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SN74ABT18504PM Datasheet(PDF) 9 Page - Texas Instruments |
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SN74ABT18504PM Datasheet(HTML) 9 Page - Texas Instruments |
9 / 30 page SN54ABT18504, SN74ABT18504 SCAN TEST DEVICES WITH 20-BIT UNIVERSAL BUS TRANSCEIVERS SCBS108B – AUGUST 1992 – REVISED JUNE 1993 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 9 state diagram description (continued) 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 provides the capability of suspending and resuming 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. register overview With the exception of the bypass and device identification registers, any test register can be thought of as a serial shift register with a shadow latch on each bit. The bypass and device identification registers differ in that they contain only a shift register. During the appropriate capture state (Capture-IR for instruction register, Capture-DR for data registers), the shift register may be parallel loaded from a source specified by the current instruction. During the appropriate shift state (Shift-IR or Shift-DR), the contents of the shift register are shifted out from TDO while new contents are shifted in at TDI. During the appropriate update state (Update-IR or Update-DR), the shadow latches are updated from the shift register. instruction register description The instruction register (IR) is eight bits long and is used to tell the device what instruction is to be executed. Information contained in the instruction includes the mode of operation (either normal mode, in which the device performs its normal logic function, or test mode, in which the normal logic function is inhibited or altered), the test operation to be performed, which of the four data registers is to be selected for inclusion in the scan path during data register scans, and the source of data to be captured into the selected data register during Capture-DR. Table 4 lists the instructions supported by the ′ABT18504. The even-parity feature specified for SCOPE™ devices is supported in this device. Bit 7 of the instruction opcode is the parity bit. Any instructions that are defined for SCOPE ™ devices but are not supported by this device default to BYPASS. During Capture-IR, the IR captures the binary value 10000001. As an instruction is shifted in, this value will be shifted out via TDO and can be inspected as verification that the IR is in the scan path. During Update-IR, the value that has been shifted into the IR is loaded into shadow latches. At this time, the current instruction is updated and any specified mode change takes effect. At power up or in the Test-Logic-Reset state, the IR is reset to the binary value 10000001, which selects the IDCODE instruction. The instruction register order of scan is illustrated in Figure 2. Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 TDO TDI Bit 7 Parity (MSB) Bit 0 (LSB) Figure 2. Instruction Register Order of Scan |
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