VSC8658 datasheet(32/95 Pages) MICROSEMI

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Functional Descriptions

VMDS-10242 VSC8658 Datasheet Revision 4.1

Figure 16 •

Test Access Port and Boundary Scan Architecture Diagram

After a TAP reset, the device identification register is serially connected between TDI and TDO by

default. The TAP instruction register is loaded either from a shift register (when a new instruction is

shifted in) or, if there is no new instruction in the shift register, a default value of 0110 (IDCODE) is

loaded. Using this method, there is always a valid code in the instruction register, and the problem of

toggling instruction bits during a shift is avoided. Unused codes are mapped to the BYPASS instruction.

4.16.8

JTAG Instruction Codes

The VSC8658 device supports the following instruction codes:

EXTEST Allows tests of the off-chip circuitry and board-level interconnections by sampling input pins

and loading data onto output pins. Outputs are driven by the contents of the boundary-scan cells, which

have to be updated with valid values (with the PRELOAD instruction) prior to the EXTEST instruction.

SAMPLE/PRELOAD Allows a snapshot of inputs and outputs during normal system operation to be

taken and examined. It also allows data values to be loaded into the boundary-scan cells prior to the

selection of other boundary-scan test instructions.

IDCODE Provides the version number (bits 31:28), part number (bits 27:12), and the manufacturer

identity (bits 11:1) to be serially read from the device.

The following table provides information about the meaning of IDCODE binary values stored in the

device JTAG registers.

Table 7 •

JTAG Device Identification Register Description

Description

Device Version

Number

Model Number

Manufacturing

Identity

LSB

Bit field

31 through 28

27 through 12

11 through 1

Binary value

0000

1000 0110 0101 1000

001 1001 1000

Boundary-Scan Register

Device Identification

Bypass Register

Instruction Register,

Instruction Decode,

Control

Test Access Port

Controller

NTRST

TMS

TDI

TCK

Select

Control

TDO Enable

Control

Mux,

DFF

TDO

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