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

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

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.

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

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

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

"Yaju Nagaonkar" <yaj_n@hotmail.com> schrieb im Newsbeitrag 
news:1138303895.360065.315910@g43g2000cwa.googlegroups.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 

Xilinx Website is a good source for information too...
http://www.xilinx.com/xlnx/xil_ans_display.jsp?BV_UseBVCookie=yes&getPagePath=19146

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.

"Peter Alfke" <peter@xilinx.com> wrote in message 
news:1138303332.041604.64010@z14g2000cwz.googlegroups.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