PM0020 datasheet(69/87 Pages) STMICROELECTRONICS

69 / 87 page

PM00020

ISC logic description

69/87

12.5

Instruction description & flows

12.5.1

Bypass

The BYPASS instruction enables the 1-bit BYPASS_REGISTER between TDI and TDO,

effectively causing the device to be "bypassed" in a JTAG chain. The Bypass Register is

always loaded with a logic-0 on the rising edge of TCK in the Capture-DR state.

12.5.2

Sample/preload

The SAMPLE/PRELOAD instruction selects the BOUNDARY_REGISTER between TDI and

TDO. It does not, however, interfere with the normal operation of the component pins or

logic in anyway (BS_ON = 0). When this instruction is active the SAMPLE function of the

instruction occurs on the rising edge of TCK that transitions the TAP from Capture-DR state

to the Shift-DR state. At this point the boundary-scan cells at each I/O pin simply sample

and store the state being driven into the pin for the case of inputs and the state being driven

by the pin for outputs. In addition, the state of the tri-state enable signals for outputs are

sampled. This information can then be shifted out within the Shift-DR state through TDO.

As that data is shifted out, data can be shifted in through TDI for the PRELOAD function of

the instruction, which basically takes effect on the falling edge of TCK in the Update-DR

state, causing the shifted-in values to be loaded onto the latched parallel outputs of the

boundary-scan cells. These values are not allowed to drive the pins, however. The purpose

is to pre-load the values at component inputs and outputs so that a following EXTEST, or

CLAMP instruction will cause these well-defined values to be driven onto the pins

immediately upon selection of those instructions, rather than having random, unknown

values at the pins.

12.5.3

Clamp

The CLAMP instruction enables the BYPASS_REGISTER between TDI and TDO. The

purpose of this instruction is to allow the component pins to be "clamped" in a known state

while the device is in "bypass" mode as part of a JTAG chain (BS_ON = 1). Normally, this

instruction should be used in conjunction with SAMPLE/PRELOAD. The desired clamp

values for the component pins should be loaded via the SAMPLE/PRELOAD instruction into

the BOUNDARY_REGISTER. When the CLAMP instruction is updated in the Update-IR

state, the pins will be driven with the values from the BOUNDARY_REGISTER. This allows

the pins of a "bypassed" device to be controlled to a known static state during board testing.

12.5.4

HIGHZ

The HIGHZ instruction enables the BYPASS_REGISTER between TDI and TDO. The

purpose of this instruction is to tri-state all component outputs. The outputs are tri-stated

after this instruction is parallel loaded into the instruction register on the falling edge of TCK

in the Update-IR state. The BYPASS_REGISTER operates exactly as it does when the

BYPASS instruction is active.

12.5.5

EXTEST

The required EXTEST instruction places the IC into an external boundary-test mode and

selects the boundary-scan register to be connected between TDI and TDO. The EXTEST

instruction forces the output pins to drive the values from the BOUNDARY_REGISTER

rather than from the normal internal logic (BS_ON = 1). The BOUNDARY_REGISTER cells

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