SN54LVT18502

3.3-V ABT SCAN TEST DEVICE

WITH 18-BIT UNIVERSAL BUS TRANSCEIVERS

SCBS669 – JULY 1996

6

POST OFFICE BOX 655303

state diagram description

The TAP controller is a synchronous finite state machine that provides test-control signals throughout the

device. The state diagram shown in Figure 1 is in accordance with IEEE Standard 1149.1-1990. The TAP

controller proceeds through its states based on the level of TMS at the rising edge of TCK.

As shown, the TAP controller consists of 16 states. There are six stable states (indicated by a looping arrow in

the state diagram) and ten unstable states. A stable state is a state the TAP controller can retain for consecutive

TCK cycles. Any state that does not meet this criterion is an unstable state.

There are two main paths through the state diagram: one to access and control the selected data register and

one to access and control the instruction register. Only one register can be accessed at a time.

Test-Logic-Reset

The device powers up in the Test-Logic-Reset state. In the stable Test-Logic-Reset state, the test logic is reset

and is disabled so that the normal logic function of the device is performed. The instruction register is reset to

an opcode that selects the optional IDCODE instruction, if supported, or the BYPASS instruction. Certain data

registers can also be reset to their power-up values.

The state machine is constructed such that the TAP controller returns to the Test-Logic-Reset state in no more

than five TCK cycles if TMS is left high. TMS has an internal pullup resistor that forces it high if left unconnected

or if a board defect causes it to be open circuited.

For the SN54LVT18502, the instruction register is reset to the binary value 10000001, which selects the

IDCODE instruction. Bits 47–44 in the boundary-scan register are reset to logic 1, ensuring that these cells that

control A-port and B-port outputs, are set to benign values (i.e., if test mode were invoked the outputs would

be at the high-impedance state). Reset value of other bits in the boundary-scan register should be considered

indeterminate. The boundary-control register is reset to the binary value 010, which selects the PSA

test operation.

Run-Test/Idle

The TAP controller must pass through the Run-Test /Idle state (from Test-Logic-Reset) before executing any test

operations. The Run-Test /Idle state also can be entered following data-register or instruction-register scans.

Run-Test/Idle is a stable state in which the test logic can be actively running a test or can be idle. The test

operations selected by the boundary-control register are performed while the TAP controller is in the

Run-Test /Idle state.

Select-DR-Scan, Select-lR-Scan

No specific function is performed in the Select-DR-Scan and Select-lR-Scan states, and the TAP controller exits

either of these states on the next TCK cycle. These states allow the selection of either data-register scan or

instruction-register scan.

Capture-DR

When a data-register scan is selected, the TAP controller must pass through the Capture-DR state. In the

Capture-DR state, the selected data register can capture a data value as specified by the current instruction.

Such capture operations occur on the rising edge of TCK, upon which the TAP controller exits the

Capture-DR state.