8 Jan
2008
8 Jan
'08
9:27 a.m.
Jim Brain schrieb:
If you didn't care about sending codes back to the
keyboard, then
you'd just need an 11 bit shift register
The previously mentioned
http://eece.ksu.edu/~eece696/beta/digital/A/stop.htm is almost there,
it just needs 1 more F/F
2 LS174s would work for the F/Fs. the CLK line needs to be inverted,
but you can probably get away without doing it.
A divide by 11 counter (LS163 with and AND on bit 0,1 and 3 driving
CLR) should do the trick.
The price you then pay is additional effort in the target machine to
parse and decode make/break codes of the keyboard which is usually
significantly more expensive than a $2 PIC or ATMEL. Think of an
Apple-II that wants certain ASCII codes at the parallel keyboard input.
You don't want to modify the system (e)prom to convert
the raw codes returned by the keyboard - no space there for tables. You
also don't want to add a simple conversion eprom at the shift register
output to translate the codes into ASCII as this won't deal with the
break (release key) stuff; it's possible to "ground" unwanted code by
translating them to e.g. 0xff and then use a 7430 to suppress the strobe
signal but yet another circuit then.
No, I am not yet at a point to use a microcontroller just to blink a
LED, which have been done with two cross coupled transistors in the
past, but a PIC is meanwhile the more compact, easier, more flexible
solution to "convert-from-A-to-B" problems.
Regards
Holger