FAQ: Under what conditions can I daisy-chain JTAG?

Filed under FAQ / JTAG IEEE 1149.1

Q: Under what conditions can I daisy-chain JTAG?


This isn't an easy questions to answer, because a lot of conditions have to be met here. First of all every IC in your chain must be electrically compatible with JTAG pins. Check the datasheet to see if your ICs meet this condition. The JTAG communication speed must be low enough even for the slowest IC, which means that if you need a high-speed JTAG but you include some very slow part in your chain, it will degrade throughput of JTAG chain dramatically and could make the debugging or programming impossible.

Another problem will occur with "extended" JTAG pins such as nTRST/nSRST/DBGRQ/etc. These pins cannot be multiplexed and only one IC can provide them (for example connecting up multiple nSRST pins together would reset all such ICs on your board).

Related links

SECONS Ltd. is not in any way connected with integrated circuit manufacturers and there is no any type of authorization, association or affiliation between SECONS and integrated circuit manufacturers. All information present in this FAQ is provided for informational purposes only, on "as-is" basis, and is intended to be useful and informative for SECONS customers.
Copyright (c) SECONS s.r.o. 2008 | Legal info