What happens when I/O signals are driven while the Spartan-3/-3E is unpowered?
Will this damage the device? Can you hot-swap a Spartan-3/-3E?
The Spartan-3/-3E supports only limited hot-swapping, if special precautions are taken, because an internal clamp diode to VCC is always present in the I/O.
As per the Absolute Maximum Ratings of the Spartan-3/-3E Data Sheet, Vin of the I/Os cannot exceed VCCO + 0.5V.
The Spartan-3 DC and Switching Characteristic Data Sheet is located at:
The Spartan-3E DC and Switching Characteristic Data Sheet is located at:
If Vin is greater than VCC + 0.5 V, the internal clamp diode to VCC will turn on and conduct a reverse current back into the associated rail.
This reverse current needs to be limited to 10 mA to prevent damage to the device. When the I/O is unpowered (VCCO = 0V or Vccaux = 0V for dedicated pins), then the clamp diodes will turn on at Vin = 0.5V.
To prevent damage to the device, a series resistor is required on all I/Os that will be driven while the device is unpowered.
The value of the series resistor can be calculated as follows: R = (Vin - 0.5)/10mA. Therefore, if Vin is 3.3V max, then R = (3.3-0.5)/10mA = 280 ohm (min).
An alternative for hot-swap applications, where Spartan-3/-3E cards are inserted into a system or system backplane, is to use sequenced connectors, where VCC and GND mate well before any signal pins can mate.
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