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U. Glaeser

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FIGURE 46.3<br />

FIGURE 46.4<br />

Boundary Scan<br />

As a widely used scan technique, boundary-scan (BS) provides an access to internal structures (blocks,<br />

IC, etc.) knowing that few logic is directly accessible from primary inputs and primary outputs. Probably<br />

one of the famous scan approaches is BS through the ANSI/IEEE 1149.1 standard. This standard is also<br />

known as JTAG (Join Action Test Group). A decade earlier, a group of test experts from industry and<br />

academia worked together in order to propose standardized protocols and architectures that help in the<br />

dissemination of the BS technique.<br />

Given a DUT such as a PCB, BS consists of as a first step in making each input and output of a circuit<br />

as a memory element. Given all memory elements, the next step consists in linking all memory elements<br />

into a register called BS register. The access to this mandatory register is possible from four standard<br />

pins. Finally, in order to be conform with the boundary scan standard ANSI/IEEE Std. 1149.1, a controller<br />

called TAP (Test Access Port) and a set of registers are implemented within each circuit. This is shown<br />

in Fig. 46.5.<br />

The BS register is mandatory in a JTAG compliant architecture. Figure 46.6 gives an example of a BS<br />

cell. Such a cell can be used as either an input or as an output cell. Such a cell supports four functional<br />

modes: normal mode (when Mode_Control = 0), where the BS cell is transparent during the normal use<br />

© 2002 by CRC Press LLC<br />

SCAN IN<br />

SCAN OUT<br />

Scan architecture with a single path.<br />

SCAN IN 1<br />

Scan architecture with several scan chains.<br />

IC 1 IC 2 IC 3<br />

IC 6 IC 5 IC 4<br />

SCAN IN 2 SCAN IN 3<br />

IC 1 IC 2 IC 3<br />

IC 6 IC 5 IC 4<br />

SCAN OUT 1 SCAN OUT 2 SCAN OUT 3

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