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AR# 62972

7-Series FPGAs: Do Xilinx still recommend the use of the 330 Ohm pull-up resistor on DONE or not?

Description

XMP277 recommends using a 330 Ohm pull-up resistor on DONE.

All configuration interface diagrams from UG470 show a 330 Ohm pull-up resistor on DONE.

However, in the latest version of UG470 (v1.9), the following information is found in Table 2-4:

"External 330 resistor circuits are not required but can be used as they have been in previous generations"

Should I use the 330 Ohm pull-up resistor on DONE or not?

Solution

Use of the 330 Ohm resistor on DONE is still recommended for some situations.

Although the FPGA DONE input is more tolerant of poor DONE transitions, DONE can often be used to drive other circuits in the system. T

As a result, it is safer to present a better low-to-high transition via the stronger pull-up to those inputs.


This statement was added to the latest version of UG470 because technically speaking, the FPGA does not need the 330 Ohm pull-up due to the "weak" internal pull-up. 

However, you must decide whether the resulting transition via the weak pull-up is good enough, considering everything else connected to DONE on the board.

AR# 62972
Date Created 11/28/2014
Last Updated 01/05/2015
Status Active
Type General Article
Devices
  • Artix-7
  • Kintex-7
  • Virtex-7