30 Aug
2008
30 Aug
'08
10:26 a.m.
On 30 Aug 2008 at 8:48, Tim Shoppa wrote:
Note that even in solid state IC form both schools are
thoroughly
established: The CMOS 4017 is a ring counter, the TTL 7490 is a BCD
counter. Usually the ring counter was used where there was no need
for any BCD coding. And the TTL nerds who didn't understand ring
counters always used 7490's followed by a 7442 decoder when they
wanted 10 decoded outputs, when they should've been using a ring
counter to begin with!
Great post, Tim!
It probably is worth mentioning that very few of the early electronic
computers that used decimal radix used BCD. 2421 13571 2+johnson
radix 5, etc. The IBM 650 used 7 bits to express one decimal digit;
while this seems wasteful, it allowed for some error checking, which
in a machine of that vintage was very important.
If all one needs is a counter and doesn't care about the actual
sequence of bit values, an LFSR can be used with fewer carry
propogation issues than a straight binary counter, as long as one can
live with one less state (e.g. 8 bits=255 states).
Cheers,
Chuck