參數(shù)資料
型號(hào)： SN54LVT8980JT
廠商： TEXAS INSTRUMENTS INC
元件分類： 微控制器/微處理器
英文描述： SPECIALTY MICROPROCESSOR CIRCUIT, CDIP24
封裝： 0.300 INCH, CERAMIC, DIP-24
文件頁(yè)數(shù)： 11/36頁(yè)
文件大?。?/td> 509K
代理商： SN54LVT8980JT
SN54LVT8980, SN74LVT8980
EMBEDDED TEST-BUS CONTROLLERS
IEEE STD 1149.1 (JTAG) TAP MASTERS WITH 8-BIT GENERIC HOST INTERFACES
SCBS676D – DECEMBER 1996 – REVISED AUGUST 2002
19
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
TCK generator
The TCK generator sources the test clock (TCK) signal required by the IEEE Std 1149.1 target(s) and the
eTBC-internal test-control logic. The fundamental TCK frequency is produced by division of CLKIN. The divisor
is programmable within a range of 1 to 128 in the configurationB register (bits 7–5, CDIV). The TCK output to
the target(s) operate in free-running or gated modes. The free-running mode toggles TCK continuously, based
on CLKIN, while the gated mode operates the TCK only when required to move the target TAP state or to perform
a run-test or scan operation.
While the eTBC is in discrete-control mode, the TCK generator is not used; instead, the state of TCK is toggled
on each alternating read and write to the discrete-control register. A falling edge of TCK is produced by write,
while a rising edge of TCK is produced by read.
Upon eTBC reset (power-up, hardware-initiated, or software-initiated), the TCK generator assumes its
free-running mode with a clock divisor of 16 (TCK = CLKIN/16).
TAP-state generator
The TAP-state generator sources the TMS signal, which sequences the TAP controllers of connected
IEEE Std 1149.1-compliant target devices. The TAP controller specified by IEEE Std 1149.1 is a synchronous
finite-state machine that provides test control signals throughout each target device; its state diagram is shown
in Figure 8. This diagram and the TAP-controller states are discussed subsequently.
The TAP-state generator operates under the control of an executing command to generate the TMS sequences
required to move connected target devices from one stable state to another, to capture and scan test data
into/out of target devices, and to operate built-in test modes of target devices in the Run-Test/Idle state.
The TAP state currently being generated always is maintained by the TAP-state generator and is constantly
available in the eTBC status register (bits 3–0, TAPST) for host read. Based on the TAP state that is current upon
command initiation, the TAP-state generator sources a defined sequence of TMS values to reach the TAP state
in which the command is progressed (e.g., Shift-IR, Shift-DR, Run-Test/Idle), and ultimately to reach the
specified end TAP state. These sequences are detailed in Tables 7–12.
While the eTBC is in free-running-TCK mode, if a currently operating scan command empties or fills a required
test data buffer, then the TAP-state generator sources the TMS sequences required to move the connected
target devices to their Pause-IR or Pause-DR states. In such case, the TAP-state generator maintains target
devices in their Pause-IR or Pause-DR states until the required test data buffer is serviced appropriately.
However, if such a buffer condition occurs while the eTBC is in gated-TCK mode, the TAP-state generator
maintains the target devices in their Shift-IR or Shift-DR states while the TCK is gated off.
While the eTBC is in discrete-control mode, the TAP-state generator is not used; instead, the state of the TMS
pin is determined by the contents of the discrete-control register. Thus, TMS sequences that cannot be
generated automatically still can be applied through the eTBC to targets that require such (e.g., near-compliant
devices).
The TAP-state generator also is not used during the operation of the special addressable shadow protocol
(ASP) scan commands. Since, by definition, ASPs operate only while the TAP is idling (maintaining one of the
TAP states Test-Logic-Reset, Run-Test/Idle, Pause-IR, or Pause-DR), the TMS pin must be maintained at the
value it held upon initiation of the ASP scan command.
For eTBC verification/debugging, in addition to continuous update of the current target TAP state in the eTBC
status register, the output of the TAP-state (TMS) generator can be selected for loopback into the TDI buffer.
When this TMS-loopback mode is selected, although a host-requested command executes in the eTBC, the
target is not affected, as both TMS and TDI are fixed at a high level.
Upon eTBC reset (power up, hardware initiated, or software initiated), the TAP-state generator assumes the
Test-Logic-Reset TAP state.
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