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As lynx said,
the e144 package has an exposed pad at the bottom of the package. this exposed pad is a ground pad that must be connected to the ground plane on your pcb. (EP3C5, Pin-Outs, Note 6).
there is not connection between this pad and gnd. if this pad is not connected, c iii doesn't work correct --- Quote End ---
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Hi, I am testing a new design that uses three Cyclone III devices including an EP3C5E144 part. I neglected to connect the exposed pad on the bottom of the E144 package to GND. The USB-Blaster connected to the onboard JTAG port is unable to recognize any devices. I have determined that the first and second devices in the JTAG chain resond to TDI with a TDO but the last device in the chain (the EP3C5E144) has no activity on TDO. Would this be the expected behavior if the exposed pad on the device bottom is not connected?
Thanks, Craig
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