01-09-2020 11:47 AM
For the Artix7 XQ7A200T-2RB676I, are there any concerns with applying voltage to the I/O pins when the FPGA is not powered? Currently, we have a power-on setup where multiple discretes at a voltage of about 2.8V are being applied to the I/O pins of the FPGA before the FPGA is powered on. A few seconds afterwards, the FPGA is powered on. The discretes go through a resistor network which limits the current to a maximum of .325mA. Is there risk of damage to the FPGA or is there some sort of input protection of the I/O pins when the FPGA is not powered?
01-09-2020 11:52 AM
Hi @darlm ,
Unpowered pins still have clamp diodes present that will be forward biased when a pin voltage is applied.
There are two aspects of the device specifications that must be considered when determining if a device could be damaged. For more information refer this AR#34347 .
Hope this helps.
01-13-2020 07:40 AM
We are not using a battery on our board, so Vccaux and VCC are both 0V, when the I/O voltages are applied to the I/O pins. Therefore, in order to not violate the (Vcco - Vccaux) spec, we want to keep Vcco at 2.625V max.
So, does this mean that the maximum Vin that can be applied to an unpowered bank in our case, assuming a .43 diode drop would be approximately 2.625V+.43V = 3.055V, to prevent risk of damage to the device?
01-16-2020 06:13 AM
When your bank is unpowered the Vcco = 0V, therefore the Vin which is recommended to be Vcco + 0.2V = 0 + 0.2V = 0.2V
Therefore when the bank is unpowered the Vin should be below 0.2V.
The Absolute Max rating for Vin = Vcco + 0.55V = 0 + 0.55V = 0.55V, when the bank is unpowered that absolute max Vin should be 0.55V.
01-17-2020 05:03 AM
I will try to write a more complete answer here.
As I said when the bank is unpowered you cannot use the expected max of 2.625V in the Vin equation. Theoretically the Vcco is 0 for an unpowered bank and the Vin becomes 0.2V.
Xilinx in general does not recommend driving I/Os when Vcco in unpowered. When the bank is unpowered and IOs are driven the clamp diode turns on.
As @ddn pointed to AR : https://www.xilinx.com/support/answers/37347.html
Where you need to consider the current through the clamp diode, Iin has a recommend limit in the datasheet of 10mA per diode with a total of 200mA per bank.
For 7 Series there is the TVCCO2VCCAUX specification that @ddn mentioned. The concern is that powering the IOs can reverse biasing the Vcco before the Vccaux and bring the TVCCO2VCCAUX specification in to be consider.
We also have a specific AR addressing the reverse biasing of the Vcco and how to limit the current to meet the TVCCO2VCCAUX specification.
If you ensure that all specifications are met there should be no issues with long term reliability.