SN54LVT18502

3.3-V ABT SCAN TEST DEVICE

WITH 18-BIT UNIVERSAL BUS TRANSCEIVERS

SCBS669 – JULY 1996

7

POST OFFICE BOX 655303

Shift-DR

Upon entry to the Shift-DR state, the data register is placed in the scan path between TDI and TDO. On the first

falling edge of TCK, TDO goes from the high-impedance state to an active state. TDO enables to the logic level

present in the least-significant bit of the selected data register.

While in the stable Shift-DR state, data is serially shifted through the selected data register on each TCK cycle.

The first shift occurs on the first rising edge of TCK after entry to the Shift-DR state (i.e., no shifting occurs during

the TCK cycle in which the TAP controller changes from Capture-DR to Shift-DR or from Exit2-DR to Shift-DR).

The last shift occurs on the rising edge of TCK, upon which the TAP controller exits the Shift-DR state.

Exit1-DR, Exit2-DR

The Exit1-DR and Exit2-DR states are temporary states that end a data-register scan. It is possible to return

to the Shift-DR state from either Exit1-DR or Exit2-DR without recapturing the data register. On the first falling

edge of TCK after entry to Exit1-DR, TDO goes from the active state to the high-impedance state.

Pause-DR

No specific function is performed in the stable Pause-DR state, in which the TAP controller can remain

indefinitely. The Pause-DR state suspends and resumes data-register scan operations without loss of data.

Update-DR

If the current instruction calls for the selected data register to be updated with current data, such update occurs

on the falling edge of TCK, following entry to the Update-DR state.

Capture-IR

When an instruction-register scan is selected, the TAP controller must pass through the Capture-IR state. In

the Capture-IR state, the instruction register captures its current status value. This capture operation occurs

on the rising edge of TCK, upon which the TAP controller exits the Capture-IR state. For the SN54LVT18502,

the status value loaded in the Capture-IR state is the fixed binary value 10000001.

Shift-IR

Upon entry to the Shift-IR state, the instruction register is placed in the scan path between TDI and TDO. On

the first falling edge of TCK, TDO goes from the high-impedance state to the active state. TDO enables to the

logic level present in the least-significant bit of the instruction register.

While in the stable Shift-IR state, instruction data is serially shifted through the instruction register on each TCK

cycle. The first shift occurs on the first rising edge of TCK after entry to the Shift-IR state (i.e., no shifting occurs

during the TCK cycle in which the TAP controller changes from Capture-IR to Shift-IR or from Exit2-IR to

Shift-IR). The last shift occurs on the rising edge of TCK, upon which the TAP controller exits the Shift-IR state.

Exit1-IR, Exit2-IR

The Exit1-IR and Exit2-IR states are temporary states that end an instruction-register scan. It is possible to

return to the Shift-IR state from either Exit1-IR or Exit2-IR without recapturing the instruction register. On the

first falling edge of TCK after entry to Exit1-IR, TDO goes from the active state to the high-impedance state.

Pause-IR

No specific function is performed in the stable Pause-IR state, in which the TAP controller can remain

indefinitely. The Pause-IR state suspends and resumes instruction-register scan operations without loss

of data.

Update-IR

The current instruction is updated and takes effect on the falling edge of TCK, following entry to the

Update-IR state.