Tuesday, November 18, 2008

Differential Signals and JTAG Boundary Scan

IEEE 1149.6
AC-coupled differential interconnections on very high speed (1+ Gbps) data paths are not testable using traditional IEEE 1149.1 techniques. The IEEE 1149.1 structures and methods are intended to test static (DC-coupled), single ended networks. IEEE 1149.6 is specifically designed for testing high speed differential, including AC coupled networks.

PCI Express Signalling/Connector Testing Support
JTAG Boundary-scan testing of PCIe connectors is pretty much out of the question. The PCIe differential signals are capacitively coupled so the boundary-scan devices on both sides of the PCIe connector would have to have some kind of dot6 compliant cells to test through the series capacitor. All of the boundary-scan compatible chips with PCIe interfaces that I saw had the PCIe signals specified as linkage bits so they wouldn't be testable anyway. If the customer has a specific device that needs to be tested we can look at the bsdl file to see if anything can be done.
We may be able to run software on the chips on both sides of the PCIe connector and test the link functionally (note that the link speed is 2.5Gb/s!!!).


Etsog said...

Hello! We are doing BScan on a project now, and we have an interface that's using differential signalling. The vendor that's doing the BScan insertion suggests using 1149.1 cells for the differential signals, instead of 1149.6. By the way, there won't be any AC coupling on the differential signals. If this is the case, will 1149.1 still work and detect board/connections defects and faults? Thanks!

JTAG said...

In a nutshell, 1149.1 will detect some faults and 1149.6 will detect additional faults.

1149.6 includes improvements to differential signal testing as well AC coupling—one specific being the option to include test receivers on both the positive and negative leg of each differential pair. This helps get around the fault immunity of the common 1149.1 methods, where a single output cell is used for each TX pair and a single input cell used for each RX pair.

For example, ScanExpress tools (made by my vendor, Corelis) will detect DC-coupled 1149.6 nets and generate appropriate test vectors utilizing the positive and negative leg receivers, allowing detection of additional faults.