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AR# 1272

Cable - How many devices can a Parallel Cable III (JTAG) program in a chain?

Description

Keywords: drive, cable, JTAG, CPLD, cable

How many devices can the Parallel Cable III (JTAG) program in a chain?

Solution

The limiting factor on the number devices that can exist in a JTAG chain is the drive capability of the TMS and TCK signals.

Due to a 5KOhm pull-up on the TMS pin, the current JTAG cable is capable of driving a maximum of seven devices. (In noisy environments, that number may be less.) If a larger chain of devices is needed, additional buffering will be required.

For more information, please refer to the ISP Checklist in the Xilinx Application Note, "A Quick JTAG ISP Checklist" (Xilinx XAPP104).
AR# 1272
Date Created 09/24/1996
Last Updated 07/25/2007
Status Active
Type General Article