18 Jun
2011
18 Jun
'11
11:10 a.m.
Another example is the SR latch
These days you typically see the NAND gate SR latch.
Of course you van cross-couple 2 NOR gats to make an SR latch which has
activ-high set and reset inputs.
Something that's not often taught in introductory electronics courses is
that if you apply De Morgan's law to one of the gates in a cross-coupled
NAND SR latch, and then elimate unecessary inverters, you end up with a
cross-coupled AND and OR gate. This (a) is easy to understand in terms of
'an AND gate can force a signal to be 0, an OR gate can force it to be 1)
nad (b) is useful if you have to fit an SR flip-flop into a pAL or GAL
and don't have CAD tools available.
you can actually build a XOR gate from only just 4
NAND gates
Sure...
I hope my skills in ascii schematics are still good
(using courier font)
+-----\
+--------------------| \
| | O---+
A input | +-----\ +-----| / | +-----\
As I mentiondi n anothre mesage, the Modem 13A makes an XOR (aor maybe
XNOR) gat from the 4 NAND gates in a 74L03, but it doesn't use that
circuit. It does it something like :
+-|\ |
A>----+--| ||>o--+-----------|\
| +-|/ ||>o-----+
+-)----------------------|/ |
| | | |
| +---------------+ +-----+----->Output
| | |
| +------|\ |
| +-|\ | ||>o-----+
B>--+----| ||>o--+-----------|/
+-|/
|
Where ---+--- meaks attach a pull-up resisstor to that point (remember,
it's a 7403, so open-collector). As somebody at HPCC said when I shwoeed
them the schematic, 'It takes a certaint amount ot style to make an XOR
gate from 4 NANDs nad not use the stnadard cirucit'.
-tony
----+---| \ | +-----/ +-----|
\
| O--| | O--> output
----+---| / | +-----\ +-----|
/
B input | +-----/ +-----| \ | +-----/
| | O---+
+--------------------| /
+-----/