04-01-2011 01:18 PM
I am using a proprietary PCI backplane (3.3V signaling) that supports hotswapping and section 126.96.36.199 of the PCI 3.0 spec lists worst case overvoltage as 7.1V which is in excess of the Absolute Max input voltage for the IO pins. The concern is that during hot-insertion a device like the Spartan-6, which does not have its PCI clamp diodes enabled until after configuration, will be exposed to large-swing signals.
Adding isolation devices like quickswitches, or external clamp diodes isn’t an option as the entire capacitance budget for each module is only 15.3pF(connector/routing/pin), and the worst case pin capacitance of the Spartan-6 is 10pF, which doesn’t leave enough wiggle room to accommodate the extra capacitance of these devices.
I need a solution that would allow hotswap in this environment without the potential for damaging the device.
Alternately, is there characterization data that shows worst case overshoot to be less than the absolute max Input voltage?
Is there any official statement that operation at these levels and conditions is acceptable, for these parts?
04-01-2011 03:01 PM - edited 04-01-2011 03:03 PM
Any device which has clamp diodes to VCC/O when unpowered is, by definition, not hot-swappable.
Spartan-6 is hot-swappable. Look at the the datasheet (DS162), Table 1. For commercial temp range parts, all IO pins tolerate voltages up to 4.25V (20% overshoot duration) or up to 4.1V (steady state) even un-powered. This is your official answer, nothing more is needed.
For 3.3V PCI (or lower), quickswitch buffers and similar remedies are not needed.
-- Bob Elkind
04-05-2011 09:26 AM - edited 04-05-2011 09:30 AM
You will not want to apply 7.1v to an unpowered Spartan-6 device. Without the clamp diodes enabled (pre-configuration), you need to respect the DS162 specs.
I agree that the IOs do not require a Vcco voltage to withstand 4.1v (steady state), but to be tolerant to higher voltages, you will need the clamp diodes enabled, which requires configuration to be completed.