Discussion:
LUT design / Transmission gates or pass transistors?
(too old to reply)
abbas
2009-01-31 02:27:31 UTC
Permalink
Hi all

I was wondering what is the design for the lookup tables of the logic
blocks in FPGAs such as the Virtex model from Xilinx? Does they use
pass transistors as most of the academic papers show? or does they use
transmission gates? Please provide reference if possible.

Thanks.
rickman
2009-01-31 06:13:27 UTC
Permalink
Post by abbas
Hi all
I was wondering what is the design for the lookup tables of the logic
blocks in FPGAs such as the Virtex model from Xilinx? Does they use
pass transistors as most of the academic papers show? or does they use
transmission gates? Please provide reference if possible.
Thanks.
If you understand the difference between pass transistors and
transmission gates, then you should be able to figure this out for
yourself.

Rick
maher
2009-01-31 18:02:24 UTC
Permalink
well, if you understand the difference then please let me know. But if
you don't, then don't pretend to be "Abo El3oreef" -- translates to: a
person who pretends to be knowledgeable while he don't know anything!
Post by rickman
Post by abbas
Hi all
I was wondering what is the design for the lookup tables of the logic
blocks in FPGAs such as the Virtex model from Xilinx? Does they use
pass transistors as most of the academic papers show? or does they use
transmission gates? Please provide reference if possible.
Thanks.
If you understand the difference between pass transistors and
transmission gates, then you should be able to figure this out for
yourself.
Rick
rickman
2009-01-31 21:36:39 UTC
Permalink
Post by maher
well, if you understand the difference then please let me know. But if
you don't, then don't pretend to be "Abo El3oreef" -- translates to: a
person who pretends to be knowledgeable while he don't know anything!
Are you asking what the difference is or are you just being rude?

This is the sort of stuff that can be figured out with a simple google
search. Every question asked here does not have to be answered here.

Search on "transmission gate vs pass transistor" and look at the first
link displayed.

Rick
maher
2009-02-01 00:56:40 UTC
Permalink
Post by rickman
Post by maher
well, if you understand the difference then please let me know. But if
you don't, then don't pretend to be "Abo El3oreef" -- translates to: a
person who pretends to be knowledgeable while he don't know anything!
Sorry if you understand my reply in a wrong way. I just replied the
same way you replied my original message!
Post by rickman
Are you asking what the difference is or are you just being rude?
This is the sort of stuff that can be figured out with a simple google
search.  Every question asked here does not have to be answered here.
Back to my original question: I don't ask what is the different
between transmission gates and pass gates; I know exactly what the
difference is. What I am asking about (as specified in my first post,
sorry I changed my nick name from "abbas" to "maher") is the circuit
level design of lookup tables in commercial FPGAs such as Vitretx-II
pro from Xilinx. Simply, this is my question. I would be grateful to
those who can reply and provide references.
Post by rickman
Search on "transmission gate vs pass transistor" and look at the first
link displayed.
Rick
rickman
2009-02-01 01:37:12 UTC
Permalink
Post by maher
Post by rickman
Post by maher
well, if you understand the difference then please let me know. But if
you don't, then don't pretend to be "Abo El3oreef" -- translates to: a
person who pretends to be knowledgeable while he don't know anything!
Sorry if you understand my reply in a wrong way. I just replied the
same way you replied my original message!
Post by rickman
Are you asking what the difference is or are you just being rude?
This is the sort of stuff that can be figured out with a simple google
search.  Every question asked here does not have to be answered here.
Back to my original question: I don't ask what is the different
between transmission gates and pass gates; I know exactly what the
difference is. What I am asking about (as specified in my first post,
sorry I changed my nick name from "abbas" to "maher") is the circuit
level design of lookup tables in commercial FPGAs such as Vitretx-II
pro from Xilinx. Simply, this is my question. I would be grateful to
those who can reply and provide references.
A transmission gate is a lot bigger, requiring two pass transistors
and an inverter to drive the complemented one. A simple pass
transistor will do nicely for nearly any digital signal gating. A
transmission gate is typically used for analog signals where you need
the resistance to remain relatively constant. A simple pass
transistor has a variable resistance as the input voltage changes...
ok for most digital stuff, but not so good for analog.

