SN54ABT8996, SN74ABT8996

10-BIT ADDRESSABLE SCAN PORTS

MULTIDROP-ADDRESSABLE IEEE STD 1149.1 (JTAG) TAP TRANSCEIVERS

SCBS489C – AUGUST 1994 – REVISED APRIL 1999

5

POST OFFICE BOX 655303

application information

In application, the ASP is used at each of several (serially-chained) groups of IEEE Std 1149.1-compliant

devices. The ASP for each such group is assigned an address (via inputs A9–A0) that is unique from that

assigned to ASPs for the remaining groups. Each ASP is wired at its primary TAP to common (multidrop) TAP

signals (sourced from a central IEEE Std 1149.1 bus master) and fans out its secondary TAP signals to the

specific group of IEEE Std 1149.1-compliant devices with which it is associated. An example is shown in

Figure 1.

ASP

IEEE Std 1149.1-

Compliant

Device Chain

A9–A0

ASP

IEEE Std 1149.1-

Compliant

Device Chain

A9–A0

ASP

IEEE Std 1149.1-

Compliant

Device Chain

A9–A0

TRST

TDO

TMS

TCK

TDI

IEEE

Std

1149.1

Bus

Master

To

Other

Modules

Figure 1. ASP Application

This application allows the ASP to be wired to a 4- or 5-wire multidrop test access bus, such as might be found

on a backplane. Each ASP would then be located on a module, for example a printed-circuit board (PCB), which

contains a serial chain of IEEE Std 1149.1-compliant devices and which would plug into the module-to-module

bus (e.g., backplane). In the complete system, the ASP shadow protocols would allow the selection of the scan

chain on a single module. The selected scan chain could then be controlled, via the multidrop TAP, as if it were

the only scan chain in the system. Normal IR and DR scans can then be performed to accomplish the module

test objectives.

Once scan operations to a given module are complete, another module can be selected in the same fashion,

at which time the ASP-based connection to the first module is dissolved. This procedure can be continued

progressively for each module to be tested. Finally, one of two global addresses can be issued to either leave

all modules unselected (disconnect address, DSA) or to deselect and reset scan chains for all modules (reset

address, RSA).

Additionally, in Pause-DR and Pause-IR TAP states, a third global address (test-synchronization address, TSA)

can be invoked to allow simultaneous TAP-state changes and multicast scan-in operations to selected modules.

This is especially useful in the former case, for allowing selected modules to be moved simultaneously to the

Run-Test-Idle TAP state for module-level or module-to-module built-in self-test (BIST) functions, which operate

synchronously to TCK in that TAP state, and in the latter case, for scanning common test setup/data into multiple

like modules.