PI7C8150BMA-33 datasheet(96/107 Pages) PERICOM

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PI7C8150B

ASYNCHRONOUS 2-PORT PCI-TO-PCI BRIDGE

Page 96 of 107

April 2015 – Revision 2.1

This mode of operation is valuable for design debugging and fault diagnosis since it

permits examination of connections not normally accessible to the test system. The

following subsections describe the boundary-scan test logic elements: TAP pins,

instruction register, test data registers and TAP controller. Figure 16-1 illustrates how

these pieces fit together to form the JTAG unit.

Figure 16-1

Test Access Port Block Diagram

16.1.1

TAP PINS

The PI7C8150B’s TAP pins form a serial port composed of four input connections (TMS,

TCK, TRST_L and TDI) and one output connection (TDO). These pins are described in

Table 16-1. The TAP pins provide access to the instruction register and the test data

registers.

16.1.2

INSTRUCTION REGISTER

The Instruction Register (IR) holds instruction codes. These codes are shifted in through

the Test Data Input (TDI) pin. The instruction codes are used to select the specific test

operation to be performed and the test data register to be accessed.

The instruction register is a parallel-loadable, master/slave-configured 5-bit wide, serial-

shift register with latched outputs. Data is shifted into and out of the IR serially through the

TDI pin clocked by the rising edge of TCK. The shifted-in instruction becomes active upon

latching from the master stage to the slave stage. At that time the IR outputs along with the

TAP finite state machine outputs are decoded to select and control the test data register

selected by that instruction. Upon latching, all actions caused by any previous instructions

terminate.

The instruction determines the test to be performed, the test data register to be accessed, or

both. The IR is two bits wide. When the IR is selected, the most significant bit is connected

to TDI, and the least significant bit is connected to TDO. The value presented on the TDI

pin is shifted into the IR on each rising edge of TCK. The TAP controller captures fixed

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