SN54LVT8980A, SN74LVT8980A

EMBEDDED TEST BUS CONTROLLERS

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

SCBS755B − APRIL 2002 − REVISED MARCH 2004

6

POST OFFICE BOX 655303

application information (continued)

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

TOE 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 noncompliance 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 can 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 automatically can adjust the incoming test-data bit stream to account

for cycle delays introduced by the pipeline.

TDI

TDO

TMS

TCK

’LVT8980A

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 addressable scan ports (ASPs). 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.