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SN74ACT8999NT Scheda tecnica(PDF) 3 Page - Texas Instruments |
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SN74ACT8999NT Scheda tecnica(HTML) 3 Page - Texas Instruments |
3 / 29 page SN54ACT8999, SN74ACT8999 SCANPATH SELECTORS WITH 8BIT BIDIRECTIONAL DATA BUSES SCANCONTROLLED IEEE STD 1149.1 (JTAG) TAP MULTIPLEXERS SCAS158D − JUNE 1990 − REVISED DECEMBER 1996 3 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 POST OFFICE BOX 1443 • HOUSTON, TEXAS 77251−1443 functional block description The ’ACT8999 implements two separate functions in one package. The primary function of the device is to include a selected secondary scan path in the system’s primary scan path to enable a PBC to perform controlling and observing test functions on the selected path. This is accomplished by driving the TMS terminal(s) of a secondary scan path with one of the DTMS pins of the device. This approach allows a system to have built-in testability at all levels without requiring that the primary-system scan path always include all subsystem scan paths. As a result, test throughput is improved and the amount of test data that must be interpreted is reduced. The device includes error-detection circuitry that prevents the user from inadvertently activating more than one secondary scan path at a time. Another function of the device is provided by the 8-bit identification bus. This bus can be hardwired with pullup and pulldown resistors to supply an identification code to the test controller(s) to verify that test operations are being performed on the proper portion of the system. The bus can also transfer data and instructions to another device, such as a local or remote bus controller, and pass control of the scan-path select function to that device. This frees the primary controller to activate another secondary scan path elsewhere in the system or perform higher-level test control functions. When the RBC is ready to return control of the device, interrupt signals alert the primary controller. The least-significant bit (LSB) of any value scanned into any register of the device is the first bit shifted in (nearest to TDO). The most-significant bit (MSB) is the last bit shifted in (nearest to TDI). The ’ACT8999 is divided into functional blocks as detailed below. test ports The test ports decode the signals on TCK, TMS, OTMS, and TRST to control the operation of the circuit. Each test port includes a TAP controller that issues the proper control instructions to the data registers according to the IEEE Standard 1149.1 protocol. The TAP controller state diagram is shown in Figure 1. Two test ports are included on the ’ACT8999, allowing different test controllers to command different sections of the device. TMS circuit The TMS circuit decodes bits in the select and control registers to determine which one, if any, of the DTMS pins (which provide mode-select signals to the secondary scan path(s)) follow the TMS pin or OTMS pin. The unselected DTMS pins are set by the circuit to a static high or low level. instruction register The instruction register (IR) is an 8-bit-wide serial-shift register that issues commands to the device. Data is input to the instruction register via TDI or DTDI and shifted out via TDO. All device operations are initiated by loading the proper instruction or sequence of instructions into the IR. data registers Six parallel data registers are included in the ’ACT8999: bypass, control, counter, boundary-scan, ID-bus, and select. The ID bus register is a part of the boundary-scan register. Each data register is serially loaded via TDI or DTDI and outputs data via TDO. Table 1 summarizes the registers in the ’ACT8999. Table 1. Register Summary REGISTER NAME LENGTH (BITS) FUNCTION Instruction 8 Issue command information to the device Remote Instruction 8 Issue command information to the select register Control 13 Configuration and enable control Counter 8 Count events on DCI, output interrupts via DCO Select 8 Select one of four DTMS pins to follow TMS or OTMS Boundary Scan 15 Capture and force test data at device periphery ID Bus 8 Pass test commands and data between a PBC and RBC(s) Bypass 1 Remove the ’ACT8999 from the scan path |
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