This is a report on a project I did during this weekend, to show up and to amuse... ;-)

In a local hw mailing list, a colleague offered some LED alphanumeric displays coming from dismantled telecom equipment, and I grabbed on the opportunity (oh yes, salvaging those precious pieces from the garbage bin... :-) ).

During the weekend, as a hobby project, I concocted a rudimentary clock from the display and an AT89S8253 (which was already wired up with some of the surroundings on a perfboard from a previous experiment - any '52 derivative with "biblical" pinout would do). Unfortunately, the camera used to make the videos was unable to capture the wonderful contrast and brightness of these displays.

These displays are "intelligent" and come integrated with a controller remembering the ASCII code of 4 characters and decoding them to the display, so controlling them is a breeze, no contortions with multiplexing and kin. Simply connect 7 data wires (as ASCII is 7 bits), 2 address wires (as there are 4 characters) and a write signal.

I connected the data to P0 and the write to the write signal of '51 (P3.6), so after setting the two address signals (connected arbitrarily to P2.6 and P2.7) I can use MOVX @R0,A with any address (or, more precisely, a variable located to __pdata address space) to write a character. This has the added bonus that P0 during the write is in push-pull mode, so there are no pullups necessary (although I threw in a 8x10k SIP R-pack just out of custom - I tried also writing through "manually" setting P0 and the write signal and everything works OK). The display is connected to the processor's (socket's) pins through an approx. 15cm ribbon, improperly wired without interleaved ground, and that works with absolutely no problemo.

Oh, and there is NO reset circuit connected to the reset pin at the moment - as a gadget with non-critical function, I could allow myself to be lousy ;-) But thanks, it works. I guess Atmel know how to make a basic integrated powerup reset by now, although based on the lack of information on it in the datasheet I wouldn't rely on it in a real commercial application.

Programmed in C using SDCC. I won't publish the sources as they are quite a mess and there is nothing interesting nor special in them. Burned by biprog (as I mentioned in other thread).

There is no RTC at the moment so it will forget when I disconnect power (wall-wart), and as it runs from the "main" crystal it is probably not very precise - I did not measure so far. RTC and battery would require another free weekend day, which is quite unlikely at the moment :-( More ideas came, including adding a buzzer and a wake-up alarm function, more buttons for easier and more complex setup, egg-cooking timer, daily reminder and similar stuff, which IMHO all fits quite well into the definition of hobby.

Jan Waclawek