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  #1 (permalink)  
Old 11-24-2007, 12:56 PM
joe
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Default using fpga as programmable connection

hi,

i would like to use a small fpga (or cpld) on a pcb to make direct
bidirectional connections between pins.
basically it should act like a programmable "cable".
is this kind of application possible using programmable logic?

regards
j
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  #2 (permalink)  
Old 11-24-2007, 01:06 PM
Jonathan Bromley
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Default Re: using fpga as programmable connection

On Sat, 24 Nov 2007 12:56:34 +0100, joe <[email protected]> wrote:

>i would like to use a small fpga (or cpld) on a pcb to make direct
>bidirectional connections between pins.
>basically it should act like a programmable "cable".
>is this kind of application possible using programmable logic?


Not with a conventional CPLD. You would need to know enough
about the signal flow to control the CPLD's output buffers.

There are plenty of bidi switch devices around (QuickSwitch
and suchlike). It might be a good idea to build a crosspoint
matrix of such switches, and then use a small CPLD to control
the enables.

Lattice have a really interesting product that might suit
your needs:
http://www.latticesemi.com/products/...ct/ispgdx2.cfm

I have no idea about price, availability and development tools
for those parts.
--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services

Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
[email protected]
http://www.MYCOMPANY.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.
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  #3 (permalink)  
Old 11-25-2007, 02:01 AM
Jim Granville
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Default Re: using fpga as programmable connection

joe wrote:
> hi,
>
> i would like to use a small fpga (or cpld) on a pcb to make direct
> bidirectional connections between pins.
> basically it should act like a programmable "cable".
> is this kind of application possible using programmable logic?
>
> regards
> j


If you are looking for analog-switch crosspoint operation, no.
You will need direction control signals, and then you can
build a digital buffered crosspoint system.

How many nodes and what delays can you tolerate ?

A 128MC device could get close to 16x16 crosspoint.

Some CPLDs are RAM based, loaded at power up, and those
variants can be RAM reloaded - meaning you could re-spin the
logic itself, via the JED info, in order to get
maximum crosspoint choice, in a small device.
Makes more sense in higher volume apps, as there is more engineering
needed.

-jg

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  #4 (permalink)  
Old 11-25-2007, 10:51 AM
glen herrmannsfeldt
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Default Re: using fpga as programmable connection

joe wrote:

> i would like to use a small fpga (or cpld) on a pcb to make direct
> bidirectional connections between pins.
> basically it should act like a programmable "cable".
> is this kind of application possible using programmable logic?


http://www.intersil.com/cda/devicein...4,00.html#data

is one of many devices made for this purpose. Though some have built in
buffers, and so are not bidirectional, others are.

You can use tristate drivers in most FPGAs, so that you could select
the direction, but internally the datapaths are programmed directionally.

-- glen

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  #5 (permalink)  
Old 11-27-2007, 07:19 PM
joe
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Default Re: using fpga as programmable connection

hi,

thanks for the answers.
because board area is very limited i was looking for a possibility to do
a (kind of) quick and dirty address data-bus extention on demand where i
additionally can write hardware peripherals in the fpga connected to
this bus but also extend the bus through the fpga if desired (as i said,
kind of "dirty")...

regards



Jonathan Bromley wrote:
> On Sat, 24 Nov 2007 12:56:34 +0100, joe <[email protected]> wrote:
>
>> i would like to use a small fpga (or cpld) on a pcb to make direct
>> bidirectional connections between pins.
>> basically it should act like a programmable "cable".
>> is this kind of application possible using programmable logic?

>
> Not with a conventional CPLD. You would need to know enough
> about the signal flow to control the CPLD's output buffers.
>
> There are plenty of bidi switch devices around (QuickSwitch
> and suchlike). It might be a good idea to build a crosspoint
> matrix of such switches, and then use a small CPLD to control
> the enables.
>
> Lattice have a really interesting product that might suit
> your needs:
> http://www.latticesemi.com/products/...ct/ispgdx2.cfm
>
> I have no idea about price, availability and development tools
> for those parts.

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  #6 (permalink)  
Old 11-28-2007, 01:30 AM
KJ
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Default Re: using fpga as programmable connection


"joe" <[email protected]> wrote in message
news:[email protected] .
> hi,
>
> thanks for the answers.
> because board area is very limited i was looking for a possibility to do a
> (kind of) quick and dirty address data-bus extention on demand where i
> additionally can write hardware peripherals in the fpga connected to this
> bus but also extend the bus through the fpga if desired (as i said, kind
> of "dirty")...
>


But now that you make known your real desire to extend a bus and have some
registers to access, an FPGA solution is plenty viable. Thinking that you
needed a bi-directional connection with no sense of which end is driving or
receiving as you stated in your original post is a completely different
problem.

In any case, extending a bus is easily doable in any FPGA that has enough
I/O pins and meets voltage standards of the things that it is connected
with.

KJ


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