Answers Database

XCHECKER cannot pull the DONE pin LOW.


Record #883

Product Family:  Software

Product Line:  FPGA Core

Problem Title:
XCHECKER  cannot pull the DONE pin LOW.


Problem Description:
XChecker reports that it could not drive DONE pin low.


Solution 1:

XCHECKER reporting that it can't pull the DONE pin LOW usually indicates
that there's something wrong with the construction of the PCB. Ie. The pin
is either being shorted to Vcc or has a very low resistance pullup. Get the
customer to try a quick experiment. See whether a 500ohm resistor with one
end connected to GND can pull the DONE pin LOW. If a 500ohm resistor doesn't
pull the DONE pin to below ~0.2V then there's now way XCHECKER ever will!
Remember that the DONE/PROG pin of an XC3000 series part is an open-drain
output so, in theory, it can't drive the pin HIGH - something else has to.

If you actually see DONE go suitably LOW then check the following: a) POWERDWN
is attached to Vcc via a pullup. b) The device is in Slave Mode - measure the
pins! c) The RESET pin on XC3000 parts is a bit sensitive to how you use it
in relation to the DONE pin. Check-out the text above the diagram on page
2-119 of the 1994 databook. There's a caveat in that you have to hold RESET
LOW for at least 6us - or 3 internal CCLKs - *AND* the DONE pin is LOW.



End of Record #883