Virtex data sheet uses R and r to represent Vref pins in the package drawings. What are the differences between R and r?
If an IO_STANDARD does not require the use of Vref, the IO_VREF sites can be used as IOs.
The R indicates that these pins are always IO_VREF, regardless of the device size. The r indicates that these pins are IO_VREF for some devices but not all, and in the smaller devices these are just IO.
If designers want to migrate a design from a smaller device to a larger device in the same package, AND they are using an IO_STANDARD that requires a VREF source, all R and r pins should be connected to the same voltage source in the larger device. They should use a PROHIBIT constraint to prevent an IO from occupying the IO_VREF sites when doing the design in the smaller device.
AR# 6110 | |
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日付 | 12/15/2012 |
ステータス | アクティブ |
種類 | 一般 |