I’ve been busy this past week on a new project centered around a Max V CPLD. Schematic isn’t final yet. Just about every time I think I have it nailed down, I find out that I could have used a cheaper part, a smaller part or a certain part ends up not meeting the requirements for one reason or another. Selecting an LDO turned out to be a chore. The one in the schematic below is being swapped out for another. Do I use two, or get one with dual output? Fixed, or adjustable output? Does the dropout line up with my input power requirements, etc? Is the one I selected a common device that’s likely to be available when it’s time to go into production, or is it something that just happened to be in stock, never to be seen again after it runs out? A part that’s unavailable means you have to wait for it to become available again, could be weeks, months, or never. Or it could mean you pay a high price to someone who has it. Or make a change to the PCB design to fit another part, that adds an extra month to the project.
The ATtiny40 I picked looked pretty good. Small, cheap, just enough IO pins to get the job done. Downsides are no on chip debugging support, slave only i2c, not supported by a lot of programmers. I was aware of the first two, they don’t bother me too much. Not working with a wide variety of programmers is a bummer. I’m swapping that out with an ATtiny48 which supports DebugWIRE, has proper i2c support and is on the supported devices list for the AVR Dragon. It costs 30 cents more, but I compensated for it by down costing another area of the circuit so I’m still within my parts budget. I’ll put those extra pins to good use, also, the pin layout is pretty nice.
I’d like to use a smaller, cheaper, lower power oscillator. There probably is one out there, but I haven’t found it. I’m using a 50MHz part because that’s what I need for some of my projects and there is no PLL on the Altera CPLDs. I wonder why they didn’t put one on there, it would be a nice thing to have. Would that significantly add to the cost of the device? Do PLL blocks take up a lot of space on the die? That would be an interesting question to ask an IC designer.
I’ll be ordering most of the parts this evening. By the time I get them in the mail I should have most of the PCB laid out. When that’s done I’ll print out the top layer to check that I got the pads laid out correctly for the parts that weren’t in the library (which was most of them). After that I’ll have some prototype boards made.