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From: Weng Tianxiang on 16 Feb 2010 19:07
On Feb 16, 12:36 pm, -jg <jim.granvi...(a)gmail.com> wrote:
> On Feb 12, 9:05 am, Weng Tianxiang <wtx...(a)gmail.com> wrote:
>
> > Hi,
> > I finally understand the reason when a flip-flops can be replaced by a
> > latch.
> > I saw the circuits before, but not realized what the basic reason was.
> > With the above paper, I now know that the technology is not a new, it
> > originated in 1980s.
>
> Even earlier than that.
>
>  Just look at the relative sales volumes of the venerable 74373 vs
> 74374.
>  In all those cases, the latch is used to buy some extra setup time.
>
>  Anywhere you find an ALE pin, you find this principle, and that goes
> back a LONG way.
>
> -jg

jg,
I checked SN74LV374 TI's manual and couldn't find what you said: ALE
pin.

For time borrowing through a pipelined stages, Intel uses Domino Logic
which was not available until 2000.

Weng
From: glen herrmannsfeldt on 16 Feb 2010 19:18
Weng Tianxiang <wtxwtx(a)gmail.com> wrote:
> On Feb 16, 12:36?pm, -jg <jim.granvi...(a)gmail.com> wrote:

>> Even earlier than that.

>> ?Just look at the relative sales volumes of the venerable 74373 vs
>> | 74374. In all those cases, the latch is used to buy some extra
>> | setup time.

>> ?Anywhere you find an ALE pin, you find this principle, and that
>> | goes back a LONG way.

> I checked SN74LV374 TI's manual and couldn't find what you
> said: ALE pin.

ALE is an output on, for example, many Intel processors. Address
Latch Enable, such that the address can be latched while the pins
are used for other purposes.

The 8085 shares the data bus with part of the address bus, for example.
With a 74S373 the address is available for decoding long before ALE
goes low.

-- glen
From: rickman on 16 Feb 2010 20:08
On Feb 16, 7:07 pm, Weng Tianxiang <wtx...(a)gmail.com> wrote:
> On Feb 16, 12:36 pm, -jg <jim.granvi...(a)gmail.com> wrote:
>
>
>
> > On Feb 12, 9:05 am, Weng Tianxiang <wtx...(a)gmail.com> wrote:
>
> > > Hi,
> > > I finally understand the reason when a flip-flops can be replaced by a
> > > latch.
> > > I saw the circuits before, but not realized what the basic reason was..
> > > With the above paper, I now know that the technology is not a new, it
> > > originated in 1980s.
>
> > Even earlier than that.
>
> >  Just look at the relative sales volumes of the venerable 74373 vs
> > 74374.
> >  In all those cases, the latch is used to buy some extra setup time.
>
> >  Anywhere you find an ALE pin, you find this principle, and that goes
> > back a LONG way.
>
> > -jg
>
> jg,
> I checked SN74LV374 TI's manual and couldn't find what you said: ALE
> pin.
>
> For time borrowing through a pipelined stages, Intel uses Domino Logic
> which was not available until 2000.


The reason twofold. One is that the pin was not called ALE on the
latch, it was called C or G or LE or something similar. ALE is from
the Intel CPUs that require the latch to hold the address bits. The
other reason is that you are looking at the wrong part. The 373 part
is the latch and the 374 part is the register. I am pretty sure the
only difference is the function of the clock input.

The latch is used with these processors for the exact reason you are
looking at latches. It allows the output of the latch to output a
stable value from the input before the clock edge rather than after.
This was used to speed memory accesses.

Rick
From: glen herrmannsfeldt on 16 Feb 2010 20:38
rickman <gnuarm(a)gmail.com> wrote:
(snip)

> The latch is used with these processors for the exact reason you are
> looking at latches. It allows the output of the latch to output a
> stable value from the input before the clock edge rather than after.
> This was used to speed memory accesses.

I still remember latches from when I first started learning about
TTL from Popular Electronics. It was usual to connect a 7490 counter,
a 7475 latch and 7447 BCD to 7 segment decoder together. You run
the counter, the display counts (maybe too fast to see), and then
latches at the appropriate time. Sort of like a lap timer in
a race, which counts up, the latch holds the value while the
counter continues on. After a short time the count continues
on for the next lap. (I think they do this on olympics races.)

-- glen
From: Weng Tianxiang on 17 Feb 2010 20:12
On Feb 16, 5:38 pm, glen herrmannsfeldt <g...(a)ugcs.caltech.edu> wrote:
> rickman <gnu...(a)gmail.com> wrote:
>
> (snip)
>
> > The latch is used with these processors for the exact reason you are
> > looking at latches.  It allows the output of the latch to output a
> > stable value from the input before the clock edge rather than after.
> > This was used to speed memory accesses.
>
> I still remember latches from when I first started learning about
> TTL from Popular Electronics.  It was usual to connect a 7490 counter,
> a 7475 latch and 7447 BCD to 7 segment decoder together.  You run
> the counter, the display counts (maybe too fast to see), and then
> latches at the appropriate time.  Sort of like a lap timer in
> a race, which counts up, the latch holds the value while the
> counter continues on.  After a short time the count continues
> on for the next lap.  (I think they do this on olympics races.)
>
> -- glen

glen,
I found a very good example on how to use a latch.

See Xilinx's patent: 5933369 "RAM with synchronous write port using
dynamic latches".

It describes the method Xilinx uses for its distributed RAM.

Weng
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