Power-up input value detection

[Nicolas Matringe]

Hi
I am working on a design involving the cloning of an obsolete chip in an
FPGA (Altera EP10K family).
The behavior of the chip depends on some input values "at power-up" (no
mention of any reset there). My problem is to reliably detect power-up.

I could still manage something with the reset condition but that's not
exactly what the datasheet says ("If <input signal> is strapped Low at
power-up...")

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____ _ __ ___
| _ \_)/ _|/ _ \ Adresse de retour invalide: retirez le -
| | | | | (_| |_| | Invalid return address: remove the -
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[fabbl]

How long are the "power-up values" stable? The EP10K has a stabilization
time that will need to be taken into account and compared against your old
part.


"Nicolas Matringe" <matringe.nicolas@numeri-cable.fr> wrote in message
news:4007BE46.6040609@numeri-cable.fr...

Hi
I am working on a design involving the cloning of an obsolete chip in an
FPGA (Altera EP10K family).
The behavior of the chip depends on some input values "at power-up" (no
mention of any reset there). My problem is to reliably detect power-up.

I could still manage something with the reset condition but that's not
exactly what the datasheet says ("If <input signal> is strapped Low at
power-up...")

--
____ _ __ ___
| _ \_)/ _|/ _ \ Adresse de retour invalide: retirez le -
| | | | | (_| |_| | Invalid return address: remove the -
|_| |_|_|\__|\___/

 

[Hal Murray]

I could still manage something with the reset condition but that's not
exactly what the datasheet says ("If <input signal> is strapped Low at
power-up...")

What do you think "at power-up" means?

Most chips I've worked with have a separate reset signal that
must be held active until the power is stable. "at power-up"
really means when reset goes away. It's the designer's
job to make sure that doesn't happen before power is stable.

Usually the signals you are looking at are not "strapped"
(or the problem would be simple) but there is a weak pullup/down
that is strong enough as long as no real driver is driving
that signal but doesn't add much load to mess up the normal
use of that pin. So the other half of the designer's job
is to make sure that nothing else drives those signals
until the chip has come out of reset.

Sometimes that last paragraph is wrong. If you are loading
the FPGA from a uProc, the uProc might have control of those
signals so the software has to get it right rather than using
pullups/downs.

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