Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
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

Ok, so the handbook indicates that the CPLD internal weak pull-ups are active in the unprogrammed state.

The signals connected to the CPLD pins will have a weak high applied to them. If an external device drives the signal on the CPLD, then it will over-ride the weak pull-up. If an external device is driven by the CPLD, then it will have a weak high applied, eg., an active low signal will be deasserted.

If you have a device signal that you would like a low on during power-on, then you need an external pull-down resistor on the signal, eg., 300-Ohms to ground. For example, the resetN signal on a controller that is controlled by the CPLD. If the CPLD is not programmed, then the external device will be held in reset.

I hope that is a little clearer.

Cheers,

Dave