AR# 7695


CPLD CoolRunner-II XPLA3 - Are weak pull-up resistors needed for a CoolRunner JTAG daisy chain?


General Description:

Do CoolRunner XPLA3 or CoolRunner-II devices require weak pull-up resistors on the JTAG pins between daisy-chained parts?


No, this is not required, but it is recommended. The internal pull-ups present on TDI and TMS (as required by the IEEE 1149.1 specification) can be as weak as 100k Ohm. As a result, it can be beneficial to apply external pull-ups to the JTAG pins to prevent noise from accidentally toggling the JTAG pins.

For XPLA3, the pull-ups are to VCCINT.

For CoolRunner-II, the pull-ups are to VCCAUX.

For more information, see (Xilinx Answer 11433).

AR# 7695
Date 12/15/2012
Status Active
Type General Article
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