Re: need small PAL with SR latch

On Fri, 24 Jun 2005 17:15:01 GMT, "colin"
<no.spam.for.me@xxxxxxxxxxxx> wrote:

>"Joel Kolstad" <JKolstad71HatesSpam@xxxxxxxxx> wrote in message
>news:11bob0f3t3k37bf@xxxxxxxxxxxxxxxxxxxxx
>> "colin" <no.spam.for.me@xxxxxxxxxxxx> wrote in message
>> news:dOVue.4085$BD2.1932@xxxxxxxxxxxxxxxxxxxxxxx
>> > I need a small prgramable logic device with a couple of SR latches,
>> > however they all seem to have D types or other type of latch on the
>macro
>> > cell with a global clock/set/reset, exept for some of the realy big
>ones.
>> > Ive looked through all of the ones that Farnell sell, and although I
>> > could obviously feed back an output to make an SR latch this wouldnt be
>> > ideal, any one know if there are any like this ?
>>
>> Why do you say that feeding back an output (internally within the part)
>isn't
>> ideal? With most of the smaller PLDs, the routing is pretty
>staightforward
>> (and fixed!) so it's reasonable to just build your own using the AND-OR
>array
>> within the part.
>
>I estimate there would be 6 gates in the circular path as well as the
>loading of the output pin and grid lines, compared to a normal latch etc
>were there is a circular path of 2 gates with minimal loading. there would
>be a significant diference in speed/generated noise. Im trying to acuratly
>measure relative edge timings so the faster the better, bit like in a type
>II phase detector, except I found that doesnt work quite the way I expected
>as it is indeterminate if a waveforem is 90' or 270' out of phase. a simple
>xor detector would not alow as much sensitivity.
>


For a 22v10 sort of architecture, an RS latch is just


Q = Q * \CLR
+ SET

which is just one macrocell.

John


.

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