OK, here's the first attempt. I can't attach it as a bitmap or image, 'cause it's too dense. (A bit like me!). So it's in PDF form.
Please go over this, and just double-check that I haven't missed anything silly! Also, if you can, please verify that it matches the correct (i.e. latest) schematic revision.
I've made a few minor, and one major (but invisible) changes to the circuit. Here are the changes I've made :
- I've renumbered the parts. (I can undo that real easy if it's too much of a headache comparing. Most of the subcircuits are blindingly obvious, so it shouldn't be a brain-baker).
- I've added a standard cap and pullup to the reset input of the MPU to meet the Atmel specs. BW was extremely lucky that the floating reset input allowed the system to work at all! I'm assuming, therefore, that he had the programming header connected all the time.
- I've changed the 24x1 SIL headers to 50-pin (25x2) headers. You can use SIL, DIL, or boxed IDC headers as connectors with no changes.
- I've changed the main reg to a 7805 instead of an L05 (this has better line rejection and lower output impedance - both good things in this circuit)
- I've also added a VCC rail to the "spare" pin of the 50-pin header.
A bit of explanation is necessary about the last two changes : I really try to avoid multiple regulators on a single small system - they're just additional points of potential failure. There's no reason why the 7805 can't supply plenty of 5V to all the cards in the system. So by making the motherboard regulator a 7805, and adding the extra bus line, it allows two options for everyone : have all the boards powered exactly the same as the original design - in this case, you don't need to do anything. Just ignore the 25th pin, and the original boards will work perfectly. The second option (that I'll be using) is to use the supplied 5V rail to power the boards.
It's entirely up to you if you want to take advantage of the single power option, and you don't have to do anything either way. Just move a jumper on the daughtercards, and that's it.
I'll provide a design for the daughtercards that does the same thing - build it with the additional regulator on each board, or you can ignore the extra regulators and use the supplied 5V.
I hope this way to a) allow anyone who really, really, really wants to use the exact, same, identical-in-every-way design of BW to do so without even thinking, and b) let anyone who wants a more reliable supply and mounting system to be able to do so with almost no changes to anything - just a jumper on each daughtercard. I think that gives us the best of both worlds, but I live to be corrected.
And of course, this needs to be tested thoroughly - this is something really basic that can catch up to you and ream you while you're not looking! So I'm not assuming anything with this change - not until it's been tested to death and verified that it's at least as reliable as the original. I hope that puts any nervousness at ease!
I hope this design looks right, and please let me know right away if you find anything wrong. As soon as I've heard back, I'll start the PCB design. In fact, I've started playing with the rat's nest already. It's going to be, er, fun to get it fitting the original PCB area...
Cheers everyone!
-PtB
Switched Cap Motherboard 3.pdf
Please go over this, and just double-check that I haven't missed anything silly! Also, if you can, please verify that it matches the correct (i.e. latest) schematic revision.
I've made a few minor, and one major (but invisible) changes to the circuit. Here are the changes I've made :
- I've renumbered the parts. (I can undo that real easy if it's too much of a headache comparing. Most of the subcircuits are blindingly obvious, so it shouldn't be a brain-baker).
- I've added a standard cap and pullup to the reset input of the MPU to meet the Atmel specs. BW was extremely lucky that the floating reset input allowed the system to work at all! I'm assuming, therefore, that he had the programming header connected all the time.
- I've changed the 24x1 SIL headers to 50-pin (25x2) headers. You can use SIL, DIL, or boxed IDC headers as connectors with no changes.
- I've changed the main reg to a 7805 instead of an L05 (this has better line rejection and lower output impedance - both good things in this circuit)
- I've also added a VCC rail to the "spare" pin of the 50-pin header.
A bit of explanation is necessary about the last two changes : I really try to avoid multiple regulators on a single small system - they're just additional points of potential failure. There's no reason why the 7805 can't supply plenty of 5V to all the cards in the system. So by making the motherboard regulator a 7805, and adding the extra bus line, it allows two options for everyone : have all the boards powered exactly the same as the original design - in this case, you don't need to do anything. Just ignore the 25th pin, and the original boards will work perfectly. The second option (that I'll be using) is to use the supplied 5V rail to power the boards.
It's entirely up to you if you want to take advantage of the single power option, and you don't have to do anything either way. Just move a jumper on the daughtercards, and that's it.
I'll provide a design for the daughtercards that does the same thing - build it with the additional regulator on each board, or you can ignore the extra regulators and use the supplied 5V.
I hope this way to a) allow anyone who really, really, really wants to use the exact, same, identical-in-every-way design of BW to do so without even thinking, and b) let anyone who wants a more reliable supply and mounting system to be able to do so with almost no changes to anything - just a jumper on each daughtercard. I think that gives us the best of both worlds, but I live to be corrected.

I hope this design looks right, and please let me know right away if you find anything wrong. As soon as I've heard back, I'll start the PCB design. In fact, I've started playing with the rat's nest already. It's going to be, er, fun to get it fitting the original PCB area...
Cheers everyone!
-PtB
Switched Cap Motherboard 3.pdf
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