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I have a PCB with the a Xilinx CPLD and the TDI, TDO, TCK, TMS pins close to the JTAG programming header (2.54mm) have ESD suppressors (BZA868AVL).
What is the purspose of these diodes on these pins?
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The ESD diodes suppress ESD, if for example a charged machine or human comes in close contact to the pins and there is an electrostatic discharge to the IO pins.

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  • \$\begingroup\$ I have never seen something like this on programming header of a Michrochips microcontrollers. It is something standard for protection reasons on JTAG or it is something optional? \$\endgroup\$ Commented Apr 2 at 21:54
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    \$\begingroup\$ @DarkKnight You can optionally put extra protection or leave it out if it is not necessary. We cannot know what was the design goal for that board, but in many occasions eval boards are expected to be handled roughly in a lab environment by students etc, and for some Microchip PIC it's just once programmed at factory and assumed to experience no ESD in a controlled environment. \$\endgroup\$ Commented Apr 2 at 22:06

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