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Altera_Forum
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14 years ago

Unused CPLD Pins

I have two CPLDs on my PCB that connect to a common SPI bus. In order to keep things simple I'm trying to talk to only one of the chips at first but I'm having some trouble with the voltages which le...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
14 years ago

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What thoughts do you have regarding the bus-contention I spoke of? MISO_Receiver is unused and so set to "output as driving ground" - I am guessing this means it's set to LOW. If that's the case, then surely two signals are trying to driving Pin 1 of the Buffer and this is causing problems. Perhaps this is why the pin is never gets to 3.3V because the other pin isn't Hi-Z.

Would appreciate your thoughts on this.

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You definitely do not want to have that setting. It should be set to "As input".

My Tcl synthesis scripts always have this setting: 

#  Tri-state unused pins#     set_global_assignment -name RESERVE_ALL_UNUSED_PINS_WEAK_PULLUP "AS INPUT TRI-STATED"
set_global_assignment -name RESERVE_ALL_UNUSED_PINS_WEAK_PULLUP "AS INPUT TRI-STATED WITH WEAK PULL-UP"
and I uncomment the line I want.

If you're using a new CPLD, then the default setting should be as input. Did you change it?

Cheers,

Dave