Re: need small PAL with SR latch


"Keith Williams" <krw@xxxxxxxxxx> wrote in message
news:MPG.1d25f957cf413709989ae9@xxxxxxxxxxxxxxxxxxxxxx
> In article <dOVue.4085$BD2.1932@xxxxxxxxxxxxxxxxxxxx>,
> no.spam.for.me@xxxxxxxxxxxx says...
> > Hi,
> > 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 ?
> >
> > Only needs a dozen gates or so to do the job, could do with being
fast
> > as posible and in circuit programable etc.
>
> The Xilinx SpartanXL had RS latch options in the CLBs, but it wasn't
> well supported. RS latches tend to cause timing headaches and aren't
> recommended. If you insist on RS latches the tools will infer one from
> VHDL, though will bitch loudly (at least Synplify would do both).
>
> --
> Keith

thanks,

Im trying to get to grips with some of the software from the manafacturers,
I tried Altera's Quartus II and that seems realy nice and easy to use, and
has no trouble taking my schematic with an rs latch. however they only do
devices many times larger than what i need. with about 100 too many pins!

I tried Lattice and finding this awkward to use but managed to create a
schematic with an rs latch, I was puzzled becuase I couldnt relate this to
the data ***, however I realise now this isnt what I need as it is using a
D type latch (with a clock wich i had ignored), so I reverted to using 2
gates to do the RS latch function.

Im confused though becuase it seems to have created a gal16v8 but with what
apears on the chip report to be negative logic outputs wich isnt what i
asked for or want.

maybe it would be better to enter it in vhdl ?
what would be an ordinary rs latch as oposed to a equivalent of a clocked D
type latch?

Colin =^.^=


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