15 Jul
2015
15 Jul
'15
7:11 p.m.
This brings up a good point: just because a D Flip Flop is clocked by
something other than a system-wide (or subsystem-wide) clock does not
turn it into a latch. Flip flops can clocked by combinatorial inputs.
This can be a problematic thing of course, as they can cause glitch
problems - had a couple of those in our student-designed 12 bit
computer, where I ended up feeding the combinatorial input into a D Flip
flop that was clocked by the FPGA-wide 50 Mhz clock, and then fed the
output of that to the flip flops (in my case JK rather than D, but the
idea would be the same).
JRJ
On 7/15/2015 9:02 PM, Jon Elson wrote:
On 07/15/2015 01:24 PM, Noel Chiappa wrote:
Well, one would have to dig into the ALDs to be sure. But, the FEMMs
have some large drawings that are essentially RTL in graphical form, and
a lot of description of how it all worked. My understanding is all
those registers were essentially D latches. So, they got one data input
from the ALU or a mux, and a latch pulse, and provided a Q output. Each
of these latches took up at least 4 SLT packages, I'm not sure exactly
how many for sure. So, the whole latch was composed of something like 4
NOR gates or the equivalent, plus one inverter.
(Sorry about being so vague, I read a bunch of IBM FEMMs about a year
ago when I had some spare time.)
Jon
On
7/14/2015 7:36 PM, Jon Elson wrote:
On the system 360 CPUs, they did not use
flip-flops like we are
used
to, today. They used latches ... Since these were
discrete
transistor
implementations, a real flip-flop was too
expensive, but a
latch could
be implemented in about 6 transistors, I think.
The 11/45 used TTL ICs, so real FFs were available in that
technology,
although they may have used latches as well.
This confused me a bit, until I realized that you were using "latch"
for what
I think of as 'SR flip-flop', and "flip-flop" for 'D and JK
flip-flops'.
Guess that shows how long ago I did hardware... :-)
To be a bit more detailed, on the 360's, were those latches 'simple'
SR flops
(i.e. un-gated), or were they gated?