I think if you actually understood the difference, you would
understand why transmission gates are not used in FPGAs. These parts
are all about density and pass transistors are used both for route
switching and in the LUTs for the output mux from the memory elements
(a RAM is just memory elements with their outputs muxed together.)
What did you find when you searched on Google?

Rick
maher
2009-02-01 01:58:03 UTC
Permalink
Post by rickman
Post by maher
Post by rickman
Post by maher
well, if you understand the difference then please let me know. But if
you don't, then don't pretend to be "Abo El3oreef" -- translates to: a
person who pretends to be knowledgeable while he don't know anything!
Sorry if you understand my reply in a wrong way. I just replied the
same way you replied my original message!
Post by rickman
Are you asking what the difference is or are you just being rude?
This is the sort of stuff that can be figured out with a simple google
search.  Every question asked here does not have to be answered here.
Back to my original question: I don't ask what is the different
between transmission gates and pass gates; I know exactly what the
difference is. What I am asking about (as specified in my first post,
sorry I changed my nick name from "abbas" to "maher") is the circuit
level design of lookup tables in commercial FPGAs such as Vitretx-II
pro from Xilinx. Simply, this is my question. I would be grateful to
those who can reply and provide references.
A transmission gate is a lot bigger, requiring two pass transistors
and an inverter to drive the complemented one.  A simple pass
transistor will do nicely for nearly any digital signal gating.  A
transmission gate is typically used for analog signals where you need
the resistance to remain relatively constant.  A simple pass
transistor has a variable resistance as the input voltage changes...
ok for most digital stuff, but not so good for analog.
I think if you actually understood the difference, you would
understand why transmission gates are not used in FPGAs.  These parts
are all about density and pass transistors are used both for route
switching and in the LUTs for the output mux from the memory elements
(a RAM is just memory elements with their outputs muxed together.)
What did you find when you searched on Google?
Rick- Hide quoted text -
- Show quoted text -
Thanks rick. You are right about the point that FPGAs are about
density, and it is important to have smaller size components, i.e.
pass transistors to implement lookup tables rather than transmission
gate, though level restorers and buffer are used with pass-transistor
based LUTs to restore the signal level since pass gates are not
perfect for all levels. What made me confused, and originally ask the
question, is that I found the following patent filed by Xilinx in
2003:
http://www.google.com/patents?id=z4IOAAAAEBAJ&dq=low+power+lookup+table+transmission+gate

If you read the abstract, you will notice that the inventors claim
that although the gate count increases in the LUT when TGs are used,
this can be mitigated by removing the level restoring circuits (they
call it half latches) required in pass-transistor based designs, and
removing initialization circuitry that is unnecessary by the TG-based
design.


Please have a look at that patent and let me know if you have any
references that describe the circuit design of LUTs for low power
operation used in the industry.
glen herrmannsfeldt
2009-02-01 06:03:14 UTC
Permalink
rickman <***@gmail.com> wrote:
(snip)
Post by rickman
A transmission gate is a lot bigger, requiring two pass transistors
and an inverter to drive the complemented one. A simple pass
transistor will do nicely for nearly any digital signal gating. A
transmission gate is typically used for analog signals where you need
the resistance to remain relatively constant. A simple pass
transistor has a variable resistance as the input voltage changes...
ok for most digital stuff, but not so good for analog.
Driven by SRAM cells, which have the complement signal available,
you don't need the inverter. So now it is down to one transistor
vs. two, something that will have to be optimized along with
the rest of the design.

In early FPGAs the routing lines could be driven from different
points through pass transistors (or transmission gates). With
the scaling to current sizes, they require intermediate buffers
such that signals can only go one direction. Presumably the
exact location for buffers along with the decision for pass
transistors vs. transmission gates are just a small part of
the design decisions that need to be made to make a high
performance FPGA design work.

-- glen

Mike Treseler
2009-02-01 01:34:25 UTC
Permalink
Post by abbas
I was wondering what is the design for the lookup tables of the logic
blocks in FPGAs such as the Virtex model from Xilinx?
I would use a block ram as a ROM.
The "design" would be a vhdl or verilog code template
something like this:
http://mysite.verizon.net/miketreseler/sync_rom.vhd

-- Mike Treseler
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