldering on that before trying it on chips I care about.

I’ve made a couple of pin assignment tweaks to the Microchip HID bootloader. You’ll need MPLAB X, the C18 Lite compiler, and the Microchip Application Libraries. The bootloader I started with is in the Application Libraries under USB/Device – Bootloaders/HID/Firmware – PIC18 Non-J. I’m not sure about the license all this stuff operates under, so here are the changes I made as a zipped diff, and here’s the zipped hex file in case you happen to like my pin assignments better than the default microchip ones (I use RE2 for the bootloader sense and RC0:2 for LEDs; this leaves all of ports B and D available, plus A0:5 and E0:1 which happen to be 8 contiguous pins; it also leaves RC4:5 for USB and RC6:7 for serial).

So I’ve decided to build a new circuit board to convert an old mechanical keyboard (NCD N97) from PS2 to USB. (A normal converter won’t work, as this keyboard only operates in “mode 3” which I very much doubt your average converter will know about). Initially I expected to use an Atmel ATmega32u4 or similar for this – but there’s no through-hole version of that chip, and I don’t want to have to produce a proper PCB and do surface mount soldering. (Atmel take note: I’d be using one of your chips if you did have a through-hole version!). My next thought was to embed a Teensy (or adafruit 32u4 breakout, or Teensy++) into the middle of some veroboard , but that means spending at least $20 on the chip, and I’d have a usb socket inside the keyboard which would then need to be routed to the outside etc.

A bit of googling led me to the Microchip PIC18F4550, which is available in 40pin DIP, and can be purchased from Futurlec for under $7 at time of writing. I order two, and a bunch of support bits (appropriate crystals etc).

Then I start to find that there are some fundamental differences between getting started in the PIC world compared to the AVR world (especially as a linux user who doesn’t want to spend the $$ on a proper programming tool like the PICkit 3).

It seems that the AVR-on-linux community has two big advantages – one is that there’s a nice GCC-based compiler which everyone uses, and the other is that avrdude supports a large number of different programmers, some of which are pretty easy to build. As a result, the community has pretty much a common toolset across the board. And thanks to the arduino project, you can even buy a bare chip with a bootloader on it fairly easily.

PIC-land is different. For a slightly-out-of-date view see this ladyada post. Apparently a GCC port isn’t possible, so there’s a bunch of different C compilers out there in use, most of which aren’t free. There also seems to be a dizzying variety of programming tools, most of which support a limited set of chips, seem to be rarely maintained, and only work with a small number of programmer hardware designs. On top of that, programming generally requires a 12V source (on a 18f4550 there is a low voltage programming mode, but it doesn’t seem to be widely supported and it eats a pin in the middle of an IO port). Which makes the programmer hardware more complicated. After several evenings googling and fruitlessly installing random unmaintained windows programmers (which is complicated by running windows 7 64bit, which doesn’t generally allow the sort of direct access to the parallel port these programs expected from Windows ME or whatever), I discovered that, at least for the 18f4550, the combination of Byron Jeff’s Trivial HVP Programmer circuit and the linux ‘pkp’ programmer which comes as part of PiKdev works nicely.

In addition, Microchip have their beta MPLAB X programming IDE available, which works on linux, and also have the “MPLAB C18 Lite Compiler for PIC18 MCUs” available for free-as-in-beer, along with the “Microchip Application Libraries”. The combination of these 3 allow you to build a bootloader which allows you to reprogram the chip using hid_bootloader. I’ve tweaked the bootloader slightly to suit me better; see my bootloader post.