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  #1 (permalink)  
Old 01-26-2006, 08:11 PM
Yaju Nagaonkar
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Default Current to sink PROG_B low?

I have desingned a board with an Atmega128 and Spartan3, such that the
Atmega128 configures the FPGA via the slave serial method.

In my current rev, the Atmega128 is powered by a 5V supply. The FPGA
power supply is correct. The HSWAP_EN is tied low to ground. An general
I/O on the microcontroller is tied to the FPGA pins.

Anyways, the problem I am having is that I am not able to pull PROG_B
low to reset the FPGA. The miroprocessor pin goes low but it is not
able to pull the PROG_B low at all?

The way I test this, is I have a JTAG port which I used to program the
FPGA. After configruation the DONE signal goes high. So my
understanding was that if I pull the PROG_B low from the
microprocessor, I should be able to manually reset the FPGA and the
DONE signal should go low.

I need to solve this problem before I move on to the actual programming
of the board using the CCLK and DIN signals, to which I will have
attach series resistors because of the Atmega128 (5V) and FPGA(3.3V).

Any suggestions or ideas appreciated.

Thanks

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  #2 (permalink)  
Old 01-26-2006, 08:22 PM
Peter Alfke
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Default Re: Current to sink PROG_B low?


Yaju Nagaonkar wrote:
>...> Anyways, the problem I am having is that I am not able to pull PROG_B
> low to reset the FPGA. The miroprocessor pin goes low but it is not
> able to pull the PROG_B low at all?
>

What do you really mean?
The output pin goes Low, but the PROG pin does not? What is between
these pins, more than a strip of copper?
Pewter Alfke, Xilinx

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  #3 (permalink)  
Old 01-26-2006, 08:31 PM
Yaju Nagaonkar
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Default Re: Current to sink PROG_B low?

There was a 242 ohm resistor to limit the current into PROG_B since,
the microcontroller is at 5V and PROG_B has to be at VCCAUX(2.5V).

I tried to remove the resistor and connected the PROG_B to the
microncontroller I/O. The FPGA still would not reset when the
Microcontroller I/O was held low. The FPGA resets only when I connect a
wire from the PROG_B pin to GND directly.

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  #4 (permalink)  
Old 01-26-2006, 08:49 PM
Peter Alfke
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Default Re: Current to sink PROG_B low?

Measure the current in that wire with a milliampere-meter. That gives
you a feel for the strength of the pull-up transistor or resistor that
you must overcome. Also, there is no need for 242 Ohm. You want to
limit the current between 5 V and 2.5V + one diode drop. 100 Ohm would
limit it to max 18 mA, which is fine.
Peter Alfke

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  #5 (permalink)  
Old 01-26-2006, 08:49 PM
Peter Alfke
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Default Re: Current to sink PROG_B low?

Measure the current in that wire with a milliampere-meter. That gives
you a feel for the strength of the pull-up transistor or resistor that
you must overcome. Also, there is no need for 242 Ohm. You want to
limit the current between 5 V and 2.5V + one diode drop. 100 Ohm would
limit it to max 18 mA, which is fine.
Peter Alfke

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  #6 (permalink)  
Old 01-26-2006, 08:49 PM
Peter Alfke
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Default Re: Current to sink PROG_B low?

Measure the current in that wire with a milliampere-meter. That gives
you a feel for the strength of the pull-up transistor or resistor that
you must overcome. Also, there is no need for 242 Ohm. You want to
limit the current between 5 V and 2.5V + one diode drop. 100 Ohm would
limit it to max 18 mA, which is fine.
Peter Alfke

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  #7 (permalink)  
Old 01-26-2006, 09:00 PM
Antti Lukats
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Default Re: Current to sink PROG_B low?

"Yaju Nagaonkar" <[email protected]> schrieb im Newsbeitrag
news:[email protected] oups.com...
> There was a 242 ohm resistor to limit the current into PROG_B since,
> the microcontroller is at 5V and PROG_B has to be at VCCAUX(2.5V).
>
> I tried to remove the resistor and connected the PROG_B to the
> microncontroller I/O. The FPGA still would not reset when the
> Microcontroller I/O was held low. The FPGA resets only when I connect a
> wire from the PROG_B pin to GND directly.
>


PROG_B is input only on FPGA with virtually no current, so if you nee strong
pulldown
to get it to low level there must be something else pulling it up, not FPGA

Antti


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  #8 (permalink)  
Old 01-26-2006, 09:01 PM
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Default Re: Current to sink PROG_B low?

Xilinx Website is a good source for information too...
http://www.xilinx.com/xlnx/xil_ans_d...PagePath=19146

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  #9 (permalink)  
Old 01-26-2006, 09:31 PM
Yaju Nagaonkar
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Default Re: Current to sink PROG_B low?

Ok. It works now. I cleaned up all the solder from the pin connections
and I disconnected a pull-up on the PROG_B pin. The pull up was tied to
2.5V(VCCAUX) vias 4.7K.
It was a simple mistake which I should have fixed on my own. Basically
I need to learn to solder things neatly.

I have already looked at the website which earlier JPdull has
mentioned.
I will move on to the other signals, names CCLK and DIN. Again, since
my micro-controller (5v) is driving these signals, I will be using
series resistors (300ohm) to limit the current to these pins.


Thanks everyone.

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  #10 (permalink)  
Old 01-27-2006, 05:16 AM
Bob
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Default Re: Current to sink PROG_B low?


"Peter Alfke" <[email protected]> wrote in message
news:[email protected] ups.com...
>
> Yaju Nagaonkar wrote:
>>...> Anyways, the problem I am having is that I am not able to pull PROG_B
>> low to reset the FPGA. The miroprocessor pin goes low but it is not
>> able to pull the PROG_B low at all?
>>

> What do you really mean?
> The output pin goes Low, but the PROG pin does not? What is between
> these pins, more than a strip of copper?
> Pewter Alfke, Xilinx
>


Pewter? Time to lay off the beer, Peter.

Bob


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  #11 (permalink)  
Old 01-27-2006, 03:53 PM
Aurelian Lazarut
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Default Re: Current to sink PROG_B low?



Yaju Nagaonkar wrote:
> Ok. It works now. I cleaned up all the solder from the pin connections
> and I disconnected a pull-up on the PROG_B pin. The pull up was tied to
> 2.5V(VCCAUX) vias 4.7K.
> It was a simple mistake which I should have fixed on my own. Basically
> I need to learn to solder things neatly.
>
> I have already looked at the website which earlier JPdull has
> mentioned.
> I will move on to the other signals, names CCLK and DIN. Again, since
> my micro-controller (5v) is driving these signals, I will be using
> series resistors (300ohm) to limit the current to these pins.
>

why not powering the atmel at 3.3V ? speed ?
Aurash

>
> Thanks everyone.
>

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  #12 (permalink)  
Old 01-31-2006, 11:39 PM
Yaju Nagaonkar
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Default Re: Current to sink PROG_B low?

That was an error in my design. I hope to run it at 3.3V in the next
revision. That would have made things much easier.

Yaju

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