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AR# 883

XCHECKER - cannot pull the DONE pin LOW


Keywords: xchecker, done, low

Urgency: Standard

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


XCHECKER reporting that it can't pull the DONE pin LOW usually indicates that there is 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 does not pull the DONE pin to below ~0.2V then there's no way XCHECKER ever will!

Remember that the DONE/PROG pin of an XC3000/4000 series part is an open-drain output so, in theory, it cannot 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 is a caveat in that you have to hold RESET LOW for at least 6us - or 3 internal CCLKs - *AND* the DONE pin is LOW.
AR# 883
日期 09/30/2005
状态 Archive
Type ??????