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U. Glaeser

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FIGURE 46.5<br />

FIGURE 46.6<br />

of the DUT. In the second mode called scan mode, all BS cells are connected into a BS chain. In this<br />

mode ShiftDR = 1 and the necessary clock pulses are applied. The third mode is the capture mode where<br />

data are loaded through the input IN into the scan path. In this mode, ShiftDR = 0 and one clock pulse<br />

ClockDR is applied. The QA flip-flop serves as a snapshot cell. The final mode is called update mode. In<br />

this mode, Mode_Control = 1 and one pulse of the UpdateDR is activated.<br />

These four functioning modes allow each of the instructions summarized below to be executed.<br />

The main motivation for this standard is to overcome the problem of physical access to circuits that<br />

becomes more and more difficult. JTAG overcomes the need of bed-of-nails fixtures with a very fine<br />

resolution, if available.<br />

As shown in Fig. 46.6, the BS architecture includes the following blocks:<br />

•<br />

•<br />

TAP controller:<br />

This block generates all clocks and signals that are necessary for the architecture.<br />

Instruction register:<br />

This register holds the instruction, which is related to the test that needs to<br />

be executed.<br />

© 2002 by CRC Press LLC<br />

Overview of the ANSI/IEEE 1149.1 architecture.<br />

I<br />

N<br />

SIN<br />

TMS<br />

CLK<br />

RST<br />

SOUT<br />

SIN<br />

A BS cell.<br />

MUX<br />

Instruction<br />

register<br />

Optional Test<br />

Data Register<br />

TAP<br />

controller<br />

bypass<br />

register<br />

MUX<br />

ShiftDR ClockDR<br />

Boundary scan<br />

register Boundary scan cell I/O<br />

QA<br />

SOUT<br />

UpdateDR<br />

QB<br />

Internal logic<br />

of the design<br />

under test<br />

MUX<br />

Mode_Control<br />

OUT

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