25 May
2007
25 May
'07
12:59 a.m.
Tony Duell wrote:
Exactly. Without knowing anything about the actual function, I knew I
could do it with a 74x151 (acutally, a 74F151) 8 input mux. The fact that
I wsa using the output to enable a data buffer rather than, say, to clock
a counter, means that odd glitches on that output of said circuit didn't
matter.
More seriosuly, I was once sort-of involved in a
piece of digital design,
and there was some combinatorial block that took in 3 bus lines and
outputted an enable signal to a buffer chip. Without even thinking about
the problem, I said 'I can do that in one chip'. One of the others said
'OK, a PROM or a PAL I suppose'. I said 'No, A normal, non-programmed
chip from the TTL data bool'.
OK, which chip was I thinking off?
If all you want to do is select a single combination of three
bits and enable based on that, a 74138 will do it. If you want
a more complex set of enable conditions, a multiplexer like the
74151 would be better. Connect the three bits to the three 'select' inputs, and wire up the eight
'data' inputs to
generate the truth table you need. It's a bit like an 8-bit
ROM, in a way.
It is _exactly_ like an 8 bit ROM, in that it consists of a fixed AND
matrix (the decoder section of the mux) follwed by a prograamble OR
matrix (here just one output).
You can use it with four input bits, too, but you might need
an inverter as well.
Yep. Or alternatively for the 3 inputs, I could have used the 74x153
4-input mux and maybe a single inverter.
-tony