Pull up/down resistors on Spartan-3E configuration inputs

[PovTruffe]

Hi,

I am new to the FPGA world and trying to design my first Spartan-3E board
(mostly using "copy and paste"...).

Why are there 2 resistors on inputs M0 and M2 on the following schematic?
http://cjoint.com/?1dis6y5LNzz (Basys 2 board)

By the way why is there a resistor on M1 ? Direct grounding is not OK ?

Thanking you in advance.

 

[Nicolas Matringe]

Le 08/03/2011 19:02, PovTruffe a écrit :

Hi,

I am new to the FPGA world and trying to design my first Spartan-3E board
(mostly using "copy and paste"...).

Why are there 2 resistors on inputs M0 and M2 on the following schematic?
http://cjoint.com/?1dis6y5LNzz (Basys 2 board)

By the way why is there a resistor on M1 ? Direct grounding is not OK ?

Hi and welcome to this wonderful world ;-)

The configuration selection pins are also user pins after configuration.
If you want to use them as IOs you won't like them to be directly
connected to ground or Vcc.

Nicolas

 

[Gabor]

On Tuesday, March 8, 2011 3:04:49 PM UTC-5, Nicolas Matringe wrote:

Le 08/03/2011 19:02, PovTruffe a �crit :
Hi,

I am new to the FPGA world and trying to design my first Spartan-3E board
(mostly using "copy and paste"...).

Why are there 2 resistors on inputs M0 and M2 on the following schematic?
http://cjoint.com/?1dis6y5LNzz (Basys 2 board)

By the way why is there a resistor on M1 ? Direct grounding is not OK ?

Hi and welcome to this wonderful world ;-)

The configuration selection pins are also user pins after configuration.
If you want to use them as IOs you won't like them to be directly
connected to ground or Vcc.

Nicolas

Be careful when using pull-down resistors on the
Spartan 3 series. The internal "weak" pullups
are not the wimpy 50K+ Ohm equivalent pullups
you find in most circuits. The actual value
depends on the I/O voltage supply, but can be as
low as 5K ohms, meaning you need 1K or less to
get a solid logic low using an external pull-down.

Anyway your best place to start is the Spartan 3
series configuration users' guide. This shows
recommended connections for all of the available
modes of configuration.

-- Gabor

 

[PovTruffe]

"Gabor" <gabor@alacron.com> a écrit :

Be careful when using pull-down resistors on the
Spartan 3 series. The internal "weak" pullups
are not the wimpy 50K+ Ohm equivalent pullups
you find in most circuits. The actual value
depends on the I/O voltage supply, but can be as
low as 5K ohms, meaning you need 1K or less to
get a solid logic low using an external pull-down.

Anyway your best place to start is the Spartan 3
series configuration users' guide. This shows
recommended connections for all of the available
modes of configuration.

Ouch ! yet another several hundred pages document...
I have thousands of pages to study. Learning FPGAs is really a
big step, even for experienced microcontroller systems designers.

 

[RCIngham]

Ouch ! yet another several hundred pages document...
I have thousands of pages to study. Learning FPGAs is really a
big step, even for experienced microcontroller systems designers.

Yes. That's why we hardware writers are so well paid!
;-)


---------------------------------------
Posted through http://www.FPGARelated.com

 

[geobsd]

hi all
stupid questions for PovTruffe (forgive me)
will you make it yourself (how) or by a pcb maker (where) ?
[not very related] and what do you think about add-on cards (where
only fpga live)
thanks

 

[PovTruffe]

hi all
stupid questions for PovTruffe (forgive me)

Why stupid ?

will you make it yourself (how) or by a pcb maker (where) ?

Yes I will design and make the PCB myself using a laser printer.
I am limited to 2 layers. I know this is not recommended for FPGAs but
this is just a learning project and reliability is not required. I could even
solder some extra wires if necessary. However I hope the FPGA will start...

and what do you think about add-on cards (where
only fpga live)

I dont understand your question.

 

[Christopher Head]

On Tue, 8 Mar 2011 19:02:52 +0100
"PovTruffe" <PovTache@gaga.invalid> wrote:

Why are there 2 resistors on inputs M0 and M2 on the following
schematic? http://cjoint.com/?1dis6y5LNzz (Basys 2 board)

By the way why is there a resistor on M1 ? Direct grounding is not
OK ?

For my Spartan 3A boards, I've always used hard connections on the mode
and variant select pins with no trouble. As pointed out, if you want to
use those pins for something after the chip is configured, obviously
you will need to arrange for them to connect to whatever external
device instead of directly to a rail.

Chris