regarding your reply to Erik's comments ... Are you staying with the mcu that you have because it's the one that you have, or could you consider acquiring one of those SiLabs boards? The 'F120-series have 128 kB (KiloBytes, not bits) of program store, which is in-application accessible, at least to some extent, and 8 kB of on-chip xRAM. I don't remember whether the xRAM includes the usual 256 bytes of on-chip RAM, but the 'F120 series is (a) very fast, and (b) generously provisioned with interfaces that you can use, as well as options in the way in which you address memory.

First of all, it's unlikely you'll need more program store than what is on-chip. Secondly, the board comes with a programmer module that interfaces serially, via the PC serial port, or, probably, as is the case with their newer chips, via the PC USB port. The device also comes with a trial version of the KEIL compiler and a useable integrated development system and debug tools, all of which you'll find quite helpful.

Now, I don't use the SiLabs parts often, as my work in the past has been focused on 40-pin DIL-packaged components that drop into my 20-30-year-old applications, so I use the Maxim/Dallas DS89C4x0's. These are also quite fast and, in the case of the 'DS89C450, provide 64kB of program store, of which the upper half is in-application-programmable and the entirety of which can be programmed from the PC-serial port. I'm quite satisfied with these parts, but most of the fellows still doing active work in the MCU arena prefer the ones of the SiLabs type, even if they're from another vendor. I'm no longer active, having retired from the business, but if you ask Maxim/Dallas for a sample of their MCU, it's likely they'll let you have one or two just as samples for your development project.

Additionally, the SiLabs 'F120-series supports I2C and SPI interfacing which will give you the option of using serial a interface flash-memory, FRAM, EEPROM, or any of a number of compatible non-memory devices.

I believe you'd be well served to study the datasheets for these devices before pinning yourself down to one or another MCU.

One other thing you might find beneficial if PCB manufacture is a problem, is that you actually can use an old technology to build your prototypes, namely wire-wrap. Tools with which to do this are becoming somewhat scarce, but still occasionally appear on eBay at very reasonable prices, and wire is plentiful and normally not costly. That way you can build your own circuits in a day or so. A circuit such as the one you've presented might take a little over an hour to fabricate, though the sockets might take a while to find. PLCC-44's are probably easier to accommodate than DIL-40's, but I've found other sizes, particularly 20- and 28-pin sockets a bit of a pain to find. I have a vast supply of "used" 14 and 16-pin wire-wrap sockets, so if you need some, I'll happily ship you a reasonable number for the price of postage, just in case you want to do that. BTW, I'd advise you never to use the more-common so-called 3-level sockets, as wire-wrap never requires more than two levels. If you need three, you're doing something wrong. You can contact me via email if you want more information about this.

RE