Embedded Design Handbook

ID 683689
Date 8/28/2023
Public
Document Table of Contents

6.3.2.1.4. JTAG

FPGAs use the JTAG interface for programming, communication, and verification. Designers frequently connect several components, including FPGAs, discrete processors, and memory devices, communicating with them through a single JTAG chain. Sometimes the JTAG signal is distorted by electrical noise, causing a communication failure for the entire group of devices. To guarantee a stable connection, you must isolate the FPGA under test from the other devices in the same JTAG chain.

The figure below Part A illustrates a JTAG chain with three devices. The tdi and tdo signals include 0 ohm resistors between each device. By removing the appropriate resistors, it is possible to isolate a single device in the chain as the figure below Part B illustrates. This technique allows you to isolate one device while using a single JTAG chain.

Figure 262. JTAG Isolation

To learn more about JTAG refer to AN39: IEEE 1149.1(JTAG) Boundary-Scan Testing in Intel FPGA Devices.