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Old 04-26-2006, 10:24 PM
Rene Tschaggelar
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Default The use of analog switches as level translators

At least Analog Devices and also Fairchild propose
analog switches as level translators to interface
eg 1.8V logic to 3.3V logic. I fail to recognize
how this is going to work. The Low state is
understood, it is just pulled down, but how
is a 1.8V output high propagated to a 3.3V
high by just a resistor ?

Apart from the analog switches, what alternative
are there to interface an 1.8V FPGA to normal 3.3V
or 5V logic ? Yes, switchable bidirectional, or
auto bidirectional is preferred.
Else what unidirectional solutions are there ?

Rene
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Old 04-27-2006, 01:18 AM
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Default Re: The use of analog switches as level translators


Rene Tschaggelar skrev:

> At least Analog Devices and also Fairchild propose
> analog switches as level translators to interface
> eg 1.8V logic to 3.3V logic. I fail to recognize
> how this is going to work. The Low state is
> understood, it is just pulled down, but how
> is a 1.8V output high propagated to a 3.3V
> high by just a resistor ?
>
> Apart from the analog switches, what alternative
> are there to interface an 1.8V FPGA to normal 3.3V
> or 5V logic ? Yes, switchable bidirectional, or
> auto bidirectional is preferred.
> Else what unidirectional solutions are there ?
>
> Rene


something like this? http://www.fairchildsemi.com/ds/74/74VCX164245.pdf

-Lasse

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  #3 (permalink)  
Old 04-27-2006, 01:38 AM
Jim Granville
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Default Re: The use of analog switches as level translators

[email protected] wrote:
> Rene Tschaggelar skrev:
>
>
>>At least Analog Devices and also Fairchild propose
>>analog switches as level translators to interface
>>eg 1.8V logic to 3.3V logic. I fail to recognize
>>how this is going to work. The Low state is
>>understood, it is just pulled down, but how
>>is a 1.8V output high propagated to a 3.3V
>>high by just a resistor ?
>>
>>Apart from the analog switches, what alternative
>>are there to interface an 1.8V FPGA to normal 3.3V
>>or 5V logic ? Yes, switchable bidirectional, or
>>auto bidirectional is preferred.
>>Else what unidirectional solutions are there ?
>>
>>Rene

>
>
> something like this? http://www.fairchildsemi.com/ds/74/74VCX164245.pdf
>
> -Lasse


and TI and Philips also have expanding families of dual-Vcc translators.
Dual Vcc works much better at low-power than the limiting resistor
kludges ( tho with FPGA icc's that probably gets lost )

Also, there are Low power CPLDs like the XC2C32A Coolrunner, and Atmel
ATF1502BE that have Dual VccIO, so can also do 'smarter' level shifting
tasks.
-jg


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