SN54ABT18245 Datasheet, PDF (8/30 Pages) Texas Instruments – SCAN TEST DEVICE WITH 18-BIT BUS TRANSCEIVERS

SN54ABT18245
SCAN TEST DEVICE WITH 18-BIT BUS TRANSCEIVERS
SGBS307A – AUGUST 1994 – REVISED JANUARY 1995
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 can 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 tells 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 3 lists the instructions supported by the ′ABT18245. 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 is 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 IR order of scan is illustrated in Figure 2.
Bit 7
TDI
Parity
(MSB)
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Figure 2. Instruction Register Order of Scan
Bit 0
(LSB)
TDO
8
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