SN54LVT8980, SN74LVT8980

EMBEDDED TEST-BUS CONTROLLERS

IEEE STD 1149.1 (JTAG) TAP MASTERS WITH 8-BIT GENERIC HOST INTERFACES

SCBS676C – DECEMBER 1996 – REVISED AUGUST 1997

6

POST OFFICE BOX 655303

application information (continued)

The eTBC also provides several capabilities that support special target application requirements. The eTBC’s

test-output enable allows its master function to be disabled so that another device (an external tester, for

example) can control the target TAP. Where required, due to target non-compliance or sensitivity to state

sequencing, discrete-control mode provides the host software with arbitrary control of TMS and TDO

sequences. Also, where targets may be sensitive to leaving Shift-DR state during scan operation, gated-TCK

mode allows the TCK output to be stopped, rather than cycling the target TAP state to Pause-DR state, when

service to TDI buffer or TDO buffer is required.

Where target devices are extremely distant (due to cabling, etc.), pipelining may be implemented at intervals

along the incoming or outgoing paths to retime (deskew) the TDI, TDO, and TMS signals. An example is shown

in Figure 2. In such applications, the eTBC can automatically adjust the incoming test-data bit stream to account

for cycle delays introduced by the pipeline.

TDI

TDO

TMS

TCK

’LVT8980

eTBC

1D

1D

1D

C1

Distant

IEEE

Std

1149.1-

Compliant

Device

Chain

Figure 2. Retimed Interface to Target

Also, in gated-TCK mode, special scan commands provide transparent support for addressable shadow

protocols. Thus, in conjunction with its high-drive outputs, the eTBC can fully support multidrop backplane TAP

configurations implemented with TI’s addressable scan ports (ASP). Figure 3 shows a multidrop TAP

configuration in a passive-backplane application implemented with a centralized (one eTBC per chassis/rack)

test-control architecture, while Figure 4 shows a passive-backplane application implemented with a distributed

(eTBC per module) test-control architecture. Figure 5 shows a multidrop TAP configuration in an

active-backplane (motherboard) application